Friday, July 24, 2009

Is Betelgeuse about to explode?

The red supergiant Betelgeuse is something of a close neighbour in galactic terms. At a distance of only 650 light years, and located prominently on the shoulder of Orion, it's well worth peering through the net curtains to see what it's up to.

In fact, Betelgeuse is sufficently close that it can even be resolved by the Hubble Space Telescope (and interferometers) as an extended disk, with a measurable diameter, rather than a mere point of light.

At 20 times the mass of the Sun, and at an age of 8.5 million years, Betelgeuse is already living on spared time, and it has long been predicted to explode as a supernova sometime within the next thousand years. As a red supergiant approaches the end of its life, it consists of a shrinking core, in which temperatures are sufficiently high for nuclear fusion to take place, surrounded by a much larger envelope. Once the core has been converted to iron, the nuclear fusion ceases, and gravitational attraction overcomes the internal pressure gradient of the core, causing it to collapse. This collapse will only be halted after electrons and protons have reacted in the core to form neutrons, and the quantum mechanical neutron degeneracy pressure then halts the collapse. The star's envelope then falls inward, and rebounds off the rigid core, causing a shock wave to propagate outwards, effectively blowing the star apart. The only residue will be either a neutron star, or if the mass of the core is greater than the Chandrasekhar limit of 1.44 solar masses, a black hole.

Now, a team of astronomers lead by Nobel laureate Charles H. Townes have recently reported that the diameter of Betelgeuse has shrunk by about 15% since 1993, and there is speculation that Betelgeuse may be in the initial stages of collapse to a supernova. John Baez calculates that if Betelgeuse does explode, it will burn in the sky up to 3 times as brightly as the full moon. That might make the news.

It's worth noting that whilst nuclear fusion is capable of synthesizing all the elements up to the iron group, heavier elements require neutron capture reactions (operating in conjunction with beta decay). These reactions go under the name of s-type and r-type processes. The former operate under fluxes of 105-1011 neutrons per cm per second, which is available over thousands of years inside larger stars which have completed their main hydrogen burning phase and have entered a red giant/asymptotic giant branch (AGB) phase. The s-process, however, is only capable of producing isotopes up to bismuth-209. To produce heavier elements such as thorium and uranium, it is necessary for the r-process to operate, which requires fluxes of the order of 1022 neutrons per cm per second. This environment is only available in a core-collapse supernova explosion (or possibly in neutron star collisions).

Hence, if Betelgeuse really is about to explode, not only will it provide a spectacular blaze of light in the sky, but it will also supply a local source of the heavier elements. It might even leave an uncomfortably local black hole in its wake...

1 comment:

Sean said...

Looks from the pics it has already gone pop. Ford Prefect rip.