Sections Review

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Vocabulary

Formulae

• Inverse square law (standard candle): distance to the standard candle = (calibration distance) × Sqrt[(calibration brightness)/(apparent brightness)]. Calibration distance is either 10 parsecs or it is the distance to a similar object at a known distance.
• Orbital speed = Sqrt[(G × (enclosed mass))/(star's distance from the galactic center)], where the enclosed mass is the mass of the galaxy inside the star's orbit, and G is the gravity constant.
• Enclosed mass = (orbital speed)² × (star's distance from the galactic center) / G, where G is the gravity constant.

Review Questions

1. What is the name for our galaxy and what kind of galaxy is it?
2. How big is our galaxy? How many stars are in it and how do we know?
3. How are Cepheids and RR-Lyrae stars considered to be standard candles? How can you find their luminosity?
4. How can you use the period-luminosity relation to find distances?
5. Why do variable stars like Cepheids, RR-Lyrae stars, and Mira variables vary in brightness?
6. Where are we in the galaxy and how do you know? How can the distribution of globular clusters tell you about our place in the Galaxy?
7. What are the four basic components of our galaxy? Where would old stars be found? Where would stars with very small amounts of ``metals'' (elements heavier than helium) be found? Where are new stars being formed? Where would stars enriched with ``metals'' be found?
8. If you could analyze the spectra of 10 stars every second, how many years would it take you to check every star inside the Sun's orbit that is in our half of the Galaxy? [Hint: used the ``enclosed mass'' graph, divide by 2, and our Sun's distance from the center = 27,000 light years (8,400 parsecs).]
9. What are the theories for how spiral arms are formed and maintained? What are the verifiable predictions made by these theories?
10. How does the density wave theory explain why stars form in spiral arms? Also, contrast it with the explanation given by the self-propagating star formation theory.
11. How do astronomers know the dark matter halo (corona) exists if it does not radiate anything our telescopes can detect?
12. Scientists are advocating a focused search for extra-terrestrial intelligence that looks at stars relatively abundant in elements heavier than helium because those are the elements from which life could possibly form. What part(s) of the Galaxy would such stars be found and what are the distinguishing orbital characteristics of such stars?
13. If very old stars tend to be metal-poor, how could you explain the presence of recently-formed stars that are metal poor?
14. How do astronomers know that there is a very massive black hole at the center of the Galaxy?
15. If there were no black hole in the center of the Galaxy, how would the orbits of the stars near the Sun be affected? (Hint: compare the mass of the black hole to the total mass inside the Sun's orbit [black hole mass/total enclosed mass]---would the gravity change significantly?)

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last updated: June 7, 2019