Astr 2100 Descriptive Astronomy
Samples Questions for Test 3


  1. What is a binary star system? Are binary stars rare or common? Two stars orbiting around each other, mutually attracted by the force of gravity. They are quite common, perhaps more than 50% of the stars in the sky.
  2. What process is responsible for generating energy in the centre of stars? Nuclear fusion
  3. Which two properties of stars are measured by the Hertzsprung Russell diagram? Temperature (or star class) and luminosity (or absolute magnitude)
  4. What property of stars can be deduced by measuring their colour? Surface tenperature
  5. How many hydrogen nuclei (protons) are destroyed in making one helium nucleus? 4
  6. What are sunspots? Regions of the Sun's surface which are cooler than their surroundings
  7. What is the name of the region of the Sun which we can see? Photosphere
  8. Large sheets of gas are sometimes seen escaping from the surface of the Sun and penetrating into (and even beyond) the atmosphere. These are known as Prominences
  9. What colour are the most massive main sequence stars? Blue
  10. How do we measure the distance to nearest stars? Stellar Parallax
  11. Our own Sun is currently a main sequence star
  12. Which two processes are responsible for transporting energy from the inside of our Sun to the outside? radiation and convection
  13. If star A has a relative magnitude of 3.2 and star B has a relative magnitude of 2.6, which one is the brighter? The one with the smaller magnitude, that is the one with a magnitude of 2.6
  14. What two other quantities do you need to know in order to determine the luminosity of a star? Relative magnitude (or intensity) and distance
  15. How is the radial speed of a star measured? Doppler Effect
  16. Where does the energy of the Sun come from? Nuclear fusion of hydrogen within its core
  17. Because of nuclear fusion, what happens to the mass of the Sun? Slowly decreases over time
  18. If a star is 50 parsecs away, what is its parallax angle? 0.02 arc seconds
  19. If the parallax angle for a star is 0.017 arc seconds, how far away is it (in parsecs)? 58.9 pc
  20. How many light years are there in one parsec? 3.26
  21. If the absolute magnitude and relative magnitude of a star are equal, how far away is it? 10 pc
  22. If a star is 100 pc away, how far is this in light years? 326
  23. How is it that a cool red star such as Aldebaran (the eye of the bull, in the constellation Taurus) is so bright? It is very large
  24. What (approximately) is the surface temperature of the Sun? 6000 K
  25. What (approximately) is the core temperature of the Sun? 25,000,000 K
  26. What (approximately) is the power output of the Sun, in watts? 4 x 1026 W
  27. Approximately, what is the diameter of the Sun in miles? 800,000 (or about 100x the diameter of the Earth)
  28. Why should you not look directly at the Sun? It is too bright, you can damage your retina.
  29. How far is the Sun from the Earth? 1 AU = 93,000,000 miles = 1.5 x 1011 m.
  30. Arcturus is 100 times as luminous as the Sun, but its surface temperature is only ½ that of the Sun. What is it radius?
    1. P = r2 T4
    2. 100 = r2 (½)4 = r2 / 16
    3. r2 = 16 * 100 = 1600
    4. r = 40 Earth radii
  31. If a star is twice the size of the Sun, but has a surface temperature of only 3000 K, what will be its luminosity (relative to that of the Sun)?
    1. T = 3000 K is about ½ the temperature of the Sun
    2. P = r2 T4
    3. P = 22 (½)4 = 0.25 the luminosity of the Sun.
  32. Stars A and B have the same relative magnitude. If star A has a luminosity equal to four times that of star B, and is 10 lyr away, how far away is star B?
    1. With the same realtive magnitude they appear to us to have equal intensity
    2. I = PA/dA2 = PB/dB2
    3. (4PB)/102 = PB/dB2
    4. dB2 = 102 /4 = 25
    5. dB = 5 lyr
  33. What is the distance (in light years) to a star whose parallax angle is 0.0033 arc seconds? 300 pc = 980 lyr
  34. A star has a surface temerature of 4500 K. If its relative magnitude is 7.7, how far away is it?
    1. From HR diagram M is about 5.5
    2. M - m = 5 - 5 log10(d)
    3. 5 log10(d) = 5 + m - M = 5 + 7.7 - 5.5 = 7.2
    4. log10(d) = 7.7 /5 = 1.44
    5. d = 101.44 = 27.5 pc
  35. A type I supernova has a relative magntiude of 12.0. How far away is it?
    1. All type I supernovae have the same M = -19.3
    2. M - m = 5 - 5 log10(d)
    3. 5 log10(d) = 5 + m - M = 5 + 12 - (-19.3) = 36.1
    4. log10(d) = 36.1 /5 = 7.22
    5. d = 107.22 = 1.7 x 107 pc (=17 Mpc)
  36. Light from a star has a measured wavelength of 500.17 nm. If the same light is measured in the lab its wavelelngth is only 500 nm.
    1. Is it moving towards us or away from us? Since the light is red shifted it is moving away
    2. What is its speed relative to us?
      1. Wavelength change = 500.17 - 500 = 0.17 nm
      2. speed = (0.17/500) * 3 x 108 = 102,000 m/s
  37. Light from a distant galaxy has a measured wavelength of 720 nm. If the same light is measured in the lab its wavelelngth is only 656 nm.What is its speed relative to us?
    1. Wavelength change = 720 - 656 = 64 nm
    2. speed = (64/656) * 3 x 108 = 2.9 x 107 m/s
  38. A pair of binary stars orbit each other at a distance of 8 AU, taking 6 years to orbit around each other. If one star has three time the mass of the other, what are their individual masses?
    1. M1 + M2 = a3 / P2 = 83 / 62 = 14.2
    2. Set M1 = 3 M2
    3. 3M2 + M2 = 14.2
    4. M2 = 14.2 / 4 = 3.6 solar masses
    5. M1 = 3 * M2 = 10.7 solar masses

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