Star cluster

Star clusters are groups of stars which are gravitationally bound.Two distinct types of star cluster can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars, while open clusters generally contain less than a few hundred members, and are often very young.Supergiant — Supergiants are the most massive stars. Supergiants can have masses from 10 to 70 solar masses and brightness from 30,000 up to hundreds of thousands . Blue supergiant star — Blue supergiants are supergiant stars (class I) of spectral type O. They are extremely hot and bright, with surface temperatures of between 20,000 .Open cluster — An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally . Galaxy formation and evolution — The formation of galaxies is still one of the most active research areas in astrophysics; and, to some extent, this is also true for galaxy..
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from:http://www.sciencedaily.com