Glossary

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Asteroid
A small solar system object composed mostly of rock. Many of these objects orbit the Sun between Mars and Jupiter. Their sizes range anywhere from 33 feet (10 meters) in diameter to less than 620 miles (1,000 kilometers). The largest known asteroid, Ceres, has a diameter of 579 miles (926 kilometers).
Black Hole
A region of space containing a huge amount of mass compacted into an extremely small volume. A black hole's gravitational influence is so strong that nothing, not even light, can escape its grasp. Swirling disks of material — called accretion disks — may surround black holes, and jets of matter may arise from their vicinity.
Brown Dwarf
An object too small to be an ordinary star because it cannot produce enough energy by fusion in its core to compensate for the radiative energy it loses from its surface. A brown dwarf has a mass less than 0.08 times that of the Sun.
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Comet
A ball of rock and ice, often referred to as a "dirty snowball." Typically a few kilometers in diameter, comets orbit the Sun in paths that either allow them to pass by the Sun only once or that repeatedly bring them through the solar system (as in the 76-year orbit of Halley's Comet). A comet's "signature" long, glowing tail is formed when the Sun's heat warms the coma or nucleus, which releases vapors into space.
Constellation
A geometric pattern of bright stars that appears grouped in the sky. Ancient observers named many constellations after gods, heroes, animals, and mythological beings. Leo (the Lion) is one example of the 88 constellations.
Dark Energy
A mysterious force that seems to work opposite to that of gravity and makes the universe expand at a faster pace.
Dark Matter
Matter that is too dim to be detected by telescopes. Astronomers infer its existence by measuring its gravitational influence. Dark matter makes up most of the total mass of the universe.
Dwarf Galaxy
A relatively small galaxy. The Large and Small Magellanic Clouds, visible in the Southern Hemisphere, are two dwarf irregular galaxies that are neighbors of the Milky Way.
Dwarf Planet
A celestial body within the solar system that shares the characteristics of planets. It orbits the Sun, is not a moon, and has a spherical or nearly spherical shape. Unlike a planet, however, a dwarf planet has not cleared away any loose cosmic rubble from its orbit. Dwarf planets include Ceres, Pluto, and Eris.
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Earth
The third planet from the Sun and one of four terrestrial planets in the inner solar system. Earth, the only planet where water exists in large quantities, has an atmosphere capable of supporting myriad life forms. The planet is 150 million kilometers (93 million miles) away from the Sun. Earth has one satellite, "the Moon."
Elliptical Galaxy
A galaxy that appears spherical or football-shaped. Elliptical galaxies are comprised mostly of old stars and contain very little dust and "cool" gas that can form stars.
Galaxy Cluster
A collection of dozens to thousands of galaxies bound together by gravity.
Galaxy Evolution
The study of the birth of galaxies and how they change and develop over time.
Globular Cluster
A collection of hundreds of thousands of old stars held together by gravity. Globular clusters are usually spherically shaped and are often found in the halos of galaxies. Each star belonging to a cluster revolves around the cluster's common center of mass.
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Irregular Galaxy
A galaxy that appears disorganized and disordered, without a distinct spiral or elliptical shape. Irregular galaxies are usually rich in interstellar matter, such as dust and gas. The Large and Small Magellanic Clouds are examples of nearby irregular galaxies.
Jupiter
The fifth planet from the Sun and the largest planet in our solar system, twice as massive as all the other planets combined. Jupiter is a gaseous planet with a very faint ring system. Four large moons and numerous smaller moons orbit the planet. Jupiter is more than five times the Earth’s distance from the Sun. It completes an orbit around the Sun in about 12 Earth years.
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Magellanic Clouds
Two dwarf irregular galaxies known as the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The galaxies are in the Local Group. The closer LMC is 168,000 light-years from Earth. Both galaxies can be observed with the naked eye in the southern night sky.
Mars
The fourth planet in the solar system and the last member of the hard, rocky planets (the inner or terrestrial planets) that orbit close to the Sun. The planet has a thin atmosphere, volcanoes, and numerous valleys. Mars has two moons: Deimos and Phobos.
Moon
A large body orbiting a planet. On Earth’s only moon, scientists have not detected life, water, or oxygen on this heavily cratered body. The Moon orbits our planet in about 28 days.
Nebula
A cloud of gas and dust located between stars and/or surrounding stars. Nebulae are often places where stars form.
Neptune
The eighth planet and the most distant giant gaseous planet in our solar system. The planet is 30 times the Earth’s distance from the Sun, and each orbit takes 165 Earth years. Neptune is the fourth largest planet and has at least eight moons, the largest of which is Triton. Neptune has a ring system, just like all the giant gaseous outer planets.
Neutron Star
An extremely compact ball of neutrons created from the central core of a star that collapsed under gravity during a supernova explosion. Neutron stars are extremely dense; they are only 10 kilometers or so in size, but have the mass of an average star (usually about 1.5 times more massive than our Sun). A neutron star that regularly emits pulses of radiation is known as a pulsar.
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Open Cluster
Also known as a galactic cluster, an open cluster consists of numerous young stars that formed at the same time within a large cloud of interstellar dust and gas. Open clusters are located in the spiral arms or the disks of galaxies. The Pleiades is an example of an open cluster.
Planet
An object that orbits a star. Although smaller than stars, planets are relatively large and shine only by reflected light. Planets are made up mostly of rock or gas, with a small, solid core. In our solar system, the inner planets (Mercury, Venus, Earth, and Mars) are the rocky objects, and the outer planets (Jupiter, Saturn, Uranus, and Neptune) are the gaseous ones.
Planetary Nebula
An expanding shell of glowing gas expelled by a star late in its life. Our Sun will create a planetary nebula at the end of its life.
Pluto
A dwarf planet whose small size and composition of ice and rock resembles the comets in the Kuiper Belt, a region beyond Neptune's orbit where Pluto resides. Pluto was considered the ninth planet until August 2006, when the International Astronomical Union reclassified it as a dwarf planet. Pluto's orbit is more elliptical than those of the eight solar system planets.
Pulsar
A neutron star that emits rapid and periodic pulses of radiation.
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Saturn
The sixth planet in the solar system, noted for its obvious ring structure. Saturn is almost ten times the Earth's distance from the Sun. The planet completes a circuit around the Sun in about 30 Earth years. Saturn is the second largest and the least dense planet in our solar system. The planet has more than 21 moons, including Titan, the second largest known moon in our solar system.
Solar System
The Sun and its surrounding matter, including asteroids, comets, planets, and moons, held together by the Sun's gravitational influence.
Spectroscopy
The study and interpretation of a celestial object’s electromagnetic spectrum. A spectrum breaks light into its component wavelengths and reveals clues to the object’s state, temperature, speed, quantity, distance, and composition.
Spiral Galaxy
A spiral-shaped system of stars, dust, and gas clouds. A typical spiral galaxy has a spherical central bulge of older stars surrounded by a flattened galactic disk that contains a spiral pattern of young, hot stars, as well as interstellar matter.
Star
A huge ball of gas held together by gravity. The central core of a star is extremely hot and produces energy. Some of this energy is released as visible light, which makes the star glow. Stars come in different sizes, colors, and temperatures. Our Sun, the center of our solar system, is a yellow star of average temperature and size.
Starburst Galaxy
A galaxy undergoing an extremely high rate of star formation. Starburst galaxies contain massive, deeply embedded stars that are among the youngest stars observed.
Star Cluster
A group of stars born at almost the same time and place, capable of remaining together for billions of years because of their mutual gravitational attraction.
Supernova
The explosive death of a massive star whose energy output causes its expanding gases to glow brightly for weeks or months. A supernova remnant is the glowing, expanding, gaseous remains of a supernova explosion.
Supernova Remnant
The glowing, expanding, gaseous remains of a supernova explosion.
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Telescope
An instrument used to observe distant objects by collecting and focusing their electromagnetic radiation. Telescopes are usually designed to collect light in a specific wavelength range. Examples include optical telescopes that observe visible light and radio telescopes that detect radio waves.
Universe
The totality of space and time, along with all the matter and energy in it. Current theories assert that the universe is expanding and that all its matter and energy was created during the Big Bang.
Uranus
The third largest planet in the solar system and the seventh from the Sun. Uranus is 19 times the Earth's distance from the Sun and completes a circuit around the Sun in about 84 Earth years. This gaseous, giant, outer planet has a visible ring system and over 20 moons, the largest of which is Titania. Uranus is tipped on its side, with a rotation axis in nearly the same plane as its orbit.
Variable Star
A star whose luminosity (brightness) changes with time.
Venus
An inner, terrestrial (rocky) planet that is slightly smaller than Earth. Located between the orbits of Mercury and Earth, Venus has a very thick atmosphere that is covered by a layer of clouds that produces a "greenhouse effect" on the planet. Venus' surface temperature is roughly 480°C (900°F), making it the hottest planet in the solar system.