You observed three different star clusters and found that the main-sequence turnoff stars in cluster 1 had spectral type A,the main-sequence turnoff stars in cluster 2 had spectral type B,and the main-sequence turnoff stars in cluster 3 had spectral type G.Which star cluster is the oldest?

Question

Quizplus · Accepted Answer

The spectral type of the main-sequence turnoff stars is an indicator of their surface temperature and hence their age. The hotter the surface temperature, the younger the star cluster is. So, cluster 1 with spectral type A has the youngest turnoff stars, followed by cluster 2 with spectral type B, and cluster 3 with spectral type G has the oldest turnoff stars. Therefore, cluster 3 is the oldest.

You Observed Three Different Star Clusters and Found That the Main-Sequence