NASA's Hubble Space Telescope has snapped a view of a stellar demolition zone in our Milky Way Galaxy: a massive star, nearing the end of its life, tearing apart the shell of surrounding material it blew off 250,000 years ago with its strong stellar wind. The shell of material, dubbed the Crescent Nebula (NGC 6888), surrounds the "hefty," aging star WR 136, an extremely rare and short-lived class of super-hot star called a Wolf-Rayet. Hubble's multicolored picture 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]. The whole structure looks like oatmeal trapped inside a balloon. The skin is glowing because it is being blasted by ultraviolet light from WR 136.
Hubble's view covers a small region at the northeast tip of the structure, which is roughly three light-years across. A picture taken by a ground-based telescope [lower right] shows almost the entire nebula. The whole structure is about 16 light-years wide and 25 light-years long. The bright dot near the center of NGC 6888 is WR 136. The white outline in the upper left-hand corner represents Hubble's view.
Hubble's sharp vision is allowing scientists to probe the intricate details of this complex system, which is crucial to understanding the life cycle of stars and their impact on the evolution of our galaxy. The results of this study appear in the June issue of the Astronomical Journal.
WR 136 created this web of luminous material during the late stages of its life. As a bloated, red super-giant, WR 136 gently puffed away some of its bulk, which settled around it. When the star passed from a super-giant to a Wolf-Rayet, it developed a fierce stellar wind - a stream of charged particles released from its surface - and began expelling mass at a furious rate. The star began ejecting material at a speed of 3.8 million mph (6.1 million kilometers per hour), losing matter equal to that of our Sun's every 10,000 years. Then 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 image. The present-day strong wind of the Wolf-Rayet star has only now caught up with the outer edge of the shell, and is stripping away matter as it flows past [the tongue-shaped material in the upper right of the Hubble image].
The stellar wind continues moving outside the shell, slamming into more material and creating a shock wave. This powerful force produces an extremely hot, glowing skin [seen in blue], which envelops the bright nebula. A shock wave is analogous to the sonic boom produced by a jet plane that exceeds the speed of sound; in a cosmic setting, this boom is seen rather than heard. The outer material is too thin to see in the image until the shock wave hits it. The cosmic collision and subsequent shock wave implies that a large amount of matter resides outside the visible shell. The discovery of this material may explain the discrepancy between the mass of the entire shell (four solar masses) and the amount of matter the star lost when it was a red super-giant (15 solar masses).
The nebula's short-term fate is less spectacular. As the stellar wind muscles past the clumps of material, the pressure around them drops. A decrease in pressure means that the clumps expand, leading to a steady decline in brightness and fading perhaps to invisibility. Later, the shell may be compressed and begin glowing again, this time as the powerful blast wave of the Wolf-Rayet star completely destroys itself in a powerful supernova explosion.
The nebula resides in the constellation Cygnus, 4,700 light-years from Earth. If the nebula were visible to the naked eye, it would appear in the sky as an ellipse one-quarter the size of the full moon. The observations were taken in June 1995 with the Wide Field and Planetary Camera 2. Scientists selected the colors in this composite image to correspond with the ionization (the process of stripping electrons from atoms) state of the gases, with blue representing the highest and red the lowest observed ionization.
Credit: NASA, Brian D. Moore, Jeff Hester, Paul Scowen (Arizona State University), Reginald Dufour (Rice University)