The Crescent Nebula is an emission nebula located in the constellation Cygnus. It has an apparent magnitude of 7.4 and lies at an approximate distance of 5,000 light years from Earth. The nebula occupies an area of 18 x 12 arc minutes of apparent sky. It has the designation NGC 6888 in the New General Catalogue.
The Crescent Nebula lies about 2.7 degrees southwest of Sadr, Gamma Cygni, the central star of the Northern Cross. It is a relatively faint object, about 26 light years across, that requires exceptionally dark skies and a UHC or OIII filter to be seen. The nebula has a low surface brightness and can only be seen in a telescope of at least 8-inch aperture. In smaller telescopes, it appears as a nebulous patch, while larger instruments reveal the crescent shape. The nebula’s shape also resembles that of the Euro sign, which is why NGC 6888 is sometimes also known as the Euro Sign Nebula.
The star responsible for the nebula’s shape and glow is the Wolf-Rayet star WR 136 (HD 192163). The nebula is formed by the star’s fast, powerful stellar wind that collides with the slower wind ejected by the star about 250,000 years ago, when WR 136 expanded to become a red giant. The collision has produced a dense shell, which continues to expand at a speed of 80 km/s, and two shock waves. One shock wave is moving outward from the shell and producing the green filamentary structure visible in images, while the other is moving inward to create a bubble of gas that is heated to X-ray emitting temperatures. The hot gas ejected by the star moves at speeds of 2,000 to 3,000 km/s and the star sheds about a solar mass of material every 10,000 years.
Crescent Nebula, image: Patrick Hsieh (CC BY-SA 4.0)
WR 136 will likely end its life in a supernova event within a few hundred thousand years. Its estimated age is about 4.7 million years. The star is 3.3 times larger than the Sun, 15 times more massive and 260,000 times more luminous. It may be a binary star with a low-mass companion of spectral classification K or M.
Crescent Nebula, image: Adam Block/Mount Lemmon SkyCenter/University of Arizona
The Crescent Nebula was discovered by William Herschel in December 1792. The nebula lies in a dense field of the Milky Way and is one of several notable deep sky objects in the area of the Northern Cross. Others include the Crescent Nebula’s smaller neighbour PN G75.5+1.7, a faint planetary nebula known as the Soap Bubble Nebula, the open cluster Messier 29, the emission nebulae NGC 7000 (North America Nebula), Pelican Nebula (IC 5070), Gamma Cygni Nebula (IC 1318) and Tulip Nebula (Sharpless 101), and the supernova remnant known as the Veil Nebula.
Gamma Cygni Nebula, Crescent Nebula, Tulip Nebula and Messier 29. Image: Wikisky
Crescent Nebula – NGC 6888
Constellation: Cygnus
Right ascension: 20h 12m 7s
Declination: +38° 21.3′
Apparent size: 18′ x 12′
Apparent magnitude: +7.4
Distance: 5,000 light years
Designations: Crescent Nebula, NGC 6888, Sharpless 105, Caldwell 27