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Wednesday
Oct272010

When Stars Explode

When a star exhausts its Hydrogen fuel by burning it into Helium, it collapses under its own weight. For small stars like our Sun, the star eventually fades away into a rather unremarkable object known as a White Dwarf - neatly placed at the center of a spectacularly colorful cosmic painting made of remnant star dust (see post on planetary nebulae). 

If the original mass of the star is large enough however, this collapse ignites the Helium - through nuclear fusion - and burns it into heavier elements such as carbon and oxygen. This process continues until the core of the star transforms into a ball of iron - with lighter elements surrounding it in layers like the layers of an onion. No nuclear fusion can burn the iron core. At that point, gravitational collapse takes on a catastrophic character and a violent (and I really mean violent) explosion tears the star apart - along with its neighborhood… The explosion can be dramatically witnessed from Earth as intense light and copious X-ray emissions. This is called a Type II Supernova and typically involves the release of an amount of energy equivalent to the detonation of 100…twenty five more zeros nuclear warheads… Another type of equally dramatic explosion that we regularly witness in the deep cosmos is known as Type Ia Supernova; this involves two orbiting stars in a dangerous gravitational dance - think of it as salsa gone wrong… in both cases, the space around the star ends up covered with star debris - evidence of a violent event in its past.

Whatever remains of the star after a Supernova further collapses until all the particles making up its atoms get converted to a single type of particle known as a neutron. Neutrons are electrically neutral but resist tight packing due to quantum mechanical effects I will talk about in another post. The end result is known as a Neutron Star - a dense ball of neutrons. This is a very very peculiar object: typically 50-100 kilometers in size, but immensely dense - heavier than our Sun… It can spin at very high rates, emitting a sweeping beam of X-rays and other cosmic radiation from its poles - like a lighthouse beacon. We can see these beams from Earth and measure the spin rate; we call these objects Pulsars.  This is the end game for a star that was originally larger than our Sun but was still lighter than five times the solar mass. For even larger stars, the intense quantum mechanical pressure generated from packing neutrons is not enough to stop the collapse. The result is instead a black hole (see post on black holes). The largest Supernova recorded so far occurred in 2006; the mass of the original star was about 150 times that of our Sun…

The first accompanying video gives a brief overview of supernovae, and talks in particular about the Type Ia kind. The second video talks about neutron stars that arise from Type II supernovae. I also prepared a short slideshow of images of famous supernovae in a third video. The soundtrack is titled "Chinar Es" - roughly translates to "You are glorious" - an ancient Armenian tune composed around year 700 A.D. by Nerses Chnorhali. I titled some of the slides with the year of the star explosion.

 

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