Keyword Topics

A yellow giant star 70 light-years away in the constellation Taurus.

A triple system of the closest stars to the Sun, about four light-years distant in the constellation Centaurus.

A blue-white main sequence star 16 light-years away in the constellation Aquila.

The largest member of the Local Group of galaxies, of which our Milky Way is a member.

A yellow-orange supergiant star 600 light-years away in the constellation Scorpius.

Aphelion is the point of greatest separation between a planet or other body and the Sun. Perihelion is the point of minimum orbital distance from the Sun and maximum orbital velocity around the Sun.

Apogee is the point of greatest separation between Earth and the Moon or an orbiting satellite. Perigee is the point of minimum orbital distance from the Earth and maximum orbital velocity around the Earth.

The NASA space program aimed at Moon exploration.

A constellation of the zodiac.

A constellation best seen in the south in late summer.

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

A yellow giant star 36 light-years away in the constellation Boötes. Much less dense than the Sun, but 25 times its size.

A natural or artificial object in orbit around another body. For examples, the Moon is the largest natural satellite of the Earth, and the planets are satellites of the Sun. In addition, there are satellite galaxies that orbit the Milky Way.

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. In our solar system, 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.

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 are trained personnel who travel in space. Historically, the word "astronaut" refers to an American space navigator and "cosmonaut" refers to a Russian space navigator.

A scientist who studies matter in outer space.

Distances in space are often measured in light-years and parsecs. A light-year is A unit of length used by astronomers to measure distance. One light-year is equal to the distance light travels in one year, which is about 5.88 trillion miles or almost 800 times the diameter of our 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, it appears to shift back and forth across the sky by exactly one second of arc (1/3600 of a degree), then its distance is one parsec.

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

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.

A small, cool, and faint red dwarf star six light years from Earth.

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

A young star system surrounded by a giant cloud of gas and dust that may harbor planet formation.

A yellow-orange supergiant star 650 light-years away in the constellation Orion.

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.

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 on the northern horizon and is often used to locate the North Star.

A pair of stars that orbit around each other.

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.

A spring constellation that trails the Big Dipper as it wheels around the North Star.

A "failed" star, larger than a planet but smaller than a star, that is not massive enough to ignite a nuclear fusion reaction at its core.

Superclusters of galaxies are arranged in 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.

A constellation of the zodiac.

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

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.

A constellation of the zodiac.

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

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 Titan. Its images showed a landscape carved by flowing liquid. Cassini's instruments have peered through Titan's atmospheric haze to discover possible pools of liquid and a possible volcano on Titan's surface. Cassini is scheduled to continue its reconnaissance of the Saturn system until 2008.

A constellation best seen in November in the northeastern sky.

A crowded six-star system about 50 light-years from Earth.

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.

An unstable star whose brightness changes periodically. 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 record the changing brightness of a Cepheid variable and plot the brightness change over time 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.

An unstable star whose brightness changes periodically. 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 record the changing brightness of a Cepheid variable and plot the brightness change over time 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.

The largest known asteroid, Ceres, is 623 miles (1,003 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."

See Asian Astronomy

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. It was jointly sponsored by the Ballistic Missile Defense Organization and NASA.

An icy, "dirty snowball" (Fred L. Whipple) that orbits the Sun. The majority of comets orbit well beyond Pluto in halos known as the Kuiper Belt and the Oort Cloud that surround the Solar System. Comet size ranges from a few meters to a few kilometers 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.

A meaningful pattern of stars visible with the unaided eye, or a region of space seen from Earth that is bounded by borders designated by the International Astronomical Union. 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.

The CONTOUR spacecraft was launched in 2002 to study the comets Encke and Schwassmann-Wachmann 3 (SW3) and bring samples back to Earth.

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

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.

See Cosmic Rays

A high-energy particle that flies through the universe at close to the speed of light. Cosmic rays are mostly bare protons and electrons. Some 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.

A cloud of glowing gas that spans several light-years. It's called the crab because its tendrils of gas 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.

A constellation best seen overhead during late summer evenings.

A term used to describe of our lack of understanding of how the universe works on the largest scales. It may be a "repulsive" force that is causing the universe to expand faster as it ages, a discrepancy in the laws of gravity, or some other phenomenon.

Matter that produces no detectable energy, but that reveals its presence through its gravitational pull on the visible matter around it. Dark matter comprises 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.

Matter that produces no detectable energy, but that reveals its presence through its gravitational pull on the visible matter around it. Dark matter comprises 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.

The Dawn probe targeted the asteroids Vesta and Ceres. The spacecraft goaled to fly past Mars to study the planet, giving scientists and engineers a chance to see how well they work. Mars then gave Dawn a gravitational "kick" toward the asteroid belt.

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.

Missions aimed to study and expand our knowledge of space outside of the solar system.

A blue-white supergiant star 1,600 light-years away in the constellation Cygnus.

See binary and multi-star systems

The third planet from the Sun in the Solar System, and the planet on which people live. The Earth orbits the Sun at an average distance of 1.0 A.U. 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, oxygen, argon, and carbon dioxide. Liquid water oceans cover 75% of the planet's surface. Earth has one moon.

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

The only natural satellite of Earth.

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 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.

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.

The full range of electromagnetic radiation in order of wavelength from longest to shortest, or frequency from lowest to highest. 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 are wider than our home galaxy, the Milky Way, and also much thicker, because they are shaped like a fat football. Some elliptical galaxies are fatter than others and rotate more slowly, thus there are two distinct categories of galaxies within elliptical galaxies.

The Milky Way's most massive star, about 8,000 light-years away and about 100 times as massive as the Sun.

Another name for Venus because it outshines all of the other stars and planets in the night sky.

NASA's longest-running series of spacecrafts, with 74 successful missions and 4 failed missions.

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

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

A vast "island" of stars, gas, and dust, which populates the universe by the billions. Galactic size and structure range from subtle ellipticals to grand spirals with the mass of at least 100 billion stars. Instead of randomly scattered throughout the universe, 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.

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.

An Italian scientist who lived from 1564 to 1642. Among many other interests, Galileo studied astronomy and constructed the first telescope for astronomy. He used the telescope to discover sunspots, craters on the Moon, and the four largest moons of Jupiter. Galileo supported the Copernican system for the Solar System, postulated by Nicholas Copernicus in 1543. As a result, he was brought up before the Catholic Inquisition, where he was forced to recant.

Galileo was the first spacecraft to enter orbit around one of the outer planets. 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 to a trickle.

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 all astronomical gamma-ray radiation.

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 all astronomical gamma-ray radiation.

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

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

Spacecraft that collected a sample of solar wind and returned it to Earth for study.

Goddard 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.

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.

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.

A British astronomer who calculated that comets follow elongated orbits around the Sun -- orbits that sometimes bring them back into view from Earth. Astronomers later named his most famous test subject, Halley's Comet, after the name who calculated its orbit and predicted that it would return back to Earth.

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

Astronauts aboard space shuttle Discovery deployed the Hubble Space Telescope on April 25th, 1990. It was the first big telescope designed to escape the blurring effects of Earth's atmosphere -- 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 the first evidence of "dark energy" -- a force that's causing the universe to expand faster as it ages.

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 collection of more than a hundred stars; the brightest form the V-shaped "face" of Taurus, the bull.

Hydrogen is an element, usually in the form of a gas, whose nucleus is made of one proton. Hydrogen is the most abundant element in the universe, accounting for about 75% of the composition of the universe. Helium is an element, usually in the form of a gas, whose nucleus is made of two protons and two neutrons. Helium is the second-most abundant element in the universe, after hydrogen, and accounts for about 25 percent of the atoms in the universe. Helium is also the product of hydrogen fusion in stars.

A wavelength, or frequency, of light that is slightly less energetic than visible light, but more energetic than radio or microwave radiation. Infrared radiation ranges in wavelength between 10^-6 to 10^-4 meters and in frequency between 10^12 and 10^14 Hz. Infrared radiation is not strong enough to ionize atoms.

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.

A research facility that began being assembled in low Earth orbit in 1998 and is scheduled for completion in 2011. The International Space Station is a program managed by the US, Europe, Russia, Canada, and Japan for the development and operation of a liveable space station.

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.

Galaxies that do not match the standard Hubble scheme of elliptical, spiral, and lenticular galaxies.

The next big space telescope — the successor to Hubble 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.

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 AU. 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 63 known moons, the largest of which are Io, Europa, Ganymede, and Callisto.

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 has 62 known moons. The four Galilean moons are Io, Europa, Ganymede, and Callisto.

Kepler is a roughly 1-meter-diameter telescope which will be staring at a hundred thousand stars in the constellations Cygnus and Lyra, easily visible from the northern hemisphere. It will stare at those hundred thousand stars for about three and a half years continuously, without ever closing its eyes. The goal is to find planets orbiting the stars -- especially Earth-like planets around Sun-like stars. In particular, 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.

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).

A German mathematician and astronomer who lived from 1571 - 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 or The New Astronomy, where he summarized planetary motion with three rules, or laws.

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: 1 step for each day of the year.

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

Two companion galaxies of the Milky Way, called the Large Magellanic Cloud and the Small Magellanic Cloud. They are visible only from the southern hemisphere. They may be gravitationally bound to the Milky Way, so that they orbit it, or they may just be making a close pass by the Milky Way.

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

A constellation of the zodiac.

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.

Like it's larger counterpart, the Big Dipper, the Little Dipper is also an "asterism" -- a group of stars that forms an easy-to-see pattern. It is said to resemble a ladle or cart and is part of the constellation Ursa Minor, which includes Polaris.

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.

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 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.

A NASA mission to return to the Moon and then travel to Mars and beyond. The objectives are to find safe landing sites, discover new resources, and test new technology.

A constellation visible high overhead in mid-summer.

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.

A classification of stars that all shine via hydrogen thermonuclear fusion, and are all in a state of hydrodynamic equilibrium. Stars spend the greatest portion of their luminous (nuclear fusion) lives on the main sequence.

In 1962, the American 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 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. Mariner found that Mercury's interior consists of a large iron-rich core, which generates a weak magnetic field around the planet.

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.

Aimed to provide detailed information about the atmospheric conditions on Mars including temperature, dust, clouds, water vapor, and carbon dioxide levels.

A European Space Agency craft set to orbit Mars. Pictures from the mission have provided evidence of volcanic eras on the Red Planet.

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.

2001 Mars Odyssey was part of NASA's Mars Exploration Program, a long-term effort of robotic exploration of the red planet. The mission globally mapped the amount and distribution of many chemical elements and minerals that make up the martian surface, which led scientists to discover vast amounts of water ice in the polar regions buried just beneath the surface.

The second mission from NASA's Discovery Program that carried a series of scientific instruments to analyze the Martian atmosphere, climate, geology and the composition of Mars' rocks and soil.

One of two exploration vehicles of the NASA Mars Surveyor '98 program that was ultimately lost within space.

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 also will provide new data for the scientists who are looking for water on Mars. The images could show small streambeds that formed in the fairly recent past. They'll also help locate minerals that formed in a watery environment. MRO also carries a radar system that will probe beneath the surface to look for deposits of frozen or liquid water.

Building on the success of the 1997 Sojourner rover, the Spirit and Opportunity rovers landed on Mars in 2003. Although their planned mission was just 90 Mars days long (about 92 Earth days), both rovers were still operating in 2007. Both found evidence that water had helped shape the Martian landscape in the distant past. Several American and European craft found similar evidence from Mars orbit.

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.

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.

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, 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. McDonald Observatory operates an international public outreach program, including the StarDate and Universo radio programs; StarDate magazine; StarDate Online, Universo Online and related websites.

The first planet from the Sun in the solar system. Mercury orbits the Sun at an average distance of 0.4 AU. The planet has a mass 0.055 times the mass of Earth and a radius 0.4 times the radius of Earth. It is made of solid, rocky material that is heavily cratered, and it has no significant atmosphere. Mercury has no moons.

The Mercury Project was one of NASA's first programs and it ran from 1958 to 1962. The project's objectives were to successfully place a manned spacecraft in orbital flight around the earth, investigate man's performance capabilities and his ability to function in the environment of space, and recover the man and the spacecraft safely.

NASA's Messenger spacecraft is now on a long, circuitous trek to Mercury, and should enter orbit in 2011 for a one-year mission to this least-explored of the solar system's inner planets.

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.

A spectacular display of meteors streaking through the sky at rates between 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. Meteor velocities usually fall within a range of about 10 km/s and 70 km/s.

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.

A meteor is a “shooting star"; a meteoroid that vaporizes in Earth's upper atmosphere as a burst of incandescent gas. A meteorite is a space rock that strikes the surface of the 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.

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.

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.

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.

Another name for the planet Venus because it rises about 90 minutes before the Sun.

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.

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 sSystem. 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 13 known moons, the largest of which is Triton.

The eighth planet from the Sun has 13 known moons as of January 2006. The largest is Triton.

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 is a spacecraft goaled to fly past Pluto in the summer of 2015.
When it arrives, New Horizons will map the surfaces of Pluto and its biggest moon, Charon. And it may scan two additional moons that appear to be showing up in recent images from Hubble Space Telescope.

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 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.

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.

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

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.

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

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

A large pattern of stars marked by a great square.

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

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 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.

A constellation of the zodiac.

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.

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

A solar system object that 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 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 –223 degrees Centigrade. Pluto has three moons. Discovered in 1930, it originally was classified as a planet, but was demoted in 2006.

A solar system object that 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 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 –223 degrees Centigrade. Pluto has three moons. Discovered in 1930, it originally was classified as a planet, but was demoted in 2006.

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

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.

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.

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.

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.

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.

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.

A small, relatively cool star of the main sequence. Red dwarfs constitute the majority of stars and have a mass of less than one-half of the Sun.

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.

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.

A planetary nebula in the constellation Lyra.

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 47 known moons, the largest of which is Titan.

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 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."

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.

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.

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.

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

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.

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

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).

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.

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.

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 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.

Monthly star charts are for sale in the StarDate store.

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.

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

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.

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.

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

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

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

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.

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

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.

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

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

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.

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 day on Uranus to equal 84 years, equal to the time it takes Uranus to orbit the Sun once.

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

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

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

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.

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.

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.

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.

A constellation of the zodiac.

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.

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

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

The hot, dense core of a once-normal star 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.

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.