A star the size of Austin is called a d. neutron star 47. We know the Sun is evolving by the simple observation that c. it is losing energy to space 48. A Hertzsprung-Russell diagram is a plot of the luminosity of a star versus its c. temperature 49. Most stars are observed to be on the Main Sequence because c. they spend the longest time there 50. The apparent brightness of a star depends on its luminosity and b. distance 51.
B stars show relatively weak optical spectral lines of hydrogen because b.
G stars show relatively weak optical spectral lines of hydrogen because c. the star is so cool that few hydrogen atoms are in the first excited state 53. The band containing most stars in a plot of the luminosity and temperature of the stars is called b. the main sequence 54. A star with very low luminosity and very high temperature is called c. a white dwarf 55. A star with very high luminosity and low temperature is called b. a red giant 56. Compared to low mass stars, high mass stars c. live a shorter time 57.