The Crescent Nebula is a Wolf-Rayet nebula in the constellation of Cygnus. Wolf-Rayet Nebulae are shell-like structures of gas found around a rare class of stars known as Wolf-Rayet stars. Wolf-Rayet stars belong to one of the most massive classes of stars in the cosmos, with incredibly powerful and forceful stellar winds. The star at the heart of the Crescent Nebula is the star WR 136, which shines 250,000 times brighter than that of the Sun (albeit primarily in ultraviolet).

 

Stellar winds are gas streams released by stars, such as the Solar wind of the Sun. In Wolf Rayet stars, however, these winds are much more forceful. The gases that make up the shell were released in the past when the star was a red giant. As it entered the Wolf-Rayet stage, however, the incredible power of the fast-moving stellar winds create shockwaves as they collide the the older, slow moving gas. This collision creates the shell-like structures we see that extends hundreds of trillions of kilometres across.