Thor's Helmet in H-Alpha
Posted: Sun Feb 05, 2006 3:20 am
http://antwrp.gsfc.nasa.gov/apod/astropix.html
Near picture center, the helmet-shaped structure with wing-like appendages is popularly called Thor's Helmet. Cataloged as NGC 2359, the striking nebula is located about 15,000 light-years away in the constellation Canis Major. The helmet is actually more like a cosmic bubble, blown as the wind from the bright, massive star near the bubble's center sweeps through the surrounding molecular cloud. Known as a Wolf-Rayet star, the energetic star is a blue giant thought to be in a brief, pre-supernova stage of evolution.
About the Star Type:
Wolf-Rayet stars (named for their discoverers) are very large, massive stars (stars which are about 20 times bigger than the sun) nearly at the end of their stellar lives. As these stars age, material which the stars have cooked up in their central nuclear furnaces (like carbon and oxygen) gradually reach the surface of the star. When enough material reaches the surface, it absorbs so much of the intense light from the star that an enormously strong wind starts to blow from the star's surface. This wind becomes so thick that it totally obscures the star - so when we look at a Wolf-Rayet star, we're really just seeing this thick wind. The amount of material which the wind carries away is very large - typically, a mass equivalent to that of the entire earth is lost from the star each year.
