What is a nebula?

Nebulae are enormous clouds of hydrogen, helium, and cosmic dust scattered through galaxies, sometimes stretching tens or hundreds of light-years across. In emission nebulae such as the Orion Nebula, radiation from nearby hot young stars ionises the gas and makes it glow, while dark nebulae simply block the light of stars behind them. Many nebulae are stellar nurseries: gravity pulls together dense pockets of gas until pressure and temperature rise enough to ignite nuclear fusion, giving birth to new stars. Other nebulae mark the end of a star life — planetary nebulae are shells of gas puffed off by dying Sun-like stars, and supernova remnants like the Crab Nebula are the expanding debris of massive stars that exploded. Despite their dramatic appearance, nebulae are so thinly spread that they are far closer to a vacuum than any gas on Earth.