H-R Diagram of the Brightest and Nearest Stars

This illustration uses lists of the brightest stars (diamonds) and the closest stars (circles).

Compare the general location of the nearest stars on the HR-Diagram to the general location of the brightest stars, and answer the following:

1. What does your comparison say about the luminosities and spectral types (temperatures) of stars in the solar neighborhood, that is, the stars that are closest to the Sun?
   
   
   Answer: Most of the stars are low luminosity, cool, K- and M-type main sequence stars.
   
   
   
2. Remembering that along the main sequence, the cooler the star the lower the mass of the star, what does the comparison say about the masses of the stars in the solar neighborhood?
   
   
   Answer: Most of the stars are low-mass stars.
   
   
   
3. What does your comparison say about the luminosities and spectral types of the brightest stars? Note in your comments some of the distances to the brightest stars as noted from the linked table.
   
   
   Answer: Most of them are very luminous and some of them are over 1000 light years away. Their spectral types are BAFGKM - all but O-type stars in this diagram.
   
   
   
4. Had we included the next 10 nearest stars (see Appendix 8 of your text), you would have noted that most of them were main sequence, spectral type M stars. In general, what might this observation say about the numbers of low mass stars in the Galaxy?
   
   
   Answer: That there are lots of them. Note that we would have a hard time actually counting all of these main sequence M-type stars because their luminosities are so low.
   
   
   
5. Note that although there is a number of spectral type B stars, there are no spectral type O stars listed. Generalize this observation to the possible numbers of O-type stars in the Galaxy.
   
   
   Answer: If our neighbor represents most neighborhoods, there are not very many of them.
   
   
   
6. M-type stars live for over 30 billion years on the main sequence and O-type stars live for under 5 million years on the main sequence. Considering these lifetimes, do the numbers of observed stars of each type make sense?
   
   
   Answer: Yes, because once an M-type star is born, it lives for a very, very long time—it sticks around. The O-type stars live for such a short time that their numbers don't build up. (Imagine a world where elephants lived for only four years, and there was only 1 baby elephant born each year. Rabbits in this world, on the other hand, live to be 60 years old, and 10 baby rabbits are born each year. You observe this world through your telescope, what will you see? Lots of rabbits, and occasionally an elephant. Galaxies like our Milky Way seem to produce more low-mass stars than high-mass stars, and the low-mass stars live around 10,000 times longer than the high-mass stars.)