General Astronomy/Star Clusters as Cosmic Laboratories

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It is estimated that the number of the stars in the universe is around 10^{20}. This is calculated based on the fact that there are about 10^9 galaxies in the universe, each of which is composed of an average of 10^{11} stars each.

A nebula is a cloudlike object. Scientists can determine what type of nebula they are observing by examing its spectrum: an emission spectrum indicates glowing gas, while an absorption spectrum indicates a mass of stars. The Pleiades, for instance, has absorption spectrum.

Two kinds of nebulae are open clusters (loose groups of a few hundred stars; e.g. Pleiades, Hyades, Ursa Major) and globular clusters (spherical nebulae with hundreds of thousands of stars, such as M13 in Hercules and M3 in Canes Venatici). Most nebulae appear as a hazy patch when viewed with a small telescope. Dark nebulae are gaseous clouds that block light. This is where stars form.

Stars are created when gravity collapses dust and gasses together.