Merging White Dwarf Stars March to Supernova Doom
Source: Discovery News
Merging White Dwarf Stars March to Supernova Doom
FEB 9, 2015 12:42 PM ET // BY IAN O'NEILL
Astronomers have discovered a stellar rarity deep inside an oddly-shaped nebula — two white dwarf stars on the verge of a cataclysmic merging event.
During a survey to try to understand the peculiar shapes of nebulae in our galaxy, astronomers using the European Southern Observatory’s Very Large Telescope (VLT) in Chile zoomed in on the planetary nebula Henize 2-428, which is lop-sided. Planetary nebulae are formed during the later stages of a star’s life after it has run out of hydrogen in its core. This violent phase sheds the star’s outermost layers through powerful stellar winds.
But some distant planetary nebula have strange asymmetries — i.e. the cloud of dust and plasma expand into unexpected, non-uniform shapes. This is a puzzle to astronomers; if the nebula was created by a single dying star, surely all the material should be ballooning out as the same speed in all directions?
Now part of Henize 2-428?s asymmetry mystery has been solved. Embedded deep inside the cloud there’s not a single star, but two stars — a fact that has led to an explosive realization.
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Read more: http://news.discovery.com/space/astronomy/merging-white-dwarf-stars-march-to-supernova-doom-150209.htm
This artist's impression of the center of the Henize 2-428 planetary nebula shows the two white dwarfs in a
tight orbital dance that will ultimately end in a supernova.
= new reply since forum marked as read
Somebody broke the stars.
A white dwarf star, the leftover core remnant of a star like the Sun, whose fuel has been long since burned up and whose mass (i.e., gravity) is insufficient to continue that process, will eventually cast off its outer layers and become a white dwarf, a small dead husk, albeit a very hot one.
There is a limit to the mass of a white dwarf, however. That is about 1.4 times the mass of our sun. Once a star has evolved to the white dwarf stage its mass will be less than that, or it would not become a white dwarf. That mass is the Chadrasekhar limit.
When a white dwarf accumulates more mass, through gobbling up mass from a neighbor star in a binary pair -- most stars are binary or more -- the white dwarf will exceed that limit. The result is a Type Ia supernova which, because they usually happens at precisely the Chandrasekhar limit, all have the same characteristics. That makes them a standard candle, a way to measure distances in the universe. And since they are amongst the brightest things in the universe they can be seen billions of light years in the distance, and the past.
Great post.
The thanks, however, belongs to Subrahmanyan Chandrasekhar. I know, that's easy for me to say. (So to speak...) 
