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The Question

(Submitted October 29, 1997)

I teach astronomy, and a student once asked me which there are more of in our galaxy -- Pop. I stars or Pop. II stars. I wasn't sure! Do you know? More specifically, what is the ratio of the number of Pop. I stars to the number of Pop. II stars in the galaxy?

The Answer

According to 'Galactic Dynamics' by Tremaine and Binney (1987: Princeton) the ratio of luminosities of the spheroid population to the disk population is 1/30. In order to translate to numbers of stars, we can use the fact that the mass/light ratio of the spheroid is greater than that for the disk (they suggest values of 5 and 12, respectively). Also, the mass distribution of disk stars has a large contribution from the more massive stars, which are not present in the halo. Taking all these things into account is tricky, and the best I can do is to suggest that the disk population probably wins, but not by a large factor (less than 10).

I hope this helps!

Tim Kallman
for the Ask an Astrophysicist Team

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