The Crescent Nebula (designated NGC6888) lies about 5000 light-years away, and it is about 25 light-years across. At its centre is a bright blue star, an unusual type of star called a Wolf-Rayet star, which is very massive, hot and intrinsically luminous. Wolf-Rayet stars are a late stage in the life of very massive stars, after passing through a red supergiant phase. Being massive, they have short lifetimes. This one, called WD136, is estimated to be only 4.7 million years old, about a thousand time less than the age of our Sun. It has only a short lifetime ahead, as it is expected to explode as a supernova within a few hundred thousand years.

When WR136 was a red supergiant, about 120,000–240,000 years ago, it blew off a shell of gas travelling at a speed of about 80 km/s and this is still expanding. When it became a Wolf-Rayet star it started emitting a very fast stellar wind due to its extremely high temperature, in which gas is ejected at about 1,700 km/s: a characteristic of these stars. The collision of these streams of gas is energising it and causing it to emit light, which is what we see as this glowing shell-like structure.

This object is set in the Milky Way, and this is clear from the density of stars around it.