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The brightest star of the constellation Taurus. It is a red giant star, and is about 67 light-years away.

Alpha Centauri

The closest star system to the Sun, at a distance of more than four light-years, Alpha Centauri consists of three stars. Two of the stars — one of which is similar to the Sun, while the other is a little smaller and cooler — form a tight pairing. The third, known as Proxima Centauri, is about one-tenth of a light-year from the other two, and is a red dwarf star. It is closer to Earth than the other two stars, but it is so faint that it is not visible to the unaided eye.


The brightest star of the summer constellation Aquila, the eagle. It is a main-sequence star, shining pure white, and is about 17 light-years from Earth. It forms one point of the Summer Triangle. Its name means “the flying eagle.”

Andromeda Galaxy, M31

The closest large galaxy to our own, at a distance of about 2.5 million light-years. It is a spiral galaxy that appears to be slightly larger but less massive than the Milky Way.

Antarctic Astronomy

Astronomers and planetary scientists conduct extensive research in Antarctica. They operate telescopes at the south pole, launch instruments on balloons, and hunt for meteorites on the Antarctic ice. The balloon missions, launched in southern summer, take advantage of the 24-hour daylight to power solar cells and maintain a steady altitude without the need to drop ballast. They climb to about 130,000 feet, and can remain aloft for weeks or months. They follow circular wind currents that circle the continent, allowing the payload to drop back on the Antarctic ice at the end of a mission. Scientists from the United States and other countries conduct weeks-long searches for meteorites on the ice. Few terrestrial rocks are found atop the ice fields deep in Antarctica, so there's a good probability that any rocks on the ice fell from space.


The brightest star of the zodiacal constellation Scorpius. It is a binary system, consisting of an orange supergiant star that is many times more massive than the Sun, and a smaller companion that is hidden in the primary star’s glare. It is about 550 light-years away.

Aphelion and Perihelion

Aphelion is the greatest distance between a planet or other body and the Sun. Perihelion is the closest distance from the Sun. Objects move slowest in their orbits when they are aphelion and fastest when at perihelion.

Apogee and Perigee

Apogee is the point of greatest separation between Earth and the Moon or an orbiting satellite, while perigee is the point of closest approach.

Apollo Project

The NASA space program that landed the first humans on the Moon. From 1969 to 1972, it put six two-man crews on the lunar surface.

Aquarius, the Water Bearer

A constellation of the zodiac.

Aquila, the Eagle

A constellation best seen in the south in late summer.


The study of astronomical practices and sky lore of ancient civilizations and cultures.


The brightest star of the constellation Boötes. It is an orange-star about 37 light-years from Earth.

Argo Navis, the Ship Argo

An extinct constellation that represented the ship that carried Jason and the Argonauts on their mythological journeys. Its pieces survive as the southern constellations Carina (the keel), Puppis (the poop deck), and Vela (the sails).

Aries, the Ram

Aries is one of the constellations of the zodiac, between Taurus and Pisces. It is a small, faint constellation, distinguished only because of its location along the ecliptic, which is the Sun’s path across the sky. The Sun passes through the constellation in late April and early May. Aries is best viewed in autumn and early winter.

Artemis Project

Artemis is NASA's next project to take astronauts to the Moon. The first test flight launched on November 16, 2022. The second flight, Artemis II, will place astronauts in lunar orbit in 2024 or later, with the third flight projected as the first lunar landing mission since Apollo 17, in December 1972. The project consists of the Space Launch System, which is NASA's most capable booster rocket since the Saturn V Moon rockets; the Orion capsule, which ferry astronauts to the Moon and back; and the SpaceX Starship rocket, which will carry Orion astronauts to the lunar surface.


A small, irregularly shaped object made of metal (iron and nickel), rocky materials, or a combination of the two that orbits the Sun or another planet. Most of the asteroids orbit in a broad band between the orbits of Mars and Jupiter. The largest is Ceres, which is about 600 miles (900 km) in diameter.


The study of the interior of a star through measuring waves that ripple across its surface. The waves are generated by the motions of giant bubbles of hot gas in a star's outer layer, known as the convective zone, which extends deep below the surface. The waves can travel all the way around the star, causing "jiggles" when they reach the surface. The individual waves can reveal details about the star's structure, interior temperatures, composition, and more. Astronomers measure the waves through tiny changes in the motion of a star's surface toward or away from Earth. Studying similar waves on the Sun is known as helioseismology.

Astrobiology and Extraterrestrial Life

Astrobiology is the study of the conditions under which life might exist on other planets or moons. Extraterrestrial life is hypothetical life beyond Earth. No life has been discovered on any other body, although several bodies in the solar system appear to have conditions that are conducive to life.

Astrology and the Zodiac

Astrology is the art of studying the relative positions of the Sun, Moon, planets, and constellations in the zodiac to explain and predict events on Earth. Not to be confused with astronomy. The zodiac is a ring of constellations that lie along the ecliptic, including Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces in order east of the vernal equinox. The constellations in the zodiac are often used by astrologers to help explain and predict events on Earth. Because the zodiacal constellations were defined 2,000 years ago, precession of the Earth’s axis has shifted the position of the Sun relative to the zodiac signs so that they are off by one month, as quoted in daily newspapers.


Measuring an object’s position in the sky.

Astronauts and Cosmonauts

Astronauts and cosmonauts are trained personnel who travel in space. “Astronaut” refers to an American or western European space traveler and “cosmonaut” refers to a Soviet or Russian space traveler. China calls its space travelers “taikonauts.”


Scientists who study the universe beyond Earth, from the planets and moons of the solar system to the birth and fate of the universe.

Astronomical Distances

Distances in space are often measured in astronomical units, light-years, or parsecs. An astronomical unit is the average from Earth to the Sun, about 93 million miles (150 million km), and is used to measure relatively short distances, such as those between the Sun and its planets or between the stars in a binary system. A light-year is the distance light travels in one year, which is about 5.88 trillion miles or almost 800 times the diameter of the solar system. The nearest star is 4.2 light-years away, while the nearest spiral galaxy lies about 2.5 million light-years from Earth. A parsec is a unit of distance equal to 3.26 light-years. The name means “PARallax-SECond,” and it refers to a way to measure the distances to other stars. The most accurate way to measure the distances to close stars is to use basic geometry. Astronomers measure the position of a star in the sky at six-month intervals, when Earth is on opposite sides of the Sun. If the star is close, then it will appear to shift a bit compared to the background stars. It’s the same effect you see if you hold your finger in front of your face and look at it with first one eye, then the other: the finger appears to move against the background of objects. This effect is called parallax. If a star has a parallax of one second — in other words, if it appears to shift back and forth across the sky by exactly one second of arc (1/3600 degree) — then its distance is one parsec.

Auriga, the Charioteer

A constellation that cruises high across the sky in January and February.

Aurorae (Northern Lights)

Beautiful ribbons of light caused by the interaction of high-energy particles in the solar wind and Earth’s magnetic field. These are common in both extreme northern (aurora borealis or northern lights) and southern latitudes (aurora australis/southern lights), near Earth’s magnetic poles.

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Barnard's Star

A small, cool, and faint red-dwarf star six light-years from Earth. It is named for its discoverer, Edward Emerson Barnard.

Beehive Cluster, M44

A bright cluster of several hundred stars that formed about 700 million years ago.

Beta Pictoris

A young star system surrounded by a giant cloud of gas and dust that may by giving birth to planets.


An orange supergiant star 650 light-years away in the constellation Orion. It forms Orion’s shoulder, and is the constellation’s second-brightest star, after Rigel. It is expected to end its life with a titanic explosion known as a supernova.

Big Bang

A theoretical model of the “birth” of the observable universe, which suggests that the universe began as a rapid expansion of space and time from a single point, or singularity.

Big Dipper

The Big Dipper is an “asterism” — a group of stars that forms an easy-to-see pattern, but that’s not a constellation in its own right. It forms the body and tail of the constellation Ursa Major, the big bear. The Big Dipper is found in the northern sky, and the outer stars in its bowl can be used to “point” to Polaris, the North Star.

Binary and Multi-Star Systems

A binary is a pair of stars that orbit each other. A multi-star system consists of three or more stars. The stars in a binary or a multi-star system all formed from a single cloud of gas and dust, so they are true “siblings.”

Black Hole

An object with such powerful gravity that nothing can escape from it, including light. The black hole’s mass is concentrated in a point of almost infinite density called a singularity. At the singularity itself, gravity is almost infinitely strong, so it crushes normal space-time out of existence. As the distance from the singularity increases, its gravitational influence lessens. At a certain distance, which depends on the singularity’s mass, the speed needed to escape from the black hole equals the speed of light. This distance marks the black hole’s “horizon,” which is like its surface. Anything that passes through the horizon is trapped inside the black hole. Black holes come in several varieties, depending on mass.

Brown Dwarf

A “failed” star, which is more massive than a planet but not massive enough to ignite nuclear fusion reactions at its core and shine as a true star.

Bubbles, Sheets and Voids

Superclusters of galaxies are arranged in structures that look like sheets or walls, surrounded by voids and filaments in the large-scale structure of the universe. Together, walls, voids, and filaments cause the large-scale structure of the universe to resemble foam bubbles. Sometimes walls, such as one dubbed the Great Wall, are the result of the intersection of several filaments.

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Camelopardalis, the Giraffe

A large but faint northern constellation. It represents the giraffe. A literal translation of its name means “the camel leopard,” indicating a combination of a camel’s long neck and a leopard’s spots.

Cancer, the Crab

A constellation of the zodiac.

Canis Major, the Greater Dog

A constellation containing the brightest star in the entire night sky, brilliant Sirius

Canis Minor, the Lesser Dog

A small constellation containing one of the brightest stars in the night sky, Procyon. Canis Minor is the smaller dog of Orion, the hunter.


A rare yellow-white supergiant star 300 light-years away in the constellation Carina.


A binary star system 46 light-years away in the constellation Auriga. Each member is 25 times larger than the Sun.

Capricornus, the Sea-Goat

A constellation of the zodiac.


An ancient astronomical observatory in the Mayan city of Chichén Itzá, in Mexico’s Yucatan peninsula.

Carina, the Keel

A southern constellation that once formed part of the larger constellation Argo Navis, the boat that carried Jason and the Argonauts in their quest for the golden fleece.

Cassini to Saturn

A spacecraft that Cassini entered orbit around Saturn in the summer of 2004, and has transmitted tens of thousands of images of the planet and its rings and moons. On January 14, 2005, a second part of the mission, the Huygens probe, parachuted to a soft landing on Saturn’s largest moon, Titan. Its images showed a landscape carved by flowing liquid. Cassini’s instruments have peered through Titan’s atmospheric haze to discover lakes of liquid methane and ethane, extensive river networks, ice volcanoes, and giant dune systems on Titan’s surface. Cassini is scheduled to continue its reconnaissance of the Saturn system until 2017.

Cassiopeia, the Queen

A constellation best seen in autumn in the northeastern sky.


A crowded six-star system about 50 light-years from Earth. It forms one of the “heads” of the twins of Gemini, and is Gemini’s second-brightest star, after nearby Pollux.

Celestial Coordinates

Astronomers measure the positions of astronomical objects using coordinates called right ascension and declination — the equivalent of longitude and latitude here on Earth. If you’re standing on Earth’s equator, the celestial equator stretches from east to west directly across the top of the sky, dividing the sky into northern and southern hemispheres.

Cepheid Variable

An unstable star that pulses in and out, causing a periodic change in brightness. In 1912, Henrietta Leavitt discovered what is known as the period-luminosity relationship for Cepheid variables, by which the period of the brightness change is related to the luminosity, and therefore the distance, of the star. An astronomer can plot a Cepheid’s changing brightness to create a light curve for the star. The distance to the Cepheid variable is then obtained by measuring the period of the light curve, which shows the length of each pulsation.

Cepheus, the King

A northern constellation that is best viewed in the evening sky during autumn.


The largest known asteroid, Ceres is less than 600 miles (975 km) in diameter, and orbits roughly 260 million miles (420 million km) from the Sun. Italian astronomer Giuseppe Piazzi discovered Ceres in 1801. Since it was the first asteroid ever discovered, it also carries the designation “1 Ceres.” In 2006, astronomers also classified Ceres as a dwarf planet.

Cetus, the Whale or Sea Monster

A constellation named for a mythological sea monster. He was sent by Poseidon, god of the sea, to punish Ethiopia for the vanity of its queen, Cassiopeia, who boasted that she was more beautiful than the sea nymphs. The king, Cepheus, ordered his daughter Andromeda chained beside the sea shore as a sacrifice. At the last moment, however, she was rescued by Perseus, the hero, who either beheaded Cetus or turned him to stone by showing him the snake-covered head of Medusa, one of the Gorgons.

Chandra X-Ray Observatory

A space-based observatory that observes the universe in X-ray wavelengths. Launched in 1999, it is one of NASA’s four Great Observatories.

Chinese Astronomy and Folklore

See Asian Astronomy

Clementine Spacecraft

The Clementine Spacecraft was launched in 1994 to test sensors and spacecraft components under extended space exposure and to make scientific observations of the Moon and an asteroid. It completed its lunar observations, producing the most detailed map of the composition of the lunar surface to date, but failed en route to the asteroid. It was jointly sponsored by the Ballistic Missile Defense Organization and NASA.

Climate change

An alteration in Earth's climate caused by human activities. Burning fossil fuels releases carbon dioxide and other "greenhouse" gases into the atmosphere, trapping the Sun's heat and causing the planet to grow warmer. (Although some areas will get cooler, the net is a planet-wide temperature increase.) That is causing widespread effects, such as the extinction of entire species of life. 

Coma Berenices

Coma Berenices is a northern constellation and the only constellation named for a real person: Queen Berenice II of Egypt, the wife of Ptolemy III. Although it is relatively faint, under a dark sky many of its stars are visible as pretty streamers that look like strands of hair.Berenice married Ptolemy, who was her cousin, in 246 B.C. As king, Ptolemy later went to war. The tale of Berenice says that to protect her husband, she promised to cut her beautiful hair and offer it to the gods if Ptolemy returned safely. He did, so she did. She placed her locks in a temple, dedicated either to the goddess Aphrodite, or to all the gods, depending on the version of the story. The hair soon disappeared. The court astronomer told the king that the locks had been taken by the gods and placed in the sky — streamers of stars near Leo, the lion. The story might be true, or might have been created — or at least embellished — to raise the king’s profile.Coma Berenices didn’t become a constellation until 1536. And it didn’t enter wide-spread use until 1602, when it was included in an atlas published by Tycho Brahe.It is best known for the Coma Cluster, a collection of thousands of galaxies centered within the constellation's borders.  


A chunk of ice and rock that orbits a star. In our solar system, the majority of comets orbit well beyond Pluto, in regions known as the Kuiper Belt and the Oort Cloud. Most comets are no more than a few miles in diameter. Upon close encounters with the Sun, comet ices vaporize, creating a coma, or cloud, around the comet, and a long tail that always points away from the Sun.

Constellations and Asterisms

Constellations are patterns of stars visible to the unaided eye, or regions of space seen from Earth that are bounded by borders designated by the International Astronomical Union. Asterisms are also naked-eye star patterns, but they do not form constellations on their own. An example is the Big Dipper, which is part of Ursa Major. Americans know northern hemisphere constellations by the names given them by ancient Babylonians and Greeks. Seafaring explorers named those in the southern hemisphere. Every culture created its own constellations, although most of those in use today came from western sources.

Contact Binaries and Merging Stars

A contact binary is two stars that are so close that their surfaces are touching. In profile, such a system would resemble a peanut. As the two stars orbit each other, they lose momentum, so they eventually spiral together. Such a merger could produce a bright outburst, known as a nova, with the two stars forming a single, more-massive star. It could also produce a supernova explosion, which would completely destroy the two stars. 

Copernicus, Nicolaus

A Polish astronomer who lived from 1473 to 1543. Copernicus is most famous for devising the Copernican system, which is also known as the heliocentric theory. The Copernican system is a model for our solar system in which Earth and all the other planets orbit the Sun and the Sun is the center of the universe. In contrast, most scientists before Copernicus subscribed to the Ptolemaic system, also known as the geocentric theory, which stated that all the planets, the Moon, and the Sun orbited Earth, which was the center of the universe.


The corona is the hot extended atmosphere of a star. In the case of the Sun, it extends millions of miles (km) into space, and is heated to more than a million degrees, compared to about 10,000 degrees Fahrenheit (5,800 Centigrade) for the surface. It is heated by magnetic activity at or near the Sun's surface, although astronomers are still trying to determine the exact mechanism. Although it is hot, the corona is so thin and faint that it's visible from Earth's surface only during a total lunar eclipse. Astronomers study it with special instruments that block out the disk of the Sun, or with space telescopes. Other stars generate their own coronas.

Cosmic Background Radiation

An electromagnetic radiation field at a black body temperature of 2.7 Kelvin that fills the entire universe uniformly to 0.00001 Kelvin. Also known as the Cosmic Microwave Background (CMB), it is thought to be the residual glow from the very hot early universe that followed the Big Bang.

Cosmic Jets

Streams of electrically particles shooting into space from the poles of some objects, such as newborn stars and black holes. A jet is powered by a strong magnetic field that captures particles from the inner edge of a rotating disks that encircles the central star or black hole. The magnetic field shoots the particles into space at close to lightspeed. The particles in a jet produce radio waves and other types of energy that can be detected by telescopes on Earth or in space. The jets around supermassive black holes can stretch across tens of thousands of light-years.

Cosmic Rays

High-energy particles that fly through the universe at close to the speed of light. Cosmic rays are mostly bare protons and electrons. Some probably are blasted into space from exploding stars, while others may come from the disks of material that encircle the supermassive black holes in the centers of galaxies.


The study of the birth, evolution, and fate of the universe.

Crab Nebula, M1

A cloud of glowing gas that is the remnant of a supergiant star that was seen to explode in 1054. It’s called the crab because its tendrils of gas, which span several light-years, resemble a crab. It is one of the most famous objects in the galaxy.


The city of Cuzco was the political, agricultural, and spiritual center of the ancient Inca empire. The city was laid out in a radial plan which mimicked the sky and pointed to specific astronomical events on the horizon. The Inca relied on astronomy for daily activities.

Cygnus, the Swan

A constellation best seen overhead during summer evenings. Its brightest stars form a pattern that looks like a crucifix, so the constellation is also known as the Northern Cross. Its brightest star, Deneb, forms a point of the large Summer Triangle.

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Dark Energy

A term that describes the accelerating expansion of the universe. The expansion may be caused by a “repulsive” force that makes the universe expand faster as it ages, a discrepancy in the laws of gravity, or some other phenomenon.

Dark Matter

Matter that produces no detectable energy, but that reveals its presence through its gravitational pull on the visible matter around it. Dark matter makes up more than 90 percent of all the matter in the universe. There are many possible forms for dark matter, but most current theories suggest a subatomic particle created in the Big Bang.

DART mission

DART (Double Asteroid Redirection Test) is a spacecraft designed to test a possible strategy for protecting Earth against an asteroid on a collision course. Launched November 23, 2021, it will encounter a double asteroid on September 26, 2022. The larger member of the system, Didymos, is a half-mile in diameter. Its companion, Dimorphos, is only about a tenth of a mile across. DART will hit Dimorphos head on, at a speed of about 15,000 miles (25,000 km) per hour. The impact should change the smaller space rock’s orbit. Astronomers will use several telescopes to track the impact and its aftermath to see how much the orbit changes. The goal is to help determine if we could deflect an asteroid on a collision course with Earth. One way to do so would be to ram objects into it at high speeds. DART is the first test to see how well that might work. It is expected to shorten the length of Dimorphos’s orbit by several minutes.

Dawn Mission

Dawn is a NASA Discovery mission that targeting Vesta and Ceres, the two most massive asteroids. It entered orbit around Vesta in July 2011, then departed a year later. It will arrive at Ceres in February 2015, making it the first probe ever to orbit two astronomical bodies other than Earth.

Deep Impact Mission

Deep Impact fired a probe at Comet Tempel 1, gouging a fresh crater and giving astronomers a look at material beneath the surface, which probably had remained unchanged since the comet’s birth.

Deep Space Missions

Missions that journey beyond the asteroid belt, into the realm of the outer solar system.

Delphinus, the Dolphin

A small summer constellation whose brightest stars form the outline of a dolphin.


A blue-white supergiant star 1,600 light-years away in the constellation Cygnus. It is the constellation’s brightest star and the most-distant bright star visible to the unaided eye. It forms one point of the large Summer Triangle.


Denebola is the third-brightest visible star of Leo, the lion. Also known as Beta Leonis, it represents the lion's tail. It is roughly 75 percent more massive than the Sun and about 15 times brighter. It is also much wider than the Sun, and it spins so rapidly that it's wider through the equator than through the poles. Denebola is 36 light-years away and less than one-tenth the Sun's age. Its surface is much hotter than the Sun's so Denebola shines pure white.

Double Stars

See binary and multi-star systems

Draco, the Dragon

An ancient but faint constellation that winds around Polaris, the North Star. Its brightest star, Thuban, once marked the north celestial pole, and was used as a celestial marker for the construction of the pyramids of Giza.

Dumbbell Nebula, M27

A planetary nebula, which is a glowing cloud of gas and dust expelled by a dying star. The Dumbbell is named for its shape, which resembles the handheld weights. It is 1,360 light-years away, in the constellation Vulpecula.

Dwarf Planet

A body in our solar system that is larger than a comet or asteroid but not large enough to qualify as a major planet. This category was created by the International Astronomical Union in 2006. A dwarf planet is large enough that its gravity can pull it into a roughly spherical shape, but not large enough to clear its orbit around the Sun of other objects. As of 2011, there are five dwarf planets in the solar system: Pluto, Ceres, Eris, Makemake, and Haumea (which is not spherical, but is shaped more like an egg or a potato). Ceres is in the asteroid belt, while the others are beyond the orbit of Neptune, the eighth planet from the Sun.

Dwarf Stars

Most stars are classified as dwarfs, including the Sun. These stars lie on the main sequence, the phase of life in which a star steadily converts the hydrogen fuel in its core to helium. When the core hydrogen has been depleted, the star gets larger, cooler, and brighter. It becomes first a subgiant, then a giant. Especially massive stars may become bright giants or supergiants. Despite the "dwarf" moniker, many main-sequence stars are quite large, with some spanning several times the Sun's diameter. The Sun is classified as a G2V star; G2 indicates its surface temperature (around 10,000 degrees Fahrenheit/5,500 C), while the Roman numeral V designates its dwarf status.

Some classes of stars that are small have special dwarf classifications. Red dwarfs are only a fraction the size and mass of the Sun, and can be as little as one ten-thousandth the Sun's brightness. Because they are so cool, their surfaces are orange or red. White dwarfs are the dead cores of once-normal stars. They typically are less massive than the Sun, and only about the size of Earth. They are extremely hot and dense, although they no longer produce nuclear reactions.


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Our home planet, the third from the Sun in the solar system. The Earth orbits the Sun at an average distance of 93 million miles (149 million km/1 astronomical unit) and has a mass of 6x10^24 kg. It is made of solid, rocky materials such as iron, nickel, and silicon, and it has an atmosphere composed mainly of nitrogen and oxygen with traces of argon, carbon dioxide, and other compounds. Liquid water covers 71 percent of the planet’s surface. Earth has one moon.

Earth's Atmosphere

The layers of gas that are gravitationally bound above the surface of our planet.

Earth's Moon

The only natural satellite of Earth.

Eclipses, Occultations and Transits

An eclipse is the result of the total or partial masking of a celestial body by another along an observer’s line of sight. Solar eclipses result from the Moon blocking the Sun relative to the Earth; thus Earth, Moon and Sun all lie on a line. Lunar eclipses work the same way in a different order: Moon, Earth and Sun all on a line. In this case the Earth’s shadow hides the Moon from view. An occultation occurs when a celestial object is eclipsed by the Moon or another solar system body. A transit is either the act of one celestial body passing in front of another or the time at which a celestial object is highest in the sky. The time at which a celestial object crosses the meridian is called the transit time.


The Sun's path against the background of stars. It is defined by the plane of Earth's orbit around the Sun. It crosses the 12 constellations of the zodiac plus Ophiuchus, the serpent bearer. The other major planets of the solar system stay close to the ecliptic as they circle through the starry background, as does the Moon. 

Einstein, Albert

A Swiss mathematician and physicist who lived from 1879 to 1955. Einstein was born in Germany to Jewish parents, and after moving to Switzerland, he took a job at a patent office. When he began publishing scientific papers, he quickly rose to fame and was recognized as a leading thinker of the time. He held several professorial positions in Switzerland, Germany, and the United States during his lifetime. In 1922, Einstein won a Nobel Prize for work he had done with the Photoelectric Effect. Over the course of his life, Einstein made major advances in physics, such as developing the Theory of Special Relativity and the Theory of General Relativity. He was arguably the most important physicist of the 20th century.

Electromagnetic Spectrum

The full range of electromagnetic radiation in order of wavelength from longest to shortest, or frequency from lowest to highest (radio waves, infrared, visible light, ultraviolet, X-rays, and gamma rays). In terms of visible light, a light spectrum runs from red to blue. Astronomers scan as much of the electromagnetic spectrum as possible to learn about the nature of the universe and how it came to be.

Elliptical Galaxies

Galaxies that are shaped like fat eggs or footballs. Ellipticals are among the largest galaxies, with some spanning more than one million light-years. They probably formed from the mergers of two or more smaller galaxies, such as spirals. Unlike spirals, where the stars all orbit the center of the galaxy in the same direction, the stars in ellipticals move in random orbits.

Eta Carinae

One of the Milky Way’s most impressive star systems, about 8,000 light-years away in the southern constellation Carina. It probably consists of two stars, each of which is many times as massive as the Sun. The stars are surrounded by a thick, dusty nebula that makes it impossible to see one of the stars. The larger star is unstable and has erupted several times over the last few centuries, most recently in 1843, when it grew brighter than every other star in the night sky except Sirius. This star is expected to end its life as a supernova.

Evening Star

A name for the planet Venus when it appears in the evening sky. It outshines all of the other stars and planets in the night sky. The name also occasionally is applied to Jupiter, the second-brightest point of light in the night sky.


ExoMars is a European space mission to study Mars and to develop the technology and capabilities for future landers and rovers. Scheduled for arrival at Mars in October 2016, it consists of the Trace Gas Orbiter, which will hunt for evidence of methane and other compounds that could be produced by living organisms, and the Schiaparelli lander, which will operate for a day or two before its batteries die. The lander will help the European Space Agency plan for ExoMars 2020, which include a rover.


A planet that orbits around a star other than the Sun.

Explorer Missions

NASA’s longest-running series of spacecraft, with more than 75 successful missions. Explorer 1, launched January 31, 1958, was the first successful American satellite. Other Explorers have studied the solar wind, Earth’s magnetic field, the “afterglow” of the Big Bang, the ultraviolet glow of distant galaxies, and many other subjects.

Extinct Constellation

An extinct constellation is one that is no longer recognized an official constellation by astronomers. In the early 20th century, astronomers created a list of 88 official constellations, each with its own boundaries. All other constellations were discarded. Many other constellations had been abandoned long before then, however. Astronomers split Argo Navis, which represented the ship that carried Jason and the Argonauts, into three smaller constellations. European astronomer created dozens of new constellations from the late 1500s through the early 1800s, either to populate the southern sky, which wasn't visible from most of Europe, or to fill in blank regions between ancient constellations. Many of those constellations survived, but many others did not. The list includes Apes, the bees; Cerberus, a three-headed serpent in the hands of Hercules; George's Harp, which honored England's King George III, a patron of astronomy; and many others.

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A blue-white main sequence star 23 light-years away in the constellation Pisces Austrinus.

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A European space telescope that is using a technique known as astrometry to provide a precise three-dimensional map of more than one billion stars in the Milky Way galaxy. It was launched December 19, 2013, from the Kourou launch center in French Guiana. It will measure the distances to stars and plot their motions across the sky, as well as providing basic information about the stars themselves. It also is expected to discover thousands of exoplanets, as well as comets and asteroids in the solar system.


A vast “island universe” of stars, gas, and dust. The universe may contain 100 billion or more galaxies. Galactic size and structure range from dwarfs and irregulars with no discernible structure and only a few million stars to giant spirals and ellipticals that span 100,000 light-years or more and contain hundreds of billions of stars. Galaxies clump together in web-like structures. Despite its great luminosity, most of a galaxy’s mass consists of dark matter, which produces no detectable energy.

Galaxy Cluster

A grouping of galaxies. The Milky Way belongs to a small cluster known as the Local Group, but other clusters can contain hundreds or thousands of galaxies. Clusters of galaxies, in turn, can group together to form superclusters.

Galileo Galilei

An Italian scientist who lived from 1564 to 1642. Galileo was the first astronomer to use the newly invented telescope to make astronomical observations. He discovered the four largest moons of Jupiter, craters and mountains on the Moon, and sunspots, and was the first to see that the Milky Way consists of a myriad of individual stars. Galileo supported the Copernican system, which held that the Sun, not Earth, was the center of the universe. As a result, he was charged with heresy by the Inquisition, and was forced to recant. He lived out his final years under house arrest.

Galileo to Jupiter

Galileo was the first spacecraft to enter orbit around an outer planet. It arrived at Jupiter in December 1995. It dropped a probe into Jupiter’s turbulent atmosphere, then observed Jupiter and its moons for seven more years. Its mission ended when it was intentionally crashed into Jupiter. Galileo’s success was limited by a failed radio antenna that reduced the amount of data it could transmit to Earth.

Gamma-Ray Astronomy

The study of the most energetic wavelength, or frequency, of electromagnetic radiation. Gamma-rays have wavelengths smaller than 10^-11 meters and frequencies greater than 10^20 Hz. They can be quite harmful to life because they are strong enough to ionize atoms and thus destroy cells. Fortunately, Earth’s atmosphere shields us from most astronomical gamma-ray radiation.

Gamma-Ray Bursts

The most powerful explosions in the modern-day universe, they most likely are caused by the explosion of a supergiant star and the accompanying collapse of its core to form a black hole. The explosion’s gamma rays are funneled into narrow beams from the star’s poles. From Earth, we see a gamma-ray burst only when such a beam sweeps across our planet. A typical gamma-ray burst produces more energy in a few hours than the Sun will produce in its entire lifetime.

Gemini Project

Designed as a bridge between Mercury and Apollo, Project Gemini primarily tested equipment and mission procedures and trained astronauts and ground crews for future Apollo missions to the Moon.

Gemini, the Twins

A constellation of the zodiac, visible high overhead in mid-winter.

Genesis probe

A spacecraft that collected a sample of solar wind and returned it to Earth for study. Its parachute failed during its descent, destroying its sample container. Scientists were able to collect the contents, however, and are working to extract bits of the solar wind from the debris.

Goddard, Robert

A scientist and engineer who pioneered much of the basic technology of modern rockets. In 1926, he launched the first liquid-fueled rocket. Over the years, Goddard developed the basic technology for rocket engines, ways to keep a rocket stable, and many other technologies that are still in use today. He was awarded more than 200 patents, many of them after his death in 1945. Goddard was reclusive, however, so much of his work was overlooked at the time and was later duplicated by other engineers.

Gravitational Waves

Ripples in space-time created by the stretching and squeezing effects of massive objects, notably supernovae, binary black holes, and binary neutron stars. Gravity waves are transverse waves, meaning that they stretch and squeeze space-time in directions perpendicular to the wave’s motion. Einstein’s Theory of General Relativity predicts the existence of gravity waves, but they have not yet been detected.


The most feeble of the four fundamental forces in the universe that affect all matter. The magnitude of gravitational attraction depends directly on mass and inversely on distance squared. For instance, the gravitational attraction between you and Earth is much greater than that between you and the Sun, even though the Sun is 333,000 times more massive than Earth. The distance separating you from the Sun weakens the mutual gravitational attraction, so as you stand on Earth’s surface, Earth’s gravitational pull on you is 1,650 times greater than the Sun’s.

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Halley, Edmund

A British astronomer who calculated that comets follow elongated orbits around the Sun — orbits that sometimes bring them back into view from Earth. Halley calculated that one comet had visited the inner solar system several times and predicted that it would return in 1758. When it did (many years after Halley’s death), astronomers named it Halley’s Comet in his honor.


The study of the interior of the Sun by measuring waves that ripple across its surface. The waves are generated by the motions of giant bubbles of hot gas in the Sun's outer layer, known as the convective zone, which extends about a third of the way from the surface to the Sun's center. The waves can travel all the way around the Sun, causing "jiggles" when they reach the surface. The individual waves can reveal details about the Sun's structure, interior temperatures, composition, and more. Astronomers measure the waves through tiny changes in the motion of the Sun's surface toward or away from Earth. Studying similar waves on other stars is known as asteroseismology.

Hercules, the Strongman

A constellation named for Hercules, a character from Roman mythology, that is best viewed in spring and summer.

Herschel, William

A British astronomer who discovered Uranus and several of its major moons, such as Titania and Oberon.

Hubble Space Telescope

The first of NASA’s four Great Observatories. Astronauts aboard space shuttle Discovery deployed Hubble Space Telescope on April 25, 1990. It was the first big telescope designed to escape the blurring effects of Earth’s atmosphere, allowing it to see the universe more clearly than any other telescope. Its observations showed that the universe is about 13.7 billion years old. They also provided some of the first evidence of “dark energy” — a force that’s causing the universe to expand faster as it ages. Several repair and refurbishing missions by shuttle astronauts have kept the telescope going and vastly improved its scientific capabilities. It is expected to continue operating until around 2015.

Hubble, Edwin

An American astronomer who lived from 1889 to 1953. Hubble discovered the expansion of the universe by measuring the red shifts of many galaxies. He also discovered that the recession velocity of a given galaxy is proportional to its distance from the Milky Way. Hubble Space Telescope is named after him.


A cluster of more than 100 stars; the brightest form the V-shaped “face” of Taurus, the bull. It is the nearest major cluster to Earth, at a distance of about 150 light-years.

Hydrogen and Helium

Hydrogen is an element, usually in the form of a gas, that consists of one proton and one electron. Hydrogen is the most abundant element in the universe, accounting for about 75 percent of its normal matter, and was created in the Big Bang. Helium is an element, usually in the form of a gas, that consists of a nucleus of two protons and two neutrons surrounded by two electrons. Helium is the second-most abundant element in the universe, after hydrogen, and accounts for about 25 percent of the atoms in the universe. Most of the helium in the universe was created in the Big Bang, but it also is the product of hydrogen fusion in stars.

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The ultra-fast expansion of the universe in the instant before the Big Bang.

Infrared Astronomy

The study of infrared energy produced by astronomical objects. Infrared is a form of energy with a longer wavelength and lower frequency than visible light. It is produced by relatively cool objects, such as planetary systems and clouds of interstellar dust. Earth’s atmosphere absorbs most infrared energy, so it is best studied with telescopes in space or aboard high-altitude balloons or aircraft.


The technique of diagnosing the properties of two or more waves by studying the pattern of interference, which is the superposition of two or more waves that results in a new wave pattern. Astronomers use this technique to combine the signals from two or more telescopes to produce images that are much sharper than those obtained by a single telescope. The technique is most often applied to radio astronomy, in which up to 30 or more radio dishes are combined into a single array. Optical interferometers, which are more difficult to align and operate, are also becoming more common.

International Space Station

An orbiting outpost for scientific, medical, and engineering experiments. Construction began in 1998. The last American component of the station was delivered in 2011. Crews of three spend several months aboard the station, with two crews inhabiting the station simultaneously. International partners in the project include the United States, Russia, Europe, Canada, and Japan.

Interstellar Gas and Dust

The gas between stars is mostly hydrogen and helium scattered at varying densities between the stars in our galaxy and other galaxies. The proportions of the gases are similar to those in the Sun. Interstellar gas supplies the raw material for star formation. Carbon monoxide and hydroxyl molecules (CO and OH) have also been detected within interstellar gas, along with highly ionized oxygen, nitrogen, carbon and silicon, which are likely remnants of past supernovae. The dust is made up of tiny grains of carbon, iron, and iron-magnesium silicates scattered at varying densities between the stars in our galaxy and other galaxies.

Irregular Galaxies

Galaxies that have no easily discernible shape. Such galaxies are generally small, but are giving birth to new stars at a rapid pace. In the early universe, such galaxies served as the building blocks for larger galaxies, merging to form larger and larger agglomerations of stars.

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James Webb Space Telescope

The next big space telescope is named in honor of James Webb, a former head of NASA who oversaw the Mercury and Gemini missions that prepared the agency for the Apollo Moon flights. It will observe the universe at infrared wavelengths, and will be deployed one million miles from Earth. That location will shield it from the warmth of Earth, which produces a lot of infrared energy.

JUICE Mission to Jupiter

Jupiter Ice Moons Explorer (JUICE) is a European spacecraft designed to study the three largest moons of Jupiter: Ganymede, Callisto, and Europa. All three moons appear to have large oceans of liquid water beneath their icy crusts. Europa is of particular because its crust is thinner and it may have more interior heat to warm the ocean, making it both a more comfortable abode for possible microscopic life and an easier world to probe. JUICE is scheduled to enter orbit around Jupiter in July 2031. It will make multiple passes by all three moons, then settle into orbit around Ganymede in December 2034. That will make it the first spacraft to orbit any planetary moon other than Earth's. 

Juno Mission

A spacecraft designed to study Jupiter’s interior and atmosphere by orbiting from pole to pole. It will obtain precise measurements of the giant planet’s magnetic and gravitational fields, and peer deep into its turbulent clouds.


The fifth planet from the Sun and the largest planet in our solar system. Jupiter orbits the Sun at an average distance of 5.2 time Earth’s distance (about 465 million miles/745 million km). The planet has a mass 318 times the mass of Earth and a radius 11.2 times the radius of Earth. Jupiter is a gas planet and has no solid surface. It is made of hydrogen and helium, with trace amounts of ammonia, water vapor, and methane. The most recognizable features of the planet are its bands of clouds and a giant storm visible from Earth called the Great Red Spot. Jupiter has more than 60 known moons, the largest of which are Io, Europa, Ganymede, and Callisto.

Jupiter's Great Red Spot

A raging red swirling storm located in Jupiter’s upper atmosphere. The storm has been observed since at least the late 1800s, and perhaps as far back as the 1600s. It appears to rotate counterclokwise once every six days. Its size varies from about 1 to 2 times the diameter of Earth, while its color varies from pale pink to vivid red.

Jupiter's Moons

Jupiter has almost 80 known moons. The four largest, known as Galilean moons for their discoverer, Galileo Galilei, are Io, Europa, Ganymede, and Callisto. Ganymede is the largest moon in the solar system.

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Kepler Mission

Kepler is a roughly 1-meter-diameter telescope that is staring at 150,000 Sun-like stars in the constellations Cygnus and Lyra. It will watch those stars for about three and a half years continuously, without ever closing its eyes. Kepler will look for events called transits, in which a planet briefly passes in front of its parent star, blocking a little bit of the star’s light. The goal is to find Earth-size planets around Sun-like stars.

Kepler's Laws

Kepler summarized planetary motion with three rules, or laws. Kepler’s First Law, the Law of Ellipses: A planetary orbit is an ellipse with the Sun at one focus. Kepler’s Second Law, the Law of Equal Areas: As a planet orbits the Sun, it sweeps out equal areas of the ellipse in equal times. This behavior means that the planet’s orbital velocity varies with distance from the Sun. At perihelion, the planet is at maximum speed and at aphelion the planet crawls along at minimum speed. Kepler’s Third Law, the Harmonic Law: Even though the orbital velocity of a planet changes constantly, one relationship does remain constant. The orbital period is directly related to the average distance between the planet and the Sun. This law implies that planetary orbital velocity decreases with increasing distance from the Sun. For instance, the orbital velocity of Mercury (47.9 km/s) is far greater than Pluto (4.7 km/s).

Kepler, Johannes

A German mathematician and astronomer who lived from 1571 to 1630. Kepler was the first person to model planetary orbits as ellipses instead of circles. He tested his theory of elliptical orbits using Danish astronomer Tycho Brahe’s observations of planetary motion in the sky. In 1609, he published his findings in a book called Astronomia nova (The New Astronomy), in which he summarized planetary motion with three laws.

Kuiper Belt

A wide belt beyond the orbit of Neptune, the most distant planet from the Sun, that is theorized to contain tens of millions of icy bodies. The most famous member of the belt is Pluto, but it also includes three other objects that have been classified as dwarf planets. Most of the Kuiper Belt’s members are leftover “building blocks” from the birth of the planets. They were thrown into distant orbits by gravitational interactions with the giant outer planets.


An ancient Mayan god whose temple’s structure relates to the Mayan calendar. Each of the four faces incorporates a broad, steep staircase consisting of 91 steps that ascend to the top platform. Counting the top platform as an additional step gives a total of 365 steps: one step for each day of the year.

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Lagoon Nebula, M8

A stellar nursery about 5,000 light-years away, toward the center of the galaxy.

Leo, the Lion

A constellation of the zodiac visible high in south in April and May.


The constellation is the hare or rabbit. It’s below the feet of Orion the hunter, and to the west of Canis Major, one of Orion’s hunting dogs. It consists of a few moderately bright stars, although you might have a hard time connecting them to see a rabbit.

Libra, the Scales

A constellation of the zodiac.

Light Pollution

The stray light cast into the sky by streetlamps, porch lights, neon signs, and other glowing objects that hinders our visibility of the night sky.


A wide, thin sheet of material that uses the pressure of light to propel a craft through space. Lightsails have been successfully demonstrated in Earth orbit using sunlight. Scientists and engineers are exploring the possibility of using lightsails to send craft to Mars and other worlds of the solar system. They also are evaluating laser-powered lightsails as a means to send tiny robotic probes to Alpha Centauri, the closest star system to our own. Although the initial thrust provided by a lightsail can be relatively small, the light pressure can be applied continuously for as long as needed, adding up to a substantial kick. 

Little Dipper

Like its larger counterpart, the Big Dipper, the Little Dipper is an “asterism” — a group of stars that forms an easy-to-see pattern but that is not a full constellation. It resembles a ladle or cart, with the tip of its handle marked by Polaris, the North Star. It is part of the constellation Ursa Minor, the little bear.

Local Group

Our galactic neighborhood, including the Milky Way, Andromeda Galaxy, M33, and more than 30 smaller galaxies. The Local Group appears to be a suburb of a supercluster of galaxies that lies 60 million light-years away, known as the Virgo Supercluster.

Lucy mission

Lucy is a spacecraft designed to study Jupiter's Trojan asteroids. These chunks of ice and rock share the orbit of Jupiter, but they aren't near the giant planet. One clump is 60 degrees ahead of Jupiter, the other 60 degrees behind. They ae held in place by the gravity of Jupiter and the Sun. Astronomers have cataloged more than 5,000 of them, although there could be millions. Trojans probably are “leftovers” from the birth of the solar system. They contain the same materials that were incorporated into the planets, so studying them should tell scientists more about how the planets formed. Launched October 21, 2021, Lucy is scheduled to fly past one asteroid in the asteroid belt, between the orbits of Mars and Jupiter. It will reach one group of Trojans in 2027, and eventually swing by five of its members. It then will skip past Earth, using our planet’s gravity to reach the other clump in 2033.

Lunar Crater Observation and Sensing Satellite (LCROSS)

A satellite that slammed into a crater at the Moon’s south pole in 2009. The impact scattered debris high above the lunar surface. Another lunar craft and telescopes on Earth observed the impact and discovered that the material it excavated included water, confirming earlier observations that shaded craters at the poles could contain large amounts of water ice.

Lunar Orbiter Project

NASA launched five Lunar Orbiter missions from 1965 to 1967. Each was designed to image a large portion of the lunar surface to help Apollo mission planners select good landing sites for astronauts. Each orbiter shot its images on film, developed the film, scanned the images, and transmitted them to Earth in strips. The technology was based in part on that developed for Earth-orbiting spy satellites. All five Lunar Orbiter missions were successful. Scientists combined their images to produce the best maps of the Moon to that time, and the images continue to provide an important resource for lunar scientists.

Lunar Phases

The apparent shapes of the Moon as seen from the Earth, which are caused by the Moon’s orbit of the Earth once every 29.5 days. The same phase will fall on the same date of the year according to the Saros cycle, every 18 years, 11 days, and eight hours. See our moon phases page for more information.

Lunar Prospector

Lunar Prospector was one of the NASA Discovery Program missions, which emphasized “Faster, Better, Cheaper” mission design and development. Lunar Prospector was designed to perform a low polar orbit investigation of the Moon, which included mapping the surface composition and locating lunar resources, measuring magnetic and gravity fields, and studying outgassing events.

Lunar Reconnaissance Orbiter

A NASA mission to map the lunar surface in unprecedented detail and make other observations from orbit. Among its accomplishments, LRO has photographed all of the Apollo landing sites, showing the abandoned lunar modules and the paths the astronauts created during their expeditions. LRO’s images may be used to find landing sites for future lunar missions.

Lunar Samples

Bits of rock and dirt collected and returned to Earth by Apollo astronauts and robotic missions by the Soviet Union and China. To date, roughly 850 pounds (385 kg) of samples have been returned to Earth, with more scheduled for return in the next few years. In the United States, most of the samples are stored in an environmentally controlled facility at Johnson Space Center in Houston. They are parcelled out (usually a few grams at a time) to researchers around the world. They have helped scientists piece together the formation history and evolution of the Moon, as well as the history of asteroid impacts on the Moon and Earth. Some samples have been held unopened since the 1970s, although a few were opened for the first time in 2021 and 2022.

Lyra, the Harp

A constellation visible high overhead in mid-summer.

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Magellanic Clouds

The Large Magellanic Cloud and Small Magellanic Cloud are companion galaxies of the Milky Way. They are visible only from the southern hemisphere. They appear to be gravitationally bound to the Milky Way, so that they orbit it. Each is less than 200,000 light-years away, making them some of our closest known galactic neighbors.


A magnetar is a rapidly spinning, highly magnetic neutron star, which is the ultra-dense corpse of a supermassive star. A magnetar's magnetic field can be trillions of times the strength of Earth's. Most magnetars are quite young. They form when a supermassive star can no longer produce nuclear reactions in its core. The core collapses to form a ball that's up to twice the mass of the Sun, but no larger than a city. It is so dense that a chunk of its matter the size of a sugarcube weighs millions of tons. As the core collapses, it spins faster and faster. The magnetic field of the entire core is then compressed into the rotating neutron star, so it is extremely powerful. 

Magnetism and Magnetic Fields

Magnetism is a force field generated by moving electrical charges. An electrical current running through a loop of wire generates a magnetic field. The strength of the field depends on the current and area of the wire loop. Plasma churning through the atmosphere of the Sun drives powerful magnetic fields that sometimes produce cool magnetic storms called sunspots. A magnetic dynamo underneath Earth’s crust generates a magnetic field around Earth.

Main Sequence

A classification of stars that shine via hydrogen thermonuclear fusion in their cores. Such stars are stable, with no major pulsations or variations. Most stars spend the greatest portion of their lives on the main sequence.

Mariner Probes

A series of American interplanetary missions. In 1962, Mariner 2 staged the first successful encounter with another planet when it flew past Venus. Its instruments recorded Venus’ surface temperature and pressure, and the composition of its atmosphere and the clouds that envelope the planet. The next successful mission in the series, Mariner 4, snapped the first close-range pictures of Mars in 1965. The images revealed a Moon-like landscape. Mariner 9 became the first spacecraft to orbit Mars, and found that the planet has a much more varied and interesting surface than scientists had expected. Mariner 10 flew by Mercury three times in 1974 and 1975, and mapped about half of its battered, Moon-like surface, which is dominated by impact craters. Other successful missions in the series included Mariner 5 to Venus and Mariner 6 and 7 to Mars.


The fourth planet from the Sun. Mars orbits the Sun at an average distance of 1.52 times Earth’s distance. The planet is about 10 percent as massive as Earth, and a bit more than half as big. It is made of solid, rocky material, and has a thin atmosphere of carbon dioxide. Iron oxide, or rust, on the surface of Mars gives it its characteristic red color. The planet has bright polar ice caps made of frozen water and carbon dioxide. Mars also has two small moons, Phobos and Deimos.

Mars Climate Orbiter

A spacecraft that was designed to provide detailed information about the atmospheric conditions on Mars, including temperature, dust, clouds, water vapor, and carbon dioxide levels. The mission failed when it approached too close to Mars and was destroyed as it plunged into the planet’s atmosphere.

Mars exploration, human

NASA is drafting plans to send astronauts to Mars as early as the late 2030s. Such a mission would require about two years using present technology, with about half of that time at Mars. Scientists will study the Martian surface and climate, and look for evidence of life, either past or present. Many scientific and technological hurdles must to overcome to undertake such a mission, however.

Mars Express

A European Space Agency craft in orbit around Mars. Pictures from the mission have provided evidence of frozen water on the Red Planet.

Mars Global Surveyor

The Mars Global Surveyor mission was designed as a rapid, low-cost recovery of the Mars Observer mission objectives. Objectives included high resolution imaging of the surface and studies of the magnetic field, topography, climate, weather, and gravity. It arrived at Mars in 1997, began its primary mission in 1999, and operated until late 2006, far exceeding its goals.

Mars Hope Mission

Hope is a Mars orbiter built by the United Arab Emirates. Scheduled for launch in July 2020, it will enter orbit around the Red Planet in February 2021. It will monitor the Martian weather and study how Mars is losing its atmosphere to space. It is the first UAE space mission beyond Earth orbit, and the beginning of a 100-year plan to establish a colony on Mars.

Mars InSight Mission

InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport) is a Mars lander, scheduled for launch as early as May 5, 2018, with landing on November 26. It will use a suite of instruments, including a seismometer and heat probe, to study the Martian interior. Its observations should reveal more about the planet's structure and its evolution.

Mars MAVEN Mission

MAVEN (Mars Atmosphere and Volatile Evolution) entered orbit around Mars on September 21, 2014. It is designed to study the planet’s upper atmosphere and its interaction with the solar wind. The craft’s findings may help planetary scientists understand how Mars lost most of its early atmosphere and water.

Mars Odyssey

2001 Mars Odyssey is part of NASA’s Mars Exploration Program, a long-term effort of robotic exploration of the red planet. The mission is mapping the distribution of many chemical elements and minerals on the martian surface. Its maps led to the discovery of vast amounts of water ice just below the surface in the planet’s polar regions.

Mars Pathfinder

The second mission from NASA’s Discovery Program. It carried a set of scientific instruments to analyze the Martian atmosphere, climate, and geology and the composition of Mars’ rocks and soil. It also carried the first Mars rover, Sojourner, which operated for about 30 days.

Mars Perseverance Rover

Perseverance is the rover that forms the Mars 2020 mission. It is scheduled for launch in July 2020 and arrival at Mars in February 2021. Based on the Curiosity rover design, Perseverance will hunt for evidence of past microscopic life in Jezero Crater, which was filled with water in the distant past. Perseverance also will collect 30 samples of Martian rock and dust for possible return to Earth by a future mission. And it carries a small helicopter to test the technology for future missions.

Mars Polar Lander

A spacecraft that was designed to land in the polar regions of Mars. It failed during final descent when its on-board computer interpreted the jolt of an engine firing as contact with the ground and shut down the engine, causing the probe to crash.

Mars Reconnaissance Orbiter

The Mars Reconnaissance Orbiter’s main instrument is a telescopic camera, which will map parts of the Martian surface with unprecedented clarity. Scientists will use the pictures to plot the landing sites for future robotic and manned missions.The pictures and other observations also are providing new data in the search for water on Mars. The observations have helped locate minerals that formed in a watery environment. MRO also carries a radar system that discovered deposits of frozen water below the surface.

Mars Rovers Spirit and Opportunity

Building on the success of the 1997 Sojourner rover, the Spirit and Opportunity rovers landed on Mars in 2004. Although their planned mission was just 90 Mars days long (about 92 Earth days), Spirit continued until March 2010, while Opportunity was still operating in 2011. Both found evidence that water had helped shape the Martian landscape in the distant past.

Mars Science Laboratory, Curiosity

Scheduled to launch in fall 2011 and land on Mars in August 2012, Curiosity is the largest rover ever sent to the Red Planet. It will carry 10 instruments that will help search Mars for environments where life might have existed and the capacity of those environments to preserve evidence of past life.

Mars' Atmosphere

The atmosphere of Mars is thin and cold. The average density is 0.7 percent the density of Earth, with average temperatures far below zero. The atmosphere consists almost entirely of carbon dioxide, with only a tiny amount of other elements and compounds. The atmosphere supports clouds of both water and carbon dioxide. The clouds sometimes produce snow. When Mars was young, its atmosphere was much thicker and warmer than it is today, allowing liquid water to flow and stand on the surface. During the last four billion years, though, most of that atmosphere has been lost. Much of it escaped into space, while some of the compounds in the atmosphere combined with others to form rocks at the Martian surface. Several spacecraft have studied the planet's atmosphere and continue to do so today.

Mars' Moons

Mars has two small moons, Phobos and Deimos. Phobos is the larger of the two. It is shaped like a potato, about 15 miles long by 10 miles wide. One end is scarred by a giant crater. The impact that created it may have fractured the entire moon, leaving big cracks that travel from one end to the other. Phobos is in such a low orbit that it circles Mars about three times per day. It rises in the west, then sets in the east just a few hours later. As seen from the equator it looks less than half as big as our Moon looks from Earth. Deimos is both smaller and farther out. It takes almost three days to complete one orbit. From the Martian surface, it looks about like Venus looks from Earth.

Martian Surface

The fourth planet from the Sun in the solar system. Mars orbits the Sun at an average distance of 1.52 AU. The planet has a mass 0.1 times the mass of Earth and a radius 0.5 times the radius of Earth. It is made of solid, rocky material, and has a thin atmosphere of carbon dioxide. Iron oxide, or rust, on the surface of Mars gives it its characteristic red color. The planet has bright polar ice caps made of frozen water and carbon dioxide. Mars also has two small moons, Phobos and Deimos.

McDonald Observatory

McDonald Observatory, a research unit of The University of Texas at Austin, is one of the world’s leading centers for astronomical research, teaching, and public education and outreach. Observatory facilities are located atop Mount Locke and Mount Fowlkes in the Davis Mountains of West Texas, which offer some of the darkest night skies in the continental United States.


The first planet from the Sun in the solar system. Mercury orbits the Sun at an average distance of 0.4 times Earth’s distance. The planet is just five percent as massive as Earth, and it about 50 percent larger than the Moon. It is made of solid, rocky material that is heavily cratered, and it has no significant atmosphere. Mercury has no moons.

Mercury Project

Project Mercury placed the first American astronauts in space, in the 1960s. The project’s objectives were to successfully place a manned spacecraft in orbit, investigate an astronaut’s performance capabilities and his ability to function in the environment of space, and recover the man and the spacecraft safely. The project successfully launched six astronauts into space, beginning with Alan Shepard on May 5, 1961.

Messenger to Mercury

NASA’s Messenger spacecraft entered orbit around Mercury in 2011 for a one-year mission to this least-explored of the solar system’s inner planets. It flew by Mercury three times before settling into orbit, with each encounter reducing its speed. Messenger will map the planet’s entire surface, measure the chemical and mineral composition of its rocks and soil, measuring its gravitational and magnetic fields, and conduct other observations.

Messier Objects

A list of about 110 astronomical objects compiled by French astronomer Charles Messier at the end of the 18th century. The catalog includes objects such as nebulae, star clusters, and galaxies, and the objects are denoted by an M followed by a number from 1 to 110.

Meteor Shower

A display of meteors streaking through the sky at rates of several to hundreds per hour. Whenever Earth intercepts a stream of comet debris in orbit around the Sun, some of the debris falls into Earth’s atmosphere, producing the meteor shower. Two of the best annual meteor showers are the Perseids in August and the Leonids in November. See our list of this year’s meteor showers for more information.

Meteors and Meteorites

A meteor is a “shooting star”; a bit of interplanetary ice or rock that vaporizes in Earth’s upper atmosphere as a burst of incandescent gas. A meteorite is a space rock that strikes the surface of Earth. Most meteorites are classified as stony because of their mineral composition, which may include olivine, pyroxene, serpentine, sulfates, organic compounds, iron, and nickel. See our list of this year’s meteor showers for more information.

Milky Way Galaxy

Our home galaxy. The Milky Way is a spiral galaxy with four major arms containing young bright stars, gas, and dust. The mass of the Milky Way is estimated to be between 400 billion and one trillion times the mass of the Sun. The Milky Way’s diameter is about 100,000 light-years, with the Sun orbiting about 27,000 light-years from the galactic center. However, most of the galaxy’s mass consists of “dark matter” which encircles the Milky Way’s disk in a vast halo. Dark matter may account for 90 percent of the Milky Way’s mass.

Mir Space Station

A Russian space station that remained in low Earth orbit from 1986-2001. It served as a research laboratory where crews conducted experiments in biology, physics, astronomy, meteorology and spacecraft systems, with an aim to develop technologies required for the permanent occupation of space.

Monoceros, the Unicorn

This faint constellation, to the east of Orion, contains some beautiful and prominent deep-sky objects, such as the Rosette Nebula and the Cone Nebula. It was created in the early 17th century to fill a void between brighter constellations.

Monte Alban

An ancient Mexican city that housed a temple used for astronomical observations. The temple was built more than 2,000 years ago, and some of its structures align with the point on the horizon where Capella made its first dawn appearance. That appearance happened on one of the two days of the year when the Sun passed directly overhead — dates that were vital to the city’s ritual life. So Capella might have acted like a warning flag, alerting the city’s leaders that the time for the year’s most important ceremonies was at hand.

Morning Star

A name for the planet Venus when it appears in the morning sky. Venus is the brightest object in the night sky after the Moon.

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NASA Discovery Missions

A NASA program aimed to explore space with lower-cost, highly focused planetary science investigations designed to enhance our understanding of the solar system. The project’s slogan is “Faster, Better, Cheaper.” Kepler, Mars Pathfinder, Lunar Prospector, and Messenger are just a few of the missions included in the Discovery program.

Near Earth Asteroid Rendezvous (NEAR) Mission

The most extensive study of any minor body to date was provided by NEAR-Shoemaker, which entered orbit around the asteroid Eros on February 14, 2000. It transmitted thousands of images of the asteroid plus extensive measurements of its shape, composition, and gravity. Although it was not designed as a lander, as its fuel neared an end, flight controllers maneuvered it to a gentle touchdown on the asteroid’s surface on February 12, 2001 — the first landing on any asteroid or comet. The craft’s instruments transmitted data from the surface for several days.


A generic term for a fuzzy, diffuse astronomical object. Astronomers have observed four different types of nebulae: H II regions, reflection nebulae, planetary nebulae, and supernova remnants.


The eighth planet from the Sun in the solar system. Neptune orbits the Sun at an average distance of 30 AU. The planet has a mass 17.1 times the mass of Earth and a radius 3.9 times the radius of Earth. Neptune is a gas planet made of hydrogen, helium, and methane and has no solid surface. It has a small ring system and 14 known moons, the largest of which is Triton.

Neptune's Moons

The eighth planet from the Sun has 14 known moons as of July 2013. The largest is Triton.


A neutrino is an almost massless particle created in abundance in the nuclear fusion reactions in the hearts of stars. They usually zip through all forms of matter without interacting, making them difficult to detect. (In fact, trillions of them pass through your body every second.) Physicists have built special detectors deep below ground. Neutrinos occasionally zap atoms and molecules in the detector chambers, producing brief flashes of light. From these reactions, physicists can determine the origin of the neutrinos. Astronomers have detected neutrinos produced in supernovas and other powerful events, enhancing our understanding of these objects.

Neutron Stars

The crushed remnant of a star that exploded as a supernova. Stars that are born with about 8 to 20 times the mass of the Sun blast most of their material into interstellar space in titanic explosions, leaving only their crushed, dense cores. Neutron stars are named after their composition: neutrons. In a star with a core that is 1.4 to 3 times the mass of the Sun, the core collapses so completely that electrons and protons combine to form neutrons. A full bathtub of neutron-star material (instead of water) would weigh as much as two Mount Everests. A neutron star is about 10-15 miles (16-24 km) in diameter, with a liquid neutron core and a crust of solid iron. Some neutron stars, called pulsars, spin rapidly (from once a second to several hundred times per second) and generate powerful magnetic fields.

New Horizons Mission

New Horizons is a spacecraft that flew past Pluto in the summer of 2015. It mapped the surfaces of Pluto and its biggest moon, Charon, and snapped pictures of the dwarf planet's three smaller moons. It is scheduled to rendezvous with another Kuiper Belt Object, 2014 MU69, on New Year's Day, 2019.

Newton, Sir Isaac

A British mathematician and physicist who lived from 1643 - 1727. Newton made major advances in the studies of optics, mathematics, astronomy, and physics, most of which were published in his two major works, Opticks and Principia. Among other innovations, he constructed the first reflecting telescope (now known as a Newtonian style telescope), invented integral calculus, and postulated a Universal Law of Gravity. Newton’s First Law: Objects in motion stay in motion, and objects at rest stay at rest, unless acted upon by an outside force. Newton’s Second Law: For every action, there is an equal and opposite reaction. Newton’s Third Law: The force exerted on an object and the acceleration of the object are directly proportional. The constant of proportionality is the mass of the object. Newton’s Law of Gravity:The gravitational force exerted on one object by another object is proportional to the product of the masses of the two objects and inversely proportional to the square of the distance between them. The constant of proportionality is the Universal Gravitational Constant, G.


A nova is an explosion from the surface of a white-dwarf star in a binary system. A nova occurs when the white dwarf, which is the dense core of a once-normal star, “steals” gas from its nearby companion star. When enough gas builts up on the surface of the white dwarf it triggers a thermonuclear explosion. For a brief time, the system can shine up to a million times brighter than normal. As long as it continues to take gas from its companion star, the white dwarf can produce nova outbursts at regular intervals.

Novae and Supernovae

A nova is an explosion from the surface of a white-dwarf star in a binary star system. A nova occurs when the white dwarf, which is the dense core of a once-normal star, “steals” gas from its nearby companion star. When enough gas builts up on the surface of the white dwarf it triggers an explosion. For a brief time, the system can shine up to a million times brighter than normal. As long as it continues to take gas from its companion star, the white dwarf can produce nova outbursts at regular intervals. A supernova is a violent stellar explosion that can shine as brightly as an entire galaxy of billions of normal stars. Astronomers divide supernovae into two groups: Type I and Type II. Type I supernovae most likely form as a white dwarf “steals” hot gas from a companion star. If enough gas piles up on the surface of the white dwarf, a runaway thermonuclear explosion blasts the star to bits, leaving nothing behind. These are the brightest supernovae, and can be used to measure the distances to other galaxies. Type II supernovae are the final stage in the evolution of stars that are at least eight times as massive as the Sun. Such a star reaches a point where it can no longer produce nuclear energy in its core. Without the outward pressure created by this energy, gravity wins out and causes the star’s core to collapse to form a neutron star or black hole. The star’s outer layers “rebound” violently, blasting into space at several percent of the speed of light.

Nuclear Fusion

A reaction that involves two atomic nuclei merging to create the nucleus of a new atom. The reaction is accompanied by a tremendous release of energy. The Sun’s energy, as with all stars, comes from nuclear fusion reactions that happen in its core. The reactions fuse hydrogen nuclei into helium nuclei. About 0.7 percent of the mass of the hydrogen is converted to energy, which makes the star shine.

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Omega Nebula, M17

A nebula about 5,500 light-years away that is giving birth to new stars.

Oort Cloud

The Oort Cloud is a hypothetical shell of icy bodies that completely surrounds the solar system. It is expected to begin about 2,000 astronomical units (AU) out and extend to 100,000 AU or farther (one AU is the average Earth-Sun distance, about 93 million miles/149 million km). Estimates say it could contain 100 billion bodies to one trillion or more. Most of its members are no more than a few miles in diameter. Passing stars may occasionally knock some of these blobs of ice and rock out of their orbits, either expelling them into interstellar space or sending them hurtling toward the inner solar system. Such bodies may account for long-period comets, which take more than 200 years to orbit the Sun. The Oort Cloud is named for Dutch astronomer Jan Oort, who first proposed its existence in 1950.


The motion of a massive body around another body, governed by the force of gravity. Planets in our solar system follow an orbit around the Sun, as first noted by Johannes Kepler, in the shape of an ellipse. An orbit is actually composed of two motions: one directly toward the other body (planet or star...) and the another that points away. In the case of a circular orbit, these two component motions are orthogonal (90 degrees apart, or at right angles). Einstein would say that an orbit is actually a geodisc in space — time, formed by the interaction between space and the mass of the two bodies.

Orion Nebula, M42

A nursery for newborn stars about 1,270 light-years away in the constellation Orion.

Orion, the Hunter

A large, well-known constellation visible high in winter’s south-southeastern sky.

OSIRIS-REx Mission

OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) is a mission to the asteroid 101955 Bennu. It launched on September 8, 2016, and is scheduled to arrive at the asteroid in December 2018.During its first month of operations, OSIRIS-REx will make several close passes by Bennu’s poles and equator. That will provide precise measurements of the asteroid’s mass and shape. After that, the craft will settle into orbit, beginning a long series of observations. The mission’s highlight will come in July 2020, when OSIRIS-REx is scheduled to scoop up a small sample from the surface for delivery to Earth three years later.

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A star's parallax is the angle it appears to move against the background of more-distant objects as Earth moves from one side of the Sun to the other. Astronomers use that angle to measure the star's distance. The technique provides highly accurate distances out to a few hundred light-years, and less-accurate ones out to thousands or tens of thousands of light-years. Astronomers generally express distances to stars and galaxies in parsecs, which is short for parallax-seconds, or the parallax in seconds of arc. One parsec equals 3.26 light-years. When describing a star's absolute magnitude, or its true brightness, astronomers use a standard distance of 10 parsecs, or 32.6 light-years. 

Parker Solar Probe

Parker Solar Probe is designed to study the Sun and its hot outer atmosphere, known as the corona, from closer range than any other spacecraft to date. Launched August 12, 2018, its primary mission will consist of 24 orbits around the Sun, shaped by seven close encounters with Venus. At its closest, in 2025, it will pass less than four million miles (6.3 million km) from the Sun's surface. It will move faster than any other spacecraft in history, reaching a maximum of 430,000 miles per hour (690,000 kph) at its closest approach. Heavy shielding will protect the spacecraft from the Sun's intense heat and radiation. The craft is named for Eugene Parker, a solar physicist. He hypothesized the existence of the solar wind in the 1950s.

Particle Physics

The branch of physics that deals with the most fundamental building blocks of matter. Much of the research is conducted in particle accelerators, which send streams of protons or other particles on collision courses with each other. The impacts produce torrents of particles and energy, many of which cannot be studied in any other way. Particle physicists also work with astrophysicists to probe conditions shortly after the Big Bang, as well as in some of the most powerful events in the universe.

Pegasus, the Flying Horse

A large pattern of stars marked by a great square.


A constellation that arcs high overhead in fall and early winter.

Phoenix Mars Lander

A craft sent to Mars’ frozen plains north of the planet’s Arctic Circle to look at conditions that are conducive for life, including the presence of liquid water, organic molecules, energy sources, and chemicals that make up our life form. Phoenix will study the conditions by digging perhaps two or three feet into the frozen soil. Phoenix will also operate a weather station. Its readings will provide new information on the interplay between the planet’s polar ice caps, its frozen ground, and its atmosphere.

Pioneer Probes

Pioneer 10 and 11 blazed the trail to the outer solar system during the 1970s. Pioneer 10 became the first spacecraft to visit any planet beyond the asteroid belt when it flew past Jupiter on December 3, 1973. Pioneer 10 found that Jupiter’s interior is hotter than previously thought, discovered that the planet’s radiation belts are strong enough to kill a human being, and discovered that Jupiter’s moon Io is embedded in a giant cloud of hydrogen that encircles Jupiter. Pioneer 11 flew past Jupiter a year later, then in 1979 became the first craft to encounter Saturn.

Pisces, the Fish

A constellation of the zodiac.

Planet Nine

Astronomers have theorized that a Neptune-sized planet orbits the Sun in the outer solar system, far beyond the realm of the eight known major planets. The planet was posited based on the alignments of several members of the Kuiper Belt, which are big chunks of ice and rock left over from the birth of the planets. The possible Planet Nine would orbit the Sun once every 10,000 to 20,000 years, at a distance equal to 400 to 800 times the distance from Earth to the Sun. It could have been ejected into its current orbit by a close encounter with Jupiter, the solar system's largest planet.

Planetary Nebulae

A bright, colorful shell of gas and dust surrounding a star in the last stages of life. A planetary nebula is created when the star puffs off its outer atmosphere. The nebula usually looks like a doughnut, sometimes with the small, hot, rapidly evolving star visible in the center. The Ring Nebula (M57) in the constellation Lyra is an example.

Pleiades Cluster, M45

A star cluster in the constellation Taurus about 440 light-years away, also known as the seven sisters.


The prototype “dwarf planet,” Pluto orbits the Sun at an average distance of 39.5 times the distance between Earth and Sun. Pluto has a mass 0.002 times the mass of Earth and a radius 0.18 times the radius of Earth. It is a small body made of water ice, and temperatures there do not rise above minus-223 degrees Centigrade. Pluto has five known moons. Discovered in 1930, it originally was classified as a planet, but was demoted in 2006.

Pluto's Moons

As of summer 2012, astronomers had discovered five moons orbiting Pluto. The largest, Charon, discovered in 1978, is roughly half the diameter of Pluto itself. Of the other moons, two have been named by the International Astronomical Union — Nix and Hydra. The other two, discovered in 2011 and 2012, have received the provisional designations P4 and P5, pending approval of formal names. Nix, Hydra, P4, and P5 are all quite small, with none larger than about 100 miles (160 km) in diameter, with the smallest only about 10 percent of that size.

Polaris, the North Star

A Cepheid variable star that marks the north celestial pole, so all the other stars seem to rotate around it.

Pole Star

Any star that aligns with Earth's polar axis. In the northern hemisphere, the pole star currently is Polaris, also known as the North Star. Thanks to precession, however, Earth wobbles on its axis, which causes the north pole to aim at different stars over a cycle of about 26,000 years. The next pole star will be Gamma Cephei, one of the leading lights of the constellation Cepheus. Other future pole stars include Vega, the brightest star of Lyra, and Thuban, in Draco, which was used to help align the Great Pyramids of Giza. In the southern hemisphere, the pole is marked by Polaris Australis, also known as Sigma Octantis. It is much fainter than Polaris, however, so it's not as useful a marker.


A yellow giant star 35 light-years from Earth. One of the twins of Gemini.


The “wobble” of the Earth’s rotational axis, which sweeps out an imaginary cone, much like a spinning top, once every 26,000 years. Precession changes the pole star as seen from Earth. Thuban, the brightest star in the constellation Draco, was the pole star while the Egyptians built the Pyramids in Egypt. Since that time, the motion of precession has rotated the Earth’s axis away from Thuban and towards Polaris, the current pole star. In 13,000 years, Earth’s rotational axis will point towards Vega, the new pole star.


A white subgiant, called the “little dog star,” 11.2 light-years away in the constellation Canis Minor.

Proxima Centauri

The closest star beyond the Sun. Proxima Centauri is a red dwarf and is far smaller, cooler, and less massive than the Sun. Proxima Centauri produces frequent outbursts of X-rays — big flares that could be deadly if you flew too close. It also has a strong magnetic field due to the hot gases rising from the star’s core, cooling as it reaches the surface, then dropping back to the center.


An ancient astronomer who codified the Greek geocentric view of the universe through his study of eccentric circles.


A spinning neutron star with a magnetic field on the order of one trillion Gauss. This magnetic field accelerates electrically charged particles along the magnetic poles, forming a beam of energy that shoots into space from the poles. If the beam shines toward Earth, astronomers see a flickering beacon. The most famous pulsar, the Crab Nebula, pulses 30 times a second.

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The most luminous and some of the most distant of all objects in the universe. They radiate between 10 and 100,000 times as much energy as our entire galaxy from an energy source that is no larger than our solar system. Such a compact source may be a supermassive black hole surrounded by an accretion disk of matter falling into the hole. The matter is heated to millions of degrees, making the accretion disk glow brightly.

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Energy that travels through space at the speed of light. The total range of electromagnetic wavelengths and frequencies is called the electromagnetic spectrum. Our eyes are sensitive to a sliver of the electromagnetic spectrum we call visible light, from red to blue. Beyond the blue end, as wavelength decreases and frequency increases, lie ultraviolet light, X-rays, and gamma-rays. In the other direction past red, as wavelength increases and frequency decreases, lies infrared, microwave, and radio regions of the electromagnetic spectrum. Radio waves are the longest, extending on the order of meters, while X-rays and gamma-rays are the shortest, on the order of atomic size. Light wavelengths are measured in microns, millionths of meters. For all forms of electromagnetic radiation, the speed of light must remain constant, regardless of wavelength or frequency.

Radio Astronomy

A very unenergetic wavelength, or frequency, of light. Radio waves are the least energetic form of radiation known. They have wavelengths longer than 10^-3 meters and frequencies less than 10^11 Hz. Radio waves are not harmful to life because they are not strong enough to ionize atoms or destroy cells. While the Earth’s atmosphere shields us from some radio radiation, it does allow radio waves in the vicinity of VHF, UHF, and FM frequencies to pass through. Astronomers study waves that pass through this “radio window” with large radio telescopes or antennas, which resemble giant satellite dishes. Often, many radio antennas are coordinated together to synthesize even bigger telescopes, such as the Very Large Array in New Mexico. In addition to astronomy, radio waves have many useful applications on Earth, such as television and radio broadcasts and RADAR.

Radio Galaxies

A particular type of active galaxy that emits more light at radio wavelengths than at visible wavelengths, also known as a radio-luminous galaxy or radio-loud galaxy. Radio galaxies are driven by non-thermal emission. Radio telescopes show that some radio galaxies, called extended radio galaxies, have lobes of radio emission extending millions of light-years from their nuclei. Centaurus A is a nearby example of an extended radio galaxy that features two outer lobes 650,000 and 1,350,000 light-years in diameter. In contrast, compact radio galaxies emit radio lobes not much larger than the galactic nucleus.

Red Dwarfs

Small, cool stars on the main sequence. Red dwarfs constitute the majority of stars and have a mass of less than one-half of the Sun.

Red Giants

A state of stellar evolution beyond the main-sequence life of a star. A red giant core is degenerate ionized helium, surrounded by a shell of hydrogen fusion, that expands the outer atmosphere in response to higher core temperatures. The hydrogen fusing shell eats through the surrounding atmosphere and deposits helium onto the shrinking core. The ballooning atmosphere cools and glows red; hence red giant. The Sun will become a red giant the size of Earth’s orbit in five to six billion years. Once the helium core reaches 100 million degrees, it explosively begins fusing helium. The birth of the active helium core is called the helium flash. The Sun as a red giant will fuse helium for about 2 billion years after the helium flash.


A blue-white star in the constellation Leo.


Einstein general theory of relativity is the best model for gravity so far, and has been confirmed in experiments and observations. According to the theory, regardless of one’s point of view (as measured by speed and direction), physical law and the speed of light are unchanged. This implies that measurements made in time and space are not absolute, but relative to your particular point of view or reference frame. General relativity led to concepts and theories such as black hole, parallel universes, worm holes, and space-time. Special relativity is Einstein’s rejection of the notion that space and time are absolute, based on the observation that the speed of light is independent of the motion of an observer. No matter how fast someone runs toward you with a flashlight, the speed of the light that flashlight emits will always remain the same. From this foundation, Einstein constructed a revolutionary model of gravity and a universe full of unexpected surprises like black hole, gravity waves, time dilation, and the equivalence of mass and energy: E=Mc^2. Astronomers and astrophysicists regularly use the theoretical tools of special relativity to interpret and analyze light.

Retrograde Motion

The temporary apparent backward motion of a planet in the sky, from east to west, caused by the geometry between the Earth and planet. Due to this geometry, only planets that orbit outside the orbit of the Earth are observed to have retrograde motion.


A young blue supergiant star 800 light-years away that marks the right foot of the constellation Orion, the hunter.

Ring Nebula, M57

A planetary nebula in the constellation Lyra.

Rosetta Mission

A European Space Agency mission to explore Comet 67P/Churyumov–Gerasimenko. Launched in 2004, the craft arrived in 2014, becoming the first spacecraft to orbit a comet. Rosetta, which will study the comet through the end of 2015, as the comet moves closer to the Sun, also carries a small probe to land on the comet.

Runaway Star

A runaway star is moving through the galaxy at a far greater speed than any of the other stars around it. In some cases, the star is moving fast enough to escape the Milky Way Galaxy and enter intergalactic space. A runaway may get a "kick" when it passes close to the supermassive black hole at the center of the galaxy, or when a companion star explodes as a supernova. Many runaway stars leave a "wake" as they plow through the gas and dust scattered between the stars.

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Sagittarius, the Archer

Sagittarius is one of the most interesting regions of the sky. The center of our Milky Way galaxy lies inside Sagittarius, about 26,000 light-years away. The constellation also contains several globular clusters — tightly packed collections of hundreds of thousands of stars. Its brightest stars form the shape of a teapot, and the constellation slides low across the southern sky of summer.


The sixth planet from the Sun in the solar system. Saturn orbits the Sun at an average distance of 9.5 AU. The planet has a mass 95.2 times the mass of Earth and a radius 9.45 times the radius of Earth. Saturn is a gas planet and has no solid surface. It is made of hydrogen, helium, ammonia, and methane. Saturn is most famous for its beautiful ring system. In addition to the rings, the planet has 62 known moons, the largest of which is Titan.

Saturn's Atmosphere

The sixth planet from the Sun in the solar system. Saturn orbits the Sun at an average distance of 9.5 AU. The planet has a mass 95.2 times the mass of Earth and a radius 9.45 times the radius of Earth. Saturn is a gas planet and has no solid surface. It is made of hydrogen, helium, ammonia, and methane. Saturn is most famous for its beautiful ring system. In addition to the rings, the planet has 47 known moons, the largest of which is Titan.

Saturn's Moons

Saturn has 62 known moons as of April 2009, the largest of which is Titan. Other major moons include Enceladus, Iapetus, and Mimas, the so-called “Death Star moon.”

Saturn's Rings

Thousands of individual rings make up Saturn’s ring system. Some rings are made of small bits of frozen water, others contain tiny grains of dust, and still others are a mixture of the two. In all, the rings are only a few hundred feet thick.

Scorpius, the Scorpion

Three bright stars form the “head” of Scorpius, the celestial scorpion, while its tail curves away below it in the southern sky of summer. The brightest star in Scorpius is Antares, which is in the middle of the scorpion’s curving body. If you placed this brilliant red star at the center of our own solar system, it would swallow Mercury, Venus, Earth, and Mars, and almost reach Jupiter.

Search for Extraterrestrial Intelligence (SETI)

An exploratory science that searches for evidence of life in the universe.


Four natural divisions in weather and daylight throughout the year resulting from Earth’s yearly revolution around the Sun and the tilt of Earth’s axis. The seasons are Spring, Summer, Fall, and Winter.

SIM-Planetquest Mission

A NASA space telescope being developed to search for Earth-sized planets orbiting in the habitable zones of nearby stars other than the Sun.

Sirius, the Dog Star

The brightest star in the northern night sky, Sirius is actually a binary star system 8.7 light-years away in the constellation Canis Major.

Solar System

The system of our Sun and its planets, including moons, asteroids, and comets.

Solar Wind

A flow of atomic nuclei and electrons from the Sun that travels at an average speed of about two million miles per hour (400 km/s).

Solstices and Equinoxes

The solstice is an event in the Earth’s orbit during which the tilt of the Earth’s axis is pointed most directly towards or away from the Sun. The summer solstice for the northern hemisphere occurs within a few days of June 21 every year. It is on this day that the position of the Sun in the sky at noon is at its highest altitude of the year, and the position of the Sun at Sunrise and Sunset is farthest north for the year. The winter solstice is around December 21, marking the date on which the Sun is lowest in the sky at noon and rises and sets farthest south. The day of the summer solstice is the longest day of the year, and the day of the winter solstice is the shortest day of the year. The equinox is the time of year when the Sun appears in the sky at the intersection of the ecliptic and the celestial equator. Two such points exist: The vernal equinox (approx. March 21) marks the start of spring. The autumnal equinox (approx. Sept. 20) marks the start of autumn. At each of these points, the Sun rises due east and sets due west.

Sombrero Galaxy, M104

Messier 104 is one of the most photogenic galaxies. A lane of dark dust outlines its edge like the brim of a hat, giving M104 a distinctive nickname: the Sombrero. It is in the constellation Virgo, and is visible through small telescopes.

Space Junk

Space junk is debris that orbits Earth, presenting hazards to operating spacecraft. The debris includes dead satellites and booster stages, pieces of exploded boosters, pieces of spacecraft, and even bits of equipment accidentally lost from manned space missions. The amount of space junk is increasing at a frenetic pace as companies launch thousands of new communication satellites into orbit. The United States Space Force tracks debris down to just a few centimeters (inches) across, and provides warnings to satellite operators when one of their craft is threatened. Debris is a special problem for the International Space Station, which is the largest craft ever placed in orbit. Astronauts aboard the station sometimes have to retreat to their return capsules when space debris is predicted to come especially close. Several space agencies are studying ways to reduce the amount of space junk, which could make space flight especially hazardous in the years to come.


A technique used by astronomers that allows them to determine the properties, such as composition, temperature, and motion through space, of celestial objects by analyzing the spectra of celestial objects. For example, because each atomic element absorbs and emits light in a unique set of wavelengths, the astronomer can sift through the spectrum of a star and determine what elements are present in the star’s atmosphere. From the shapes and depths of spectral lines, the astronomer can calculate fundamental qualities of a star, such as how fast the gases churn through the stellar atmosphere or the star’s effective temperature. An astronomer may also be interested in correlations between the abundances of certain elements and the physical behavior of the star, the age of the star, or the abundances of other elements. For instance, compared with the Sun, stars with low amounts of iron are also low in almost every other element with respect to hydrogen. Spectroscopy is performed by astronomers with instruments called spectrographs.

Speed of Light

The maximum velocity for everything in the universe; 186,282.397 miles (299,792.458 km) per second, or fast enough to go to the Moon and back in less than three seconds.


A blue-white giant star 250 light-years away in the constellation Virgo.

Spitzer Space Telescope

Spitzer Telescope has provided a dramatic new view of the Milky Way galaxy. It has mapped giant stellar nurseries, and revealed thousands of infant stars, as well as thousands of old stars, which have wrapped themselves in cocoons of gas and dust. Spitzer has also helped produce the best map of the Milky Way to date. The map reveals that the Milky Way probably has two major spiral arms wrapping around a long “bar” of stars in its center.

Star Catalogs and Charts

Monthly star charts are for sale in the StarDate store.

Star Clusters

A grouping of the same types of astronomical objects. For example, stars in the Milky Way can group together into open clusters or globular clusters. Galaxies also group together into cluster of galaxies, and the clusters of galaxies group together into superclusters of galaxies.


The process in which a cloud of interstellar gas and dust collapses to form a new star. The collapse may be triggered by the shockwave from an exploding star, the gravity of a passing star cluster, or some other external source. The process fragments a cloud that may span many light-years into smaller pockets of matter, which collapse under their own gravitational pull. As such a pocket of matter collapses, its core gets hotter and hotter. When it gets hot enough, it triggers the process of nuclear fusion, which powers the newborn star.

Stardust probe

Stardust flew through the coma of Comet Wild 2, gathering grains of comet dust that it returned to Earth in early 2006.

Stargazing and Skywatching

For more information about what to see in the nightsky tonight and this month, visit our Stargazing page.


A dense, glowing ball of hydrogen, helium, and trace quantities of heavier elements that shines with energy released from a hydrogen thermonuclear fusion reaction in the center, or core. Our Sun is an example of a star. Stars can appear red, orange, yellow, blue, or white, according to their masses and effective temperatures. Stars are born from clouds of gas and dust, live for millions or billions of years, depending on their masses, and die sometimes spectacular deaths. Stars usually exist in very large collections called galaxies.

Stereo probes

Solar TErrestrial RElations Observatory is the third mission in NASA’s Solar Terrestrial Probes program. This two-year mission will trace the flow of energy and matter from the Sun to Earth. It will reveal the 3D structure of coronal mass ejections; violent eruptions of matter from the sun that can disrupt satellites and power grids, and help us understand why they happen.

String Theory

A theory that proposes that subatomic particles are one-dimensional strings.

Subatomic Particles

Having a size smaller than the size of an atom, which is about one angstrom, or 10^-10 meters.

Summer Triangle

A pattern of three bright stars that stands highest in the sky during the nights of summer and early autumn. Its brightest member is Vega, in the constellation Lyra, which rises first. Deneb, the tail of Cygnus, the swan, is the northernmost point of the triangle, with Altair, in Aquila, the eagle, marking the southern point.



The closest star to the Earth and the center of the solar system.

Sunrise and Sunset

The point in time at which the Sun first appears in the morning sky or sets in the evening sky.There are several ways to define sunrise and sunset. They can be the points at which the Sun is bisected by the horizon (half in view, half not), or the points when the Sun has dropped below the horizon, or the points when the Sun is just out of view.You might think those last two would be the same, but they’re not. Earth’s atmosphere acts as a lens, bending the Sun’s rays, so when you see the Sun standing just atop the horizon, it’s actually below the horizon, but the atmosphere has projected an image of the Sun into view.The official timekeeper for the United States, the U.S. Naval Observatory, defines sunrise and sunset as the moments when the center of the Sun is physically 50 minutes of arc below the horizon, which is a bit less than one degree. That accounts for the size of the Sun itself, and the “bending” properties of the atmosphere. The Naval Observatory's web site provides a tool for calculating sunrise and sunset times at any point on Earth's surface.The atmosphere can bend the Sun’s rays at different angles at different times, though. So the predicted times of sunrise and sunset can be off by a minute or so. And when the Sun rises and sets at a low angle to the horizon, they can be off by several minutes. 

Sunspots and Solar Flares

A sunspot is a cooler, and therefore darker, region of the Sun’s photosphere caused by a solar magnetic disturbance. Strong, dense magnetic fields generated by circulating plasma sometimes become entangled and surge through the photosphere, creating the sunspot. The knot causes the temperature to fall to around 1000 K in the sunspot region, which darkens the photosphere. The dark center of the sunspot is called the umbra, and it is surrounded by a dim filamentary area called the penumbra. Sunspots range from Earth-size “pimples” to swollen scars halfway across the surface. Sunspot activity generally follows an 11-year cycle, called the “sunspot cycle.” A solar flare is a violent eruption of plasma from the chromosphere of the Sun that is whipped up by intense magnetic activity. During the eruption, flares rise thousands of kilometers above the chromosphere, and the plasma temperatures quickly soar to 20 million degrees. Large flares release 10^25 Joules, or about the energy of a few million volcanic eruptions on the Earth. Sunspot and solar flare frequency are strongly related. In addition, flares often disturb the Earth’s atmosphere electrically, thus interfering with radio transmissions. The aurora borealis and aurora australis are results of flare activity that injects energetic particles into Earth’s magnetic field.

Supergiant Stars

A state of stellar evolution beyond the main-sequence life of a star. A red giant core is degenerate ionized helium, surrounded by a shell of hydrogen fusion, that expands the outer atmosphere in response to higher core temperatures. The hydrogen fusing shell eats through the surrounding atmosphere and deposits helium onto the shrinking core. The ballooning atmosphere cools and glows red; hence red giant. The Sun will become a red giant the size of Earth’s orbit in five to six billion years. Once the helium core reaches 100 million degrees, it explosively begins fusing helium. The birth of the active helium core is called the helium flash. The Sun as a red giant will fuse helium for about 2 billion years after the helium flash.


A supernova is a violent stellar explosion that can shine as brightly as an entire galaxy of billions of normal stars. Astronomers divide supernovae into two primary groups: Type Ia and Type II. Type Ia supernovae form as a white dwarf “steals” hot gas from a companion star. If enough gas piles up on the surface of the white dwarf, a runaway thermonuclear explosion blasts the star to bits, leaving nothing behind. These are the brightest supernovae, and can be used to measure the distances to other galaxies. A Type II supernova is the final stage in the evolution of a star that is at least eight times as massive as the Sun. Such a star reaches a point where it can no longer produce nuclear energy in its core. Without the outward pressure created by this energy, gravity wins out and causes the star’s core to collapse to form a neutron star or black hole. The star’s outer layers “rebound” violently, blasting into space at several percent of the speed of light.

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Taurus, the Bull

A constellation of the zodiac marked by a V-shaped pattern of stars that outlines the bull’s face.

Telescopes and Modern Observatories

A telescope is an optical instrument that gathers and focuses light into a camera, CCD, spectrograph, or an astronomer’s eye. Two major types of telescopes dominate astronomy: reflectors and refractors. Many of the world’s largest telescopes are housed at observatories: The entire complex of buildings, telescopes, equipment, and staff involving scientific astronomical observations. Small observatories might consist of just one telescope situated in a dome, one instrument and computer, and one observer. McDonald Observatory is an example of a major observatory, and consists of five major telescope in five different domes, numerous instruments, computers, living quarters, outbuildings, astronomers, and engineers.


A former “North Star” 300 light-years away in the constellation Draco, the dragon.


A stretching force that is caused by the difference between gravitational forces on opposite sides of an object, such as a planet or moon. For example, because the Moon pulls on opposite sides of the Earth with different strengths, water on the Earth is pulled either toward or away from the Moon, resulting in the ocean tides. Elsewhere in the Solar System, the magnitude of gravitational force on Jupiter’s moon Io is greatest on the side of Io that faces Jupiter and least on the back side. The difference between the two forces compresses and stretches Io, which in turn heats up Io’s interior. As a result, Io is the most geologically active body in the solar system.


The largest moon of Saturn and the second-largest moon in the solar system. Titan has a thick, cold, nitrogen-rich atmosphere that supports winds, clouds, and rain. The rain, which consists of droplets of liquid methane and ethane, fills hundreds of lakes and seas in Titan’s polar regions, and appears to carve river channels. Tiny grains of methane ice form long, tall dunes across parts of Titan, and “volcanoes” may spurt frozen water onto the surface. A vast ocean of liquid water may lie far beneath Titan’s crust. The Huygens probe landed on Titan in 2006, with its images showing rounded pebbles on the surface, perhaps smoothed by water. Titan is difficult to study from afar because the atmosphere is topped by “smog” — a layer of hydrocarbons that turn orange when exposed to sunlight. The Cassini spacecraft, which ferried Huygens to Titan, has made by dozens of passes by the big moon, using radar and instruments sensitive to infrared and ultraviolet light to peer through the haze and map much of Titan’s surface.

Tombaugh, Clyde

An American astronomer who discovered Pluto in 1930. He was the first American to discover a planet in our solar system.

Transiting Exoplanet Survey Satellite (TESS)

TESS is a space telescope, launched April 18, 2018, designed to follow up on the discoveries of planets orbiting stars other than the Sun. In particular, it is studying planets around particularly bright stars, which generally are among the closest to Earth. Its observations will allow astronomers to determine the sizes and masses of the planets it studies, which reveals their density. That can tell us whether the planets are dense and rocky, like Earth; big and fluffy, like Jupiter and Saturn; or somwhere in between, like Neptune. Additional observations can reveal whether a planet has an atmosphere, and perhaps tell us something about the atmosphere's composition. The TESS observations also will reveal supernova explosions, variable stars, and other astronomical phenomena.

Trifid Nebula, M20

An area of active star formation in our Milky Way galaxy 2,200 light-years away.

Trojan asteroid

A Trojan asteroid is a chunk of rock, metal, and ice that shares an orbit with a planet. Trojans can inhabit two regions, 60 degrees ahead of or behind the planet. They are held in place by the combined gravitational pull of the Sun and the planet with which they share an orbit. Earth has two known Trojans. The giant outer planets may have thousands of them. Jupiter, for example, has at least 5,000. Many Trojans contain organic compounds. Similar compounds could have been incorporated into the young Earth, so the Trojans could tell us more about the development of life on our planet and the possibility of life on other worlds of the solar system.

Tycho Brahe

A Danish astronomer who lived from 1546 to 1601. In 1576, the King of Denmark granted him the island of Hveen on which to establish an observatory. Since this was before the invention of the telescope, he designed his own equipment for unaided eye observations. Until 1597 he made very accurate observations at Hveen, then moved to Prague. His very detailed records were later used by Johannes Kepler to discern Kepler’s three laws of planetary motion.

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Ultraviolet Astronomy

A wavelength, or frequency, of light that is more energetic than visible light, but less energetic than x-ray radiation. Ultraviolet, or UV, radiation ranges in wavelength between 10^-8 and 10^-7 meters and in frequency between 10^15 and 10^17 Hz. UV rays can be very harmful to life because they are strong enough to ionize atoms and destroy cells. Fortunately, the Earth’s atmosphere shields us from most UV radiation. Astronomers who want to study UV light from celestial sources must do so from space-borne telescopes, such as the Hubble Space Telescope.


The seventh planet from the Sun in the solar system. Uranus orbits the Sun at an average distance of 19.2 AU. The planet has a mass 14.5 times the mass of Earth and a radius 4.0 times the radius of Earth. Uranus is a gas planet made of hydrogen, helium, and methane and has no solid surface. The planet’s axial tilt is an extreme 98 degrees, causing one year on Uranus to equal 84 years, equal to the time it takes Uranus to orbit the Sun once.

Uranus' Moons

Uranus has 17 known moons, the largest of which are Titania and Oberon.

Ursa Major, the Great Bear

A constellation always visible from northern latitudes, containing the stars of the Big Dipper.

Ursa Minor, the Lesser Bear

A constellation containing the stars of the Little Dipper and Polaris, the North Star.

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Variable Stars

A star whose brightness changes periodically. There are many types of variable stars and their brightnesses vary for many different reasons. For example, the brightness of a Cepheid variable star changes because its luminosity changes; the luminosity change is related to the internal structure of the star and the processes that occur there. Variable stars can also be caused by eclipsing dark or bright companions.


A blue-white star 26 light-years away in the constellation Lyra; a dust disk that surrounds it may harbor a planet.


The second planet from the Sun in the solar system. Venus orbits the Sun at an average distance 0.7 AU. The planet has a mass of 0.08 times the mass of Earth and a radius 0.95 times the radius of Earth. It is made of solid, rocky material and has a predominantly carbon dioxide atmosphere. Clouds of sulfuric acid cover the entire planet. Venus has no moons.

Venus Express

The first Venus exploration mission of the European Space Agency. Launched in 2005, Venus Express has been continuously sending back science data from its polar orbit around Venus. Equipped with seven science instruments, the main objective of the mission is the long term observation of the Venusian atmosphere.

Venusian Surface

The second planet from the Sun in the solar system. Venus orbits the Sun at an average distance 0.7 AU. The planet has a mass of 0.08 times the mass of Earth and a radius 0.95 times the radius of Earth. It is made of solid, rocky material and has a predominantly carbon dioxide atmosphere. Clouds of sulfuric acid cover the entire planet. Venus has no moons.

Viking to Mars

NASA’s Viking 1 and 2 landers became the first spacecraft to touch down safely on Mars in 1976. From opposite sides of Mars, they photographed and analyzed their environments, finding a surface of iron-rich clay and a frigid atmosphere battered by giant dust storms. Experiments designed to look for signs of life produced inconclusive results, which scientists interpreted as meaning the Martian surface is sterile.

Virgo, the Virgin

A constellation of the zodiac.

Voyager Probes

Voyager 1 and 2 conducted a “grand tour” of the outer planets during the 1970s and ’80s. Voyager 1 flew past Jupiter and Saturn, discovering new moons and rings, compiling movies of the motions of both planets’ atmospheres, and conducting other observations. Voyager 2 followed the same path, but was then targeted to fly past Uranus and Neptune. Its encounters provided the first detailed looks at both giant planets and their moons. No other spacecraft has flown past either planet. Both Voyagers still operate today, and have detected signs of the “edge” of the solar system.

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Weather and Atmospheric Phenomena

The layers of gas that are gravitationally bound above the surface of a planet, moon, or outer layers of a star.

Whirlpool Galaxy, M51

A beautiful galaxy about 30 million light-years away, in the constellation Canes Venatici.

White Dwarfs

The hot, dense cores of once-normal stars like the Sun. At the end of such a star’s life, it can no longer produce the nuclear-fusion reactions that power it. Its outer layers drift away into space, while its core collapses into a ball that is as about as massive as the Sun but no bigger than Earth. This is the fate of stars that do not exceed about four to eight times the mass of the Sun. The Sun reaches this stage in a few hundred million to several billion years, depending on the star’s original mass. A white dwarf may spin rapidly, is extremely hot, and may generate a strong magnetic field.


A theoretical “shortcut” between two points in space-time made possible by a singularity. In science fiction, wormholes allow people and starships to travel from one part of the galaxy to another almost instantaneously. While theory allows wormholes to exist, they would make poor passageways because they should close up as soon as anything tries to enter them.

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X-Ray Astronomy

A very energetic wavelength, or frequency, of light. X-rays are more energetic than ultraviolet light, but less energetic than gamma-ray. They range in wavelength from 10^-8 meters to 10^-12 meters, and in frequency from 10^17 to 10^20 Hz. X-rays can be quite harmful to life because they are strong enough to ionize atoms and thus destroy cells. The Earth’s atmosphere shields us from all astronomical x-ray radiation. X-rays produced by people do have useful applications, however; for example, they are used to help identify broken bones and to treat certain types of cancer.

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Zubeneschamali, Zubenelgenubi

The brightest and second-brightest stars of Libra, the balance scales, and also known as Beta Librae and Alpha Librae, respectively. Their names are derived from Arabic phrases that mean the northern and southern claws of the scorpion, which is an adjacent constellation. Thousands of years ago, the stars were considered members of Scorpius. By about 2,500 years ago, however, they had been severed from the scorpion and incorporated into the new constellation Libra. They retain their older names, however, which are among the most lyrical in all the night sky. Zubeneschamali is about 185 light-years away. It is about 3.5 times the Sun's mass, five times its diameter, and more than 100 times its total brightness. Zubenelgenubi is a binary system about 76 light-years from Earth. Both of its stars are bigger and more massive than the Sun.

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