What are nebulas?

neb·u·la
noun
noun: nebula; plural noun: nebulae; plural noun: nebulas
1.
Astronomy
a cloud of gas and dust in outer space, visible in the night sky either as an indistinct bright patch or as a dark silhouette against other luminous matter.
(in general use) any indistinct bright area in the night sky, for example, a distant galaxy.
2.
Medicine
a clouded spot on the cornea causing defective vision.
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