Astronomy Picture of the Day
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 April 15 
Posted on 04/14/2003 10:34:50 PM PDT by petuniasevan
Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2003 April 15
Explanation: What caused the Crescent Nebula? Looking like an emerging space cocoon, the Crescent Nebula, visible on the right, was created by the brightest star in its center. A leading progenitor hypothesis has the Crescent Nebula beginning to form about 250,000 years ago. At that time, the massive central star had evolved to become a Wolf-Rayet star (WR 136), shedding its outer envelope in a strong stellar wind, ejecting the equivalent of our Sun's mass every 10,000 years. This wind impacted surrounding gas left over from a previous phase, compacting it into a series of complex shells, and lighting it up. The Crescent Nebula, also known as NGC 6888, lies about 4,700 light-years away in the constellation of Cygnus. Star WR 136 will probably undergo a supernova explosion sometime in the next million years.
These stars are very rare; only around 150 are known to exist in the Milky Way. There are several reasons for this. First, hot, massive stars form relatively rarely in proportion to overall star formation. Second, a hot, massive star uses up its nuclear "fuel" at a rapid rate. They are short-lived stars! Third, the Wolf-Rayet phase is a short transition between a star's existence as a main sequence O or B star, and its violent end as a supernova.
By the way, a most fascinating characteristic of many Wolf-Rayet stars is that they produce laser light. Here is a link to an amateur astronomer's Wolf-Rayet page. There are lots of links (some broken) and lots of information which won't require a degree in physics for readers to understand. Observe Laser Stars
The mysterious and massive star Eta Carinae seems to be in the process of becoming a Wolf-Rayet star. It was "Variable Star of the Month" at AAVSO in April 2000. Unfortunately for us in the temperate northern latitudes, Eta Carinae is located WAY south at RA 10 45.1, dec. -59 41.
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