Monster of the Milky Way

Catalogue of the Cosmos

Black holes are just one of myriad marvels in outer space. In this feature, learn the difference between pulsars and quasars, antimatter and dark matter, brown dwarfs and white dwarfs, and many other extraterrestrial wonders.


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—A small rocky or metallic body that orbits a star.

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.

Binary star system—Two stars orbiting their common center of gravity.

Black hole—A region in space where gravity is so strong that space closes back on itself, allowing nothing, not even light, to escape.

Blue straggler—A star that forms in a globular cluster from the collision or merger of two stars. It is hotter and bluer than its non-coalesced counterparts.

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.

Comet—A small solar system body made of ice and dust that moves in an elliptical orbit around the sun. A typical comet has a solid nucleus a few kilometers in diameter. When it nears the inner solar system, ices evaporate and form an extended and diffuse atmosphere, which is blown away from the sun by the solar wind and radiation pressure to form a prominent tail of gas and dust.

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.

Elliptical galaxy—A galaxy that appears round or elliptical and lacks a disk with spiral arms. Such galaxies have little dust and gas, and show few signs of ongoing star formation.

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

Galaxy—A massive, gravitationally bound assembly of stars, interstellar clouds, and dark matter.

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

Gamma-ray burst—Sudden, unpredictable flashes of high-energy photons coming from space.

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.

Globular cluster—A dense spherical cluster of hundreds of thousands of stars bound together by gravity.

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.

Magellanic clouds—Two irregular satellite galaxies of the Milky Way, visible even to the naked eye in the southern skies.

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.

Milky Way—The faint band of light stretching across the sky, due to myriad faint stars and nebulas; the name of the spiral galaxy containing our solar system.

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.

Nebula—An immense, cloud-like mass of interstellar gas and dust, generally in the spiral arms of a galaxy.

Neutron star—An extremely dense collapsed star consisting mainly of neutrons. A neutron star is what often remains after the supernova explosion of a massive star.

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.

Orion nebula—A large interstellar cloud of gas and dust giving birth to stars in the constellation of Orion, about 1,500 light-years away.

Planet—An astronomical body with enough mass for its gravity to make it spherical but not enough to generate nuclear energy. Planets have non-intersecting orbits around stars or drift freely in space.

Planetary nebula—An expanding shell of luminous gas that surrounds a small white dwarf star. The ionized shell receives ultraviolet light from the hot white dwarf and re-emits it as colorful visible light by fluorescence. (Planetary nebula have nothing to do with planets; the term is a historical misnomer.)

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.

Quasar—The highly luminous core of a remote galaxy, thought to be powered by a supermassive black hole. Quasars look like stars on an ordinary photograph but have very different spectra.

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).

Spiral galaxy—A system of billions of stars with a central bulge of older stars surrounded by a flat disk with spiral arms of gas and dust nebulas and young stars. The Milky Way is a spiral galaxy.

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.

Supernova—The catastrophic explosion of a star, which blows off most of its mass, increasing in brightness by as much as a billion times. A Type I supernova is due to the thermonuclear detonation of a compact white dwarf star that becomes unstable by accreting mass from an orbiting companion star. A Type II supernova results from the gravitational collapse of a massive star that has exhausted its nuclear fuel.

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.

White dwarf—The hot, collapsed core of a red giant star after it has expelled its outer layers and ceased to produce energy by fusion. A white dwarf has a mass comparable to that of the sun but is no larger than the Earth.

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Asteroid 951 Gaspra

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Black hole

Galaxies possibly containing black holes, which can appear bright as they suck in all nearby light.

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Elliptical galaxies

An elliptical concentration of stars in globular cluster M15

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Gamma-ray burst

GRB 971214 field, about four months after a gamma-ray burst, as seen by the Hubble Space Telescope

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Globular cluster

Located about 28,000 light-years from Earth, M-80 (NGC 6093) is one of the densest of 147 known globular clusters in the Milky Way.

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Magellanic clouds

A Hubble Space Telescope image of supernova 1987A, in the Large Magellanic Cloud

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Planetary nebula

Hubble Space Telescope image of planetary nebula NGC 6543, the so-called "Cat's Eye Nebula"

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White dwarf

White dwarf stars in globular cluster M4 as seen by Hubble Space Telescope

Glossary excerpted with permission from Cosmic Horizons: Astronomy at the Cutting Edge, edited by Steven Soter and Neil deGrasse Tyson, New York: The New Press, 2001. Definition of gamma-ray burst courtesy of Dr. Jerry Bonnell, Goddard Space Flight Center.

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