Point a telescope at a dark patch of sky and you may find a nebula: a glowing cloud, sometimes spanning light-years, where the raw material of stars hangs in space. The word comes from the Latin for cloud, and the name fits. But nebulae are not all the same. Some are stellar nurseries, some are stellar graveyards, and telling them apart reveals the whole life cycle of stars.

The most photogenic nebulae are emission nebulae, clouds of hydrogen gas lit from within by hot young stars. The stars' ultraviolet light makes the gas glow, usually pink or red in photographs, like a neon sign powered by starlight. The Orion Nebula, visible to the naked eye as a fuzzy patch in Orion's sword, is the classic example and one of the best first targets for a small telescope.

Reflection nebulae do not glow on their own. They shine by reflecting the light of nearby stars, usually with a blue tint, because dust scatters blue light more efficiently than red, the same physics that makes Earth's sky blue. Dark nebulae do the opposite: they are cold, dense clouds that block background starlight, appearing as black silhouettes against the Milky Way. All three types are the same basic stuff, seen under different lighting.