Wednesday, November 21, 2012

Fast Stars

Supernovae are very violent events.  Very very very violent events.  Burning for just few days, a supernova emits as much light and other radiation as the sun will emit in its lifetime; so much light that it briefly outshines the combined luminosity of every other star in its galaxy.  There are two basic types of supernova: the first is triggered by the collapse of a supermassive star; the second by the reignition of nuclear fusion in a white dwarf star.  White dwarfs are the remnants of stars of average mass that have used up their hydrogen and, because the heat of fusion processes is no longer countering gravity, have collapsed into dense spheres of electron-degenerate matter and are slowly radiating away their stored energy (more massive stars collapse into even denser neutron stars).  But if a white dwarf is orbiting close to a companion star it can draw off and accumulate material until a runaway carbon fusion process ignites and destroys the white dwarf.  The properties and luminosity of these supernovae, called Type 1a, are so uniform that they can be used as standard candles to determine the distance to the galaxy in which they briefly flare.

But sometimes, like a misfiring firework, Type 1a supernovae sputter out before they reach peak luminosity.  A new computer simulation model suggests that these failed supernovae contain multiple ignition points that expand the white dwarf too quickly and prevent full detonation of the star.  Instead, there's an asymmetric explosion, something like a rocket jet.  The kick of this explosion could, apparently, accelerate the white dwarf to speeds of hundreds of kilometres per second, enough to rip it out of orbit around its companion star, or even to turn it into a hypervelocity star travelling at a speed that would enable it to escape from the Milky Way.  Imagine weaponising a supernova, turning a white dwarf into a bullet of electron-degenerate matter with the mass of the sun . . .

And if that isn't weird enough, it's not the only way that hypervelocity stars can be created.  Stars orbiting close to the supermassive black hole at the centre of the Milky Way accelerate as they swing around it.  Here's a neat simulation of the actual stars tracing their orbits:

If a multiple star system swings too close to the black hole, one of its members could gain enough momentum to escape its orbit, and zoom away at high speeds.  When I wrote about this in Eternal Light, back in 1991, this was just a hypothesis. Since then, the Hubble telescope has spotted a massive hypervelocity star heading out from the galactic centre at some 2.6 million kilometres per hour, three times the sun's velocity as it traces its orbit around the galaxy. You really don't need to make it up . . .


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