A nebula is an interstellar cloud of gas and dust. Some are stellar nurseries, where gravity draws the material together into new stars; others are the glowing debris a dying star has thrown off. Emission nebulae shine because nearby hot stars ionize their gas, reflection nebulae scatter the light of stars beside them, and dark nebulae are dense enough to blot out whatever lies behind. The word once meant any fuzzy patch in the sky, including distant galaxies, before telescopes grew sharp enough to tell them apart.