There is a main-sequence mass-luminosity relation because

There is a main-sequence mass-luminosity relation because
 
  a. helium fusion produces carbon.
  b. stars expand when they become giants.
  c. more massive stars support their larger weight by making more energy.
  d. the helium flash occurs in degenerate matter.
  e. all stars on the main sequence have about the same radius.

Question 2

The mass of the black hole at the center of our galaxy can be estimated using the ____ of a star orbiting it.
 
  a. semi-major axis
  b. orbital period
  c. semi-major axis and orbital period
  d. neither semi-major axis nor orbital period

Question 3

The H-R diagram main-sequence of stars has a limit at the lower-mass or energy-output end because
 
  a. low-mass stars form from the interstellar medium very rarely.
  b. low-mass objects are composed primarily of solids, not gases.
  c. pressure does not depend on temperature in degenerate matter.
  d. the lower limit represents when the radius of the star would be zero.
  e. there is a minimum temperature for hydrogen nuclear fusion (the definition of a star).

Question 4

The center of our galaxy lies in the direction of the constellation of
 
  a. Ursa Minor.
  b. Ursa Major.
  c. Sagittarius.
  d. Orion.
  e. Monoceros.

Question 5

Which of the following is NOT evidence of the existence of an interstellar medium?
 
  a. extinction of light from distant stars
  b. very narrow calcium absorption lines in the spectra of O and B stars
  c. reddening of more distant stars
  d. dark cloud silhouetted against background stars
  e. molecular absorption lines in the spectra of cool stars