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Stellar Astronomy Unit 3 Key Terms and Matching Definitions _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ 21-cm radiation AGN barred spiral Cepheid variable dark matter Edwin Hubble elliptical galaxy galactic cannibalism galactic merger galaxy galaxy cluster Local Group Milky Way Galaxy nebula redshift Sag A* spiral galaxy 1. Our home galaxy. 2. A cloud of interstellar gas and dust. What early astronomers thought distant galaxies were. 3. Stars that pulsate at a rate proportional to their luminosity. They are used to determine the distance to galaxies outside our own Milky Way. 4. Radiation from hydrogen that has been used to map the structure of the Milky Way. 5. A radio source thought to be at the center of our galaxy. 6. Unseen matter that likely accounts for the rotation of our galaxy and others as though they are solid disks with the inner stars rotating at the same rate as the outer stars. 7. A flattened galaxy reminiscent of a pinwheel. 8. An oval galaxy with no distinguishing spiral arms. 9. A collection of galaxies held together by their mutual gravitational attraction. 10. The collection of approximately 40 galaxies that includes the Milky Way Galaxy. 11. When two similarly-sized galaxies collide and merge together into one larger galaxy. 12. When a small galaxy collides with a large galaxy and is “swallowed up” by the larger one. 13. When spectral lines move to longer wavelengths as a result of a galaxy moving away from us. Used to determining the speed of a receding galaxy. 14. The astronomer who first determined that most galaxies are moving away from us, indicating an expanding universe. 15. An active galactic nuclei where matter is likely spiraling into a central black hole, generating energetic electromagnetic radiation that we can observe. 16. A spiral galaxy with a bar of star through its center. 17. A large collection of stars, nebulae, interstellar gas and dust.