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Study Notes for Chapter 30: Stars, Galaxies, and the Universe Directions: Use the following to complete your notes to prepare for your test. 1. A galaxy is made of stars, dust, and gas bound together by gravity. 2. A large, bright star whose hot core has used most of its hydrogen is a nova. 3. A light-year is the distance light travels in a year. 4. A small, hot, extremely dense core left after a star collapses is a white dwarf. 5. A star moving away from the Earth has a spectrum that is shifted toward red. 6. A star with the sun’s mass would stay on the main sequence of the H-R diagram for about 10 billion years. 7. A white dwarf is a hot, extremely dense core of matter left after a star collapses. 8. About 6,000 stars are visible from Earth without a telescope. 9. Absolute magnitude is the true brightness of a star. 10. After its temperature rises to 10,000,000°C, a protostar becomes a star when nuclear fusion begins. 11. Apparent magnitude is the brightness of a star as it appears from Earth. 12. Astronomers believe that cosmic background radiation formed shortly after the big bang. 13. By analyzing the light that a star emits, astronomers can determine the composition and temperature of a star. 14. During the main sequence stage, hydrogen fuses into helium to generate energy in a star’s core. 15. Hubble’s discovery that there was red shift in the spectra of galaxies lead to an understanding that the universe is expanding. 16. Many astronomers believe that black dwarfs do not exist. 17. Most of the universe is made of dark energy and dark matter. 18. Red shift in the spectra of galaxies told Hubble that the universe is expanding. 19. Scientists determine the composition and temperature of stars by analyzing the spectra of the light that stars emit. 20. Stars appear to move in the sky because Earth is moving. 21. The big bang occurred about 14 billion years ago. 22. The coolest stars are red in color, the hottest stars are blue.