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Thursday, December 22, 2011

How to make a supernova

Two international teams of astronomers have confirmed a leading theory about how type Ia supernovae form. This is significant not only for supernova aficionados, but for anyone trying to understand the nature of the universe.   Type Ia supernovae were used to demonstrate the expansion of the universe and the existence of dark energy.


 
Caption: This is SN 2011fe in the Pinwheel Galaxy (M101) at maximum brightness, a composite of optical data from the Las Cumbres Observatory Global Telescope Network 0.8m Byrne Observatory Telescope at the Sedgwick Reserve and (purple) hydrogen emission data from the Palomar Transient Factory.
Credit: B.J. Fulton (LCOGT) / PTF

The current theory is that type Ia supernovae require a binary system. The exploding star is thought to be a white dwarf, the normally quiescent stage at the end of a small star’s lifetime.  When a star below a certain mass runs out of fuel, it first expands dramatically and then collapses into a dense package that can no longer generate energy.  This is the fate that awaits our Sun, in about five billion years.

For most white dwarfs, this is the end of the line, a quiet retirement of slowly dissipating heat and energy.  However, if the white dwarf has a companion star (thought to be a red giant), it can steal matter from that star, effectively rebooting itself.  If it steals too much energy too quickly, the white dwarf will explode into a type Ia supernova.

In August, there was a supernova in the Pinwheel galaxy that we were lucky enough to observe just eleven hours after the explosion.  Actually, since supernova SN 2011fe is 21 million light years away, the event really occurred 21 million years minus eleven hours ago, last August.  Don’t you love cosmology? 

In any case, analysis of the event showed that the explodee couldn’t have been more than a tenth of the radius of the Sun.  In other words, it was indeed a white dwarf.  On the other hand, while the identity of the companion star is not known, it was definitely not a red giant, as previously assumed.  This means that although type Ia supernovae do appear to involve white giants in binary systems, the second star in the system may be more variable than expected.