KE-JUNG CHEN/SCHOOL OF PHYSICS AND ASTRONOMY/UNIV. MINNESOTA

A stellar temper tantrum that occurred just weeks before the star went supernova has offered a rare peek inside a star on the brink of exploding.

For years, researchers have relied on supernova explosions to measure the expansion of the Universe and to reveal the presence of faint, faraway galaxies. But they still do not completely understand how stars blow up. Although supernovae are detected fairly frequently — their extreme brightness means that they can outshine entire galaxies — researchers rarely see what a star looks like before it explodes. Such 'supernova precursors' have been seen in only a handful of cases.

In August 2010, Eran Ofek of the Weizmann Institute of Science in Rehovot, Israel, and his colleagues discovered a supernova using the Palomar Transient Factory (PTF), a wide-field sky survey based at the Palomar Observatory in California.The PTF constantly sweeps the sky, often observing the same swaths nearly every night for months on end. The repeated observations make it ideally suited to pick up stars' dying throes before they explode.

Two properties of the supernova, dubbed SN 2010mc, prompted them to search for an outburst that might have preceded the supernova explosion. First, the supernova belonged to a class known as type II, which involves the explosion of a massive star; several lines of evidence suggest that these heavyweight stars hurl some of their outer layers into space, starting years, decades or even longer before they explode.

Second, SN 2010mc went off relatively close — 500 million light-years (153 megaparsecs) — to Earth, increasing the odds that the Palomar survey might have recorded a faint pre-supernova eruption.

Looking back at earlier PTF images from the same patch of sky, the authors found a massive star blowing off into space a parcel of gas equivalent to about one per cent of our Sun’s mass. To the authors' surprise, that eruption occurred a mere 40 days before the supernova itself, the researchers report in Nature1.

By contrast, the pre-explosion outbursts that have been associated with the supernovae SN 2006jc and SN 2009ip seem to have occurred two to three years before the final eruptions. Because 40 days is a tiny fraction of the roughly ten-million-year lifetime of a massive star, and such outbursts cannot happen too often without whittling the star down to nearly nothing, the pre-explosion eruption must be directly linked to the SN 2010mc, says study co-author Eran Ofek at the Weizmann Institute of Science in Rehovot, Israel.

“Any theory for the explosion itself must therefore explain the presence of the precursor”, says another of the study's co-authors, Mark Sullivan at the University of Southampton, UK.

Stellar outburst

The mass and velocity of the material ejected before the SN 2010mc explosion seems to match those in a model proposed by Eliot Quataert and Joshua Shiode of the University of California, Berkeley2. Those researchers suggested that, during the very late stages of evolution of a massive star, oscillations in the stellar interior carry outwards some of the enormous amount of energy that is generated by nuclear burning at the core, prompting the star to expel some of its outer layers. Whether such an outburst triggers a star to go supernova or is a requirement for an explosion needs to be investigated further, says Shiode.

“I fully agree with the idea that looking for evidence for pre-supernova mass loss can tell us a lot about the final days of the star,” comments supernova theorist Stan Woosley at the University of California Santa Cruz.

Woosley, however, favours a different explanation for the outburst: a type of stellar pulsation that can only be generated within extremely massive stars, and grows stronger with time3. The first pulsations would expel some of the star’s outer layers, and the later, more violent ones would lead to an unusually bright, full-blown supernova. If Woosley is right, pre-explosion outbursts would be rare events — restricted to the most massive supernovae — and would not shed light on the origin of the more common, lower mass supernovae.