The Crescent Nebula

Posted on Friday, 5 December, 2014Monday, 21 August, 2017 by Zheluo Cai

The Crescent Nebula, also known as NGC 6888, is an emission nebula found in the nebula rich constellation Cygnus, about 5000 light-years away (red dot, left off centre of the sky map). It was discovered by Friedrich Wilhelm Herschel in 1792, and is about 25 light-years across.

Blown by winds from its central, bright, massive star, this colorful portrait of the nebula uses narrow band image data combined in the Hubble palette.It shows emission from sulfur, hydrogen, and oxygen atoms in the wind-blown nebula in yellow, green and blue hues. NGC 6888’s central star is classified as a Wolf-Rayet star (WR 136).

This image contains the Crescent Nebula NGC 6888 and surrounding nebulosity belonging to the γ Cygni Nebula Region. Although it does look like a miniature version of the well-known Veil Nebula, NGC 6888 is in fact neither a supernova remnant nor a planetary nebula, but one of the rare Wolf-Rayet nebulae. At the heart of this nebula is a Wolf-Rayet star (the bright star located centrally within the nebula); these stars rank among the most massive and hottest stars known. Characteristic for Wolf-Rayet Stars are their strong stellar winds, approximately 2.000 kilometers per second, and the enormous mass-loss associated with them, they can lose an entire solar mass in only 10,000 years. The central star exhibits an absolute magnitude of -4.4, radiating thus 5,000 times more brightly as our sun, however the maximum energy output lies in the UV range. These stars will erupt in supernovae during the next million years.

This composite X-ray (blue), optical (red and green) image reveals dramatic details of a portion of the Crescent Nebula. The intense radiation from the exposed hot, inner layer of the central star began pushing gas away at high speeds that rammed into the slower red giant wind, a dense shell (shown in red) was formed. The force of the collision created two shock waves: one that moved outward from the dense shell to create the green filamentary structure, and another that moved inward to produce a bubble of million degree Celsius X-ray emitting gas (blue). The brightest X-ray emission is near the densest part of the compressed shell of gas, indicating that the hot gas is evaporating matter from the shell. The massive star WR 136 that has produced the nebula appears as the bright dot at the center of the full-field image. The surrounding area contains several patches of emission nebulosity. The brightest parts are designated LBN 215 (upper left), LBN 208 (upper right, filamentary structure) and LBN 193 (lower right) in Lynd’s Bright Nebulae catalog.