Question

• Consider a star cluster that starts out with main sequence stars of spectral type O, B, A, F, G, K, M.
• Explain which spectral types will be present when the cluster is 10⁸ years old.
• Explain which will be present when it is 10¹⁰ years old. (Note: Review the slide in the "Stars" lecture that shows the lifetimes of stars on different parts of the HR diagram to help you answer this question)

Answer 1

Both these are answered with reference to the HR diagram for stars.

Given that all the stars in the cluster are main sequence stars.

Now if we take a look at the HR diagram, we see that clusters belonging to the main sequence region and which are aged around 108 years, the following spectral types are present - since a 108 year old cluster is still a young cluster, therefore, the spectral types of G, K and M are majorly present at that age, with a few of F type stars.

The older clusters belong to the topmost level of the main sequence region. Here only the stars with lowest masses survive. So for a 1010 years old cluster, it is suggestive that the cluster will lie somewhere near the topmost level of the sequence. So, the spectral types of B and A are dominant with a few traces of O and F types of stars.