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Unformatted text preview: ASTR 100 Midterm 3 Version 1 Solutions April 19, 2006 Multiple Choice, 2 pts each. 1. B 6. A 11. B 16. A 21. B 2. A 7. B 12. B 17. D 22. C 3. D 8. C 13. A 18. C 23. D 4. A 9. D 14. B 19. D 24. C 5. C 10. D 15. A 20. A 25. D Short Answer, 10 pts each. 1. The nearest star to us is actually a triple star system (one star orbits a binary star). One of the stars in the system, Alpha Centauri A, has a luminosity of 1.6 times the luminosity of the Sun. Another star in the system, Alpha Centauri B, has a luminosity of 0.5 times the luminosity of the Sun. What is the ratio of their apparent brightnesses, and which one is brighter? (Make sure to show your work, and give your answer as a decimal.) In order to do this calculation, you had to make an assumption. What was it? We need to use the inverse-square law for light to solve this problem. The equation is b = L 4 πd 2 b is the apparent brightness of a star, L is the star’s luminosity, and d is the distance to the star. In this problem, we know the luminosities of the stars. We need to find the apparent bright- nesses. But how can we use the equation without the distances to the stars? Read the problem carefully: the stars are in a triple star system. Since they are all orbiting each other, they can’t be too far apart from each other. That means they are all approximately the same distance away from us. So our assumption is that the distance to each star is the same. Now we can find the ratio of the apparent brightnesses of the stars. First, write the equation for both stars – I will denote the stars as A and B, according to the end of their names....
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- Spring '06
- Astronomy, L Sun