SN2008bk: The last moments of a star

Supernova 2008bk in the spiral galaxy NGC 7793 was discovered at the end of March 2008 by amateur astronomer Berto Monard from South Africa. The galaxy is 13 million light-years away in the direction of Sculptor constellation. This supernova is a typical example of a massive star that exploded at the end of its life. But this time, the astronomers were able not only to see the explosion, but to precisely pinpoint the star that exploded, a rather rare achievement.

This was made possible because the explosion site had been observed several times with the help of ESO's Very Large Telescope, the latest images having been observed only a few months before the explosion with the new HAWK-I instrument.

An European team of astronomers, led by Seppo Mattila of University of Turku, Finland, then observed the explosion site again two months after the supernova's discovery, this time with the VLT's NACO instrument, which uses adaptive optics to resolve the finest details. Adaptive optics is a technique that allows astronomers to overcome the blurring effect of the atmosphere, thereby producing very sharp images. By comparing these sets of images, the team found the doomed star from the early images.

The colours and brightness of the star revealed it to belong to the family of red supergiants — to which the very bright star Betelgeuse also belongs — and that it had initially a mass of about eight to nine times the mass of our Sun.

Just before exploding, it was about 500 times larger than our Sun, meaning that if it were placed where the Sun is, it would engulf all the planets up to the planet Mars.

This is only the fifth time astronomers have been able to directly trace back the star that exploded as a supernova. Of all these red supergiants, four of them have about eight times the mass of our Sun, which is thought to be the minimum mass needed to produce such explosions.

This research appears today in print as a Letter to the Editor of the Astrophysical Journal (ApJ, 2008, vol. 688, L91). The team is composed of S. Mattila (Tuorla Observatory, University of Turku, Finland), S. Smartt and Mark Crockett (Queen's University Belfast, UK), J. Eldridge (University of Cambridge, UK), J. Maund (University of Copenhagen, Denmark), and J. Danziger (Universita di Trieste, Italy).

The image shows the star before it exploded (left), as observed with ISAAC and FORS on ESO's Very Large Telescope, and after the explosion (right), as seen by the very sharp NACO.