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dark matter  Matter that cannot be detected or has not yet been detected by the radiation it emits. The presence of dark matter can be deduced from its gravitational interaction with other bodies.
dark nebula  A dense, interstellar cloud containing enough gas and dust to block the light of background stars. The dimming of background stars gives the appearance of a region with no stars.
density wave theory  A theory to account for the spiral arms of spiral galaxies. According to the density wave theory, spiral arms are the crests of waves moving through a galaxy like water waves move through water.
extinction  The dimming of starlight due to absorption and scatteringby interstellar dust particles.
galactic bulge  A somewhat flattened distribution of stars, about 6 kiloparsecs in diameter, surrounding the nucleus of the Milky Way.
galactic disk  A disk of matter, about 30 kiloparsecs in diameter and 2 kiloparsecs thick, containing most of the stars and interstellar matter in the Milky Way.
galactic equator  The great circle around the sky that corresponds approximately to the center of the glowing band of the Milky Way.
galactic halo  The roughly spherical outermost component of the Milky Way, reaching to at least 30 to 40 kiloparsecs from the center.
galactic latitude  The angular distance of a body above or below the galactic equator.
galactic longitude  The angular distance, measured eastward around the galactic equator, from the galactic center to the point on the equator nearest the direction to a body.
galactic nucleus  The central region of the Milky Way.
globular cluster  A tightly packed, spherically shaped group of thousands to millions of old stars.
HII region  A region of ionized hydrogen surrounding a hot star.
interstellar reddening  The obscuration, by interstellar dust particles, of blue starlight more strongly than red starlight.
kiloparsec (kpc)  A unit of distance, equal to 1000 parsecs (pc), often used to describe distances within the Milky Way or the Local Group of galaxies.
microlensing event  The temporary brightening of a distant object that occurs because its light is focused on the Earth by the gravitational lensing of a nearer body.
reflection nebula  A cloud of interstellar gas and dust that is luminous because the dust it contains reflects the light of a nearby star.
rotation curve  A plot of the speed of revolution of the stars and gas in a galaxy versus distance from the center of the galaxy.
scattering  The redirection of light in random directions when it strikes atoms, molecules, or solid particles.
Sgr A  A small, bright source of radio emission, possibly the accretion disk of a black hole, that probably marks the exact center of the Milky Way.
stellar population  A group of stars that are similar in spatial distribution, chemical composition, and age.

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