Stars are classified by their spectra (the elements that they absorb) and their temperature. There are seven main types of stars. In order of decreasing temperature, O, B, A, F, G, K, and M. Type O stars are the stars with the highest temperature and biggest luminosity. So, if there is a new born star cluster : 1. with one O star,
2. with 10 A stars,
3. with 10 G stars,
4. with 1000 M stars
the stellar type O will dominate the light output from the cluster.