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Unformatted text preview: Doppler-velocity measurements tell us we live in a spiral galaxy 5. Orbits of stars near the center of our galaxy indicate that it contains a black hole with 4 million times the mass of the Sun 6. Best hypothesis for how galaxies form: 1. A giant cloud of H and He condensed after the Big Bang 2. Halo stars (oldest) formed first, in clumps, as gravity caused cloud to contract. These clumps later merged. 3. Bulge stars form next 4. Remaining gas settled into spinning disk 5. Star formation continues as long as there is material in the ISM (interstellar medium) (stars continuously form in disk as galaxy grows older)...
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- The Milky Way