File:Caldwell 27.jpg

English: Commonly called the Crescent Nebula, Caldwell 27 looks more like a prehistoric dinosaur egg. Hubble’s vibrantly colored image zooms in on a small region of the nebula that is “only” 3 light-years across, or about 17.6 trillion miles. The cloudy, mottled shell surrounds an extremely hot and short-lived type of star called a Wolf-Rayet. Dubbed WR 136, this colossal star is unleashing a powerful stellar wind of charged particles from its surface, which is tearing apart the shell of surrounding material that the star blew off 250,000 years ago.

WR 136 created this web of luminous material during the late stages of its life. As a bloated, red supergiant, WR 136 puffed away some of its bulk, which then settled around it in a vast, roughly spherical cloud. When the star evolved from a supergiant to a Wolf-Rayet star, it developed an even fiercer stellar wind and began expelling mass at a furious rate. The stellar wind collided with the material around the star and swept it up into a thin shell. That shell broke apart into the network of bright clumps seen in the Hubble image.

Hubble’s close-up of the nebula reveals with unprecedented clarity that the shell of matter is a network of filaments and dense knots, all enshrouded in a thin “skin” of gas (seen in blue). Hubble’s sharp vision allows scientists to probe the intricate details of the complex system. Such details are crucial to understanding the life cycle of stars and their impact on the evolution of our galaxy. The observations were taken in June 1995 with Hubble’s Wide Field and Planetary Camera 2. Scientists selected the image’s colors to correspond with the ionization state of the gases (how many atoms have been lost or gained in the atoms of each gas), with blue representing the highest and red the lowest observed ionization.

Also cataloged as NGC 6888, Caldwell 27 was discovered by William Herschel in 1792. This stellar demolition zone lies within our own galaxy, about 4,700 light-years from Earth in the constellation Cygnus, the Swan. For best viewing, observe Caldwell 27 with a moderate to large telescope equipped with a light-pollution filter during the late summer from the Northern Hemisphere (or during the winter in the Southern Hemisphere). With a magnitude of 8.8, the Crescent Nebula is not visible to the naked eye — but if it were, it would appear in the sky as an ellipse one-quarter the size of the full moon. In the future, the nebula’s shell may become compressed and begin glowing again, this time as a powerful blast wave moves outward from the Wolf-Rayet star when it completely destroys itself in a supernova explosion.

Credit: Brian D. Moore and J. Jeff Hester (Arizona State University)

For more information about Hubble’s observations of Caldwell 27, see:

hubblesite.org/contents/news-releases/2000/news-2000-23.html

For Hubble's Caldwell catalog site and information on how to find these objects in the night sky, visit:

www.nasa.gov/content/goddard/hubble-s-caldwell-catalog