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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.


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.

Astrobiology and Extraterrestrial Life

Astrobiology is the study of the conditions under which life might exist on other planets or moons.