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dark matter type of matter that has been detected only by its gravitational effects on ordinary matter dark nebulae nebulae that con be observed when they block the light from more distant stars or nebulae galactic disk an immense, circular, flattened region containing most of our galaxy's luminous stars and interstellar matter HII region a cloud of ionized hydrogen formed when young stars heat the surrounding gas interstellar matter the gas and dust located between the stars in a galaxy kiloparsec A distance equal to 1000 parsecs. reflection nebula An interstellar cloud in which the dust particles reflect starlight, making the cloud visible. Sgr A It is a complex radio source at the center of the Milky Way Galaxy. It stands for Sagittarius A. spiral arm Long, spiral pattern of bright stars, star clusters, gas and dust that extends from the center to the edge of the disk of spiral galaxies