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Key Terms

Astronomy 2eKey Terms

central bulge: (or nuclear bulge) the central (round) part of the Milky Way or a similar galaxy

dark matter: nonluminous mass, whose presence can be inferred only because of its gravitational influence on luminous matter; the composition of the dark matter is not known

dark matter halo: the mass in the Milky Way that extends well beyond the boundary of the luminous stars to a distance of at least 200,000 light-years from the center of the Galaxy; although we deduce its existence from its gravity, the composition of this matter remains a mystery

differential galactic rotation: the idea that different parts of the Galaxy turn at different rates, since the parts of the Galaxy follow Kepler’s third law: more distant objects take longer to complete one full orbit around the center of the Galaxy

halo: the outermost extent of our Galaxy (or another galaxy), containing a sparse distribution of stars and globular clusters in a more or less spherical distribution

Milky Way Galaxy: the band of light encircling the sky, which is due to the many stars and diffuse nebulae lying near the plane of the Milky Way Galaxy

population I star: a star containing heavy elements; typically young and found in the disk

population II star: a star with very low abundance of heavy elements; found throughout the Galaxy

spiral arm: a spiral-shaped region, characterized by relatively dense interstellar material and young stars, that is observed in the disks of spiral galaxies

supermassive black hole: the object in the center of most large galaxies that is so massive and compact that light cannot escape from it; the Milky Way’s supermassive black hole contains 4.6 millions of Suns’ worth of mass

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- Authors: Andrew Fraknoi, David Morrison, Sidney Wolff
- Publisher/website: OpenStax
- Book title: Astronomy 2e
- Publication date: Mar 9, 2022
- Location: Houston, Texas
- Book URL: https://openstax.org/books/astronomy-2e/pages/1-introduction
- Section URL: https://openstax.org/books/astronomy-2e/pages/25-key-terms

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