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Catalogue of the Cosmos—Additional Entries
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Accretion—The collection of material together to form a star, planet, or moon, usually mediated by a rotating disk.

Active galaxy—A galaxy with an unusually strong output of energy, thought to be powered by a supermassive black hole in its core.

Andromeda Galaxy—The nearest large spiral galaxy to the Milky Way, about 2.5 million light years away. Also known as M31.

Antimatter—Matter made of particles with the same mass as the corresponding particles of conventional matter, but with an opposite electrical charge.

Asteroid belt—The region of the solar system where most of the asteroids orbit. It lies between the orbits of Mars and Jupiter.

Big bang—The fiery birth of the observable universe in an explosion of space itself, which occurred at some time between 12 and 15 billion years ago. According to the prevailing theory, the big bang launched the observed expansion of the universe that continues to this day.

Brown dwarf—An astronomical object with mass in the range between a planet and a star (greater than 1.3 percent and less than 8 percent the mass of the sun). Brown dwarfs have a brief phase of weak nuclear fusion of deuterium (heavy hydrogen), but never become hot enough to fuse hydrogen, as do stars.

Circumstellar disk—Dust and gas forming a disk in orbit around a star. Some circumstellar disks may contain planetary systems.

Cosmic background radiation—The microwave energy observed from all directions in the sky, at an equivalent temperature of 2.7 degrees above absolute zero, and interpreted as the residual glow from the big bang.

Cosmic rays—Fast-moving, high-energy, subatomic particles, mainly protons, that permeate the galaxy.

Dark matter—Unseen matter that is detected only by its gravitational pull on visible matter. Most of the universe is evidently made of dark matter. Its nature is yet to be determined.

Dark matter halo—A roughly spherical halo of dark matter that surrounds a galaxy, including the Milky Way, and extends far beyond the region of luminous stars.

Dwarf galaxy—The smallest and most common kind of galaxy.

Galactic disk—The flat disk of a spiral galaxy, which includes young stars and the gas and dust clouds from which they are formed.

Gamma rays—Highly energetic photons, having the shortest wavelengths and the highest frequencies in the [electromagnetic spectrum].

Gas-giant planet—A giant planet with a massive and deep atmosphere that surrounds a relatively small rocky core.

Giant star—A highly luminous star, approaching the end of its life, with an extended, tenuous atmosphere surrounding a hot core depleted in hydrogen.

Gravitational waves—Disturbances or ripples in the fabric of space produced by violent events in the cosmos.

Kuiper Belt—A donut-shaped region of comets in orbit beyond Neptune, assumed to be the oldest surviving remnant of the original solar nebula and the source of short-period comets.

Kuiper Belt objects (KBOs)—The comets that populate the Kuiper Belt.

Local Group—A small group of about two dozen galaxies, including its two largest members, the Milky Way and the Andromeda Galaxy.

Long-period comet—A comet with an orbital period exceeding about 200 years. Such long-period comets have very elongated elliptical orbits, and can have periods of more than a million years. They originate from the Oort cloud in the outermost reaches of our solar system.

Macho (Massive Compact Halo Object)—An unseen stellar or planetary body that may contribute to the dark matter in galaxies.

Meteor—A bright streak of light produced by a small fragment of rock or metal that burns up as it enters the atmosphere.

Meteorite—A fragment of rock or metal that has landed on the Earth from interplanetary space. Most meteorites come from the asteroids, but a few are from other planets or satellites.

Molecular cloud—A large interstellar cloud of gas and dust with temperatures low enough for atoms to combine as molecules. Giant molecular clouds are the main regions of star and planet formation in galaxies.

Oort cloud—A spherical cloud of trillions of comets extending about halfway to the nearest stars and weakly bound by the sun's gravity. Long-period comets originate from the Oort cloud.

Planetesimal—One of the family of asteroid-sized bodies that first condensed out of the disk of the solar nebula and later collided to form the planets.

Plutino—A subclass of Kuiper Belt objects which, like Pluto, orbit the sun twice during every three orbits of Neptune.

Protoplanetary disk—A disk of dust, gas, and perhaps developing planets orbiting a young star. A transitional stage between a solar nebula and a solar system.

Protostar—A gravitationally contracting gas cloud in the early stage of star formation, before fusion begins in its core.

Pulsar—A rapidly spinning neutron star that emits radio energy at regular intervals and is thereby observed on Earth as a pulsating radio source.

Red giant—A large, highly luminous and relatively cool (red) star at a late stage of its life, once it has exhausted its core hydrogen fuel.

Rogue planet—A planet-sized body that escaped its host planetary system and is not gravitationally bound to a star.

Short-period comet—A comet with an orbital period less than about 200 years, the most famous example being Halley's Comet, which appears every 76 years. Short-period comets come from the Kuiper Belt and typically orbit the sun in the same direction as the planets.

Solar nebula—The cloud of gas and dust that formed the young sun and the surrounding planets.

Solar system—The sun and all the objects bound to it by gravity (planets, satellites, asteroids, comets).

Stellar wind—The outflow of charged particles that a star emits into interstellar space.

Supercluster—A congregation of clusters of galaxies.

Supergiant—An extremely luminous star with an extended tenuous atmosphere.

Trajectory—The curving path of a body in motion through space.

T Tauri star—A class of young star with variable luminosity, thought to be in the process of gravitational contraction before its arrival at the main sequence where it will begin to fuse hydrogen into helium.

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