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.
2002 August 25 
Posted on 08/24/2002 10:33:25 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.
2002 August 25
Explanation: Old photographs show no evidence of the above nebula. In 1992, a white dwarf star toward the constellation of Cygnus blew off its outer layers in a classical nova explosion: an event called Nova Cygni 1992. Light flooded the local interstellar neighborhood, illuminated this existing gas cloud, excited the existing hydrogen, and hence caused the red emission. The only gas actually expelled by the nova can be seen as a small red ball just above the photograph's center. Eventually, light from the nova shell will fade, and this nebula will again become invisible!
The nova/supernova puzzle remained a mystery until it dawned on researchers that they might not be similar phenomena. And they're not, as far as we can tell now. A supernova is indeed a massive star meeting its catastrophic end. A nova is - apparently - the result of a white dwarf star with a close companion star, most likely a giant. The giant's swollen outer layers are sucked into an "accretion disk" and then onto the dwarf's surface. When a critical mass of about 1/100,000 a solar mass is reached, the pressure and temperature of the gases triggers a nuclear explosion. The layers are blown off violently into space at 500 miles/second. The brightness goes way up, then drops back down over a period of weeks. Then the cycle starts again. Some novae have been seen multiple times.
Artist's conception of a white dwarf's accretion disk
accumulating matter from a companion star.
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That's SUPPOSED to be Astronomy Fun Fact. Sheesh. 
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