Tuesday, February 16, 2010

Light Shed on Mysterious Supernova and the Star Behind It

A supernova that lasted an unprecedented 555 days, SN2007bi, appears to have been caused by a star that technically should not exist....today. The monster that led to this supernova ought to have been more than 300 solar masses, but this seems to contradict current theories, backed by observational data. A protostellar cloud would gravitationally collapse to form a star at 150 solar masses or less, and would never get even close to 300 solar masses. Stars this large just can't form.

However, that's in todays universe.

Back in the early days, when the universe was less than a few hundred million years old, the universe consisted only of hydrogen, helium and a little lithium which were forged in the first three furious minutes following the Big Bang. Heavier elements had not yet been formed as the universe cooled too quickly after the Big Bang to continue building elements, and the stars that would eventually synthesise them didn't yet exist. Huge amounts of these light elements had to come together before reaching the critical mass necessary for gravitational collapse that leads to stellar ignition, hence the truly enormous mass of these early stars. These first stars burned quickly (quickly for stars, ie a few million years) and brightly before exhausting their reserves of hydrogen and exploding in supernovae, creating heavier elements and sending them out into the universe, in the process reducing and eventually killing off any possibility of these types of stars from forming in the future. But why did it it burn so long? A hypothetical "pair-instability"-type supernova occurrs in stars this big. Hypothetical because it wasn't believed that stars this big could exist.

So how could stars this big exist today?

This supernova was seen in a dwarf galaxy 1.6 billion light years away, too nearby to be from the early universe. From observations, these dwarf galaxies seem to have significantly less of the heavier-than-lithium elements than they should have for their size. The implication here is that these dwarf galaxies may in fact be leftover "islands" of the early universe! The low mass could explain why they never accumulated to form fully sized galaxies.

This is why I love cosmology. The timescales, the distances and the concepts are so awesome, the processes so ferociously elegant....it's humbling to think that a complex glob of carbon compounds on a rock orbiting a yellow star in an insignificant galaxy can even exist to contemplate such things.

For lots more on this amazing story: http://www.newscientist.com/article/mg20527470.900-primordial-giant-the-star-that-time-forgot.html?full=true


Posted via web from John's posterous

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