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Nebulae are clouds of interstellar gas where stars are born as the gaseous material condenses into a tightly packed mass. Ancient and medieval man could not see the colors in nebulae, as they appear to the naked eye as a grayish smudge against the dark night sky. The Eta Carina image has the nebula in the center, with the Southern Cross to the left, and the False Cross to the right.
Loose star clusters appear to be associated as stellar groupings in the sky, but often are separated by greatly different distances from the earth. Note the blue nebulosity around the stars of the Pleiades. Globular clusters do form a spatially connected group, and often there more than a million stars in a single cluster. Galaxies contain hundreds of billlions of stars.