Crescent Nebula Imaged by Chris Thayer

The Crescent Nebula (also designated NGC 6888 and Caldwell 27 on Patrick Moore’s Caldwell catalogue) is an emission nebula in the constellation of Cygnus. It was formed by fast stellar winds from the Wolf-Rayet star WR 136 (HD 192163) colliding with and energizing the slower moving wind (ejected by the star when it became a red giant around 400,000 years ago) creating a shell and two shock waves.  One shock wave is moving outward and the other inward; the inward moving shock wave energises the stellar wind to temperatures where X-rays are emitted.

Wolf-Rayet stars are hot massive stars (more than 25 solar masses) which are undegoing heavy mass loss. They are characterised by an extraordinary spectrum consisting of very wide emission bands as well as absorption lines in the violet region. These lead astronomers to conclude that these stars are surrounded by rapidly expanding shells of gas. Wolf-Rayet stars are classified as irregular variable stars, and are sometimes also called W stars. They are extremely rare, reflecting their short lifespan.

Lookup NGC 6888 on SEDS

Crescent nebula by Chris Thayer - click to expand

Imaged using an ED 80 and Mead DSI 3mono and guided with LX200 GPS at f 6.3 with Mead DSI3c.  Exposure: 9 frames x 4min (Hydrogen alpha)