Astronomy 122&, Fall 2009

Homework E Answers

Part I: Multiple choice. One point each.

1. A 2nd magnitude star is how many times brighter than a 12th magnitude star?
d) 10,000 (2 sets of 5 magnitudes, 100X100)

2. The absolute magnitude of a star is what the apparent magnitude would be at how many parsecs?
b) 10

3. The primary distances to the stars are found by
e) parallax

4. The parallax of a star is 0.02 seconds of arc. What is its distance?
c) 50 pc (Distance = 1/p)

5. What characterizes the spectrum of an A star?
b) strong hydrogen

6. What characterizes the spectrum of an M star?
a) titanium oxide

7. The Sun has what absolute magnitude and is of what spectral class?
c) +5, G2

8. A class F dwarf star (ignoring the core) is typically made mostly of
e) hydrogen

9. The standard spectral sequence is a result of
a) different stellar temperatures

10. The most luminous and hottest dwarf stars below are of class
a) O

11. The most common stars are
e) M, L, and T dwarfs

12. White dwarfs are typically about the size of
c) Earth

13. We know that supergiants are really larger than giants because supergiants
b) are brighter than giants that have the same temperatures

14. Stellar masses are found from
b) double stars

15. As you go down the main sequence from class A, stellar masses
b) decrease

16. What is the minimum mass for a star that will run full hydrogen fusion (in solar units)?
c) 0.08

17. The masses of white dwarfs are similar to that of
d) the Sun

18. The coolest and brightest M supergiants are about at big as the orbit of
b) Earth
d) Jupiter, even approaching that of Saturn

19. Brown dwarfs are
a) stars below the proton-proton chain mass cutoff

20. The spectra of class T stars are characterized by
a) methane

Part II

1. (10 points)

Star	Proper Name	Class	Spectrum lines	Temperature (K)
Alpha Lyrae	Vega	A0 V	hydrogen, ionized metals	9500
Alpha Cygni	Deneb	A2 Ia	hydrogen, ionized metals	9000
Alpha Orionis	Betelgeuse	M2 Ia	TiO, neutral and ionized Ca, neutral metals	3500
Alpha Bootis	Arcturus	K1 III	Ionized calcium, neutral metals	4500

2. (5 points)If all the values are in solar units, L = (R**2)(T**4) = (5**2)(60,000/6000)**4 = 25 X 10**4 = 250,000 solar luminosities.

3. (5 points) If all values are in solar units, R**2 =L/(T**4)=(1/10,000)/(3000/6000)**4 = (1/10,000)/(1/2**4) = (1/10,000)/(1/16) = 16/10,000. R is the square root of 16/10,000 = 4/100 = 0.04 times the radius of the Sun.

4. (10 points) The HR diagram will be discussed in class. Note that the stars from the two tables fall in very different places. The nearest stars are mostly red dwarfs, whereas the visually brightest stars are A and B dwarfs and K and M giants and/or supergiants. The red dwarfs, though very common, are just too faint to be seen, which is an example of observational selection.