Rare Star Explosion Follows A Flash Of Light

A brief flash of light had been observed two years before this rare cataclysm occurred. This precursor signal, which had never been seen before, raises hopes that astronomers will be able to "predict" explosions and observe stars as they enter the very last moments of their existence. 

On 9 October 2006, two years after a Japanese amateur astronomer observed a flash of light in the constellation Lynx in UGC4904, the appearance of an object ten times brighter in the same spot attracted the attention of a European consortium, which mobilized a bank of telescopes(2). According to the first observations obtained at the La Palma Observatory in Spain, the explosion was exceptional: not a single trace of hydrogen or helium – the most abundant elements in stars – could be seen in the emitted light. It was only some ten days later that the first traces of helium were detected in the star's spectrum (i.e. the distribution of light according to energy).

These findings were confirmed by observations that followed over a period of nearly three months. The supernova,  named SN2006jc(3), reached the maximum luminosity that is characteristic of the most powerful star explosions, more than a billion times brighter than the Sun. Astronomers divide these explosions into two broad categories – supernovae types I and II, which refer to two completely different types of phenomenon. The type I category refers to the disintegration of a small, compact star, known as a white dwarf, which has been made unstable by the accumulation of matter coming from a companion.

The type II category, on the other hand, refers to the explosion of a massive star. In the first case, very little hydrogen and helium is seen in the explosion, whereas in the second type of explosion, these two elements are predominant. SN2006jc does not fit into either category and so has been catalogued in a special sub-category, Ic.

These very rare cases have only been discovered very recently. This rarity can no doubt be explained by the great mass of the star concerned. It is probably a star of 60 to 100 solar masses which has lost a great quantity of mass previously. Here it is only the central part, a core of carbon and oxygen of 15 to 25 solar masses that explodes. Thus, most of the elements in the explosion come from the core of the star, while the helium observed is only found around the edge and comes from the star's envelope, which was ejected earlier. 

The flash of light observed in 2004 also leaves many questions unanswered. As with earthquakes, scientists know very few precursor events capable of warning them that a star explosion is imminent. Supernova SN2006jc is the only known example of a star explosion for which a flash of light was observed two years earlier. For this reason, it opens up new horizons for predicting massive star explosions. Eta Carinae could be one example of a star similar to SN2006jc close to our galaxy. It also experienced an outburst of luminosity making it the second brightest star in the sky in 1843. A future outburst could be the sign of an imminent explosion.

Regular monitoring of such objects is a fine example of how small telescopes can be used and makes an excellent programme for collaboration between amateur and professional astronomers. It will no doubt be possible for us now to detect some of the most massive stars just before they break up.

The results will be published in the 14 June 2007 issue of Nature.

Additional Information

1) The observations were made at the Haute-Provence Observatory (1.93m telescope, CNRS, France), the Asiago Observatory (1.82 m Copernico telescope, Italy), the National Astronomical Observatory (2.16 m telescope, BAO, Xinglong Observatory, Beijing, China) and La Palma Observatory (Telescopio Nationale Galileo 3.58 m, Nordic Optical Telescope 2.56 m, Liverpool Telescope 2.0 m and Herschell Telescope 4.2m, Canaries, Spain).

2) Several hundred supernovae are discovered each year. The supernova's name is composed of the year it was discovered, followed by letters indicating its chronological order in the year. Thus, "SN2006a" was the first supernova discovered in 2006, "SN2006aa" the 27th and "SN2006jc" the 263rd of the year.