The Supernova That Cried Wolf

A STAR THAT has mimicked two supernova explosions since 2009 has finally blown itself apart for real. The discovery should clarify how massive stars behave in the years before their violent deaths.

The first bright explosion of the star, dubbed SN 2009ip, was detected in 2009 in a galaxy 67 million lightyears away. Soon after that, however, astronomers discovered that the star had survived the outburst intact.

Archived observations revealed that the star was a very hot, massive star known as a luminous blue variable (LBV). These stars have nearly exhausted their hydrogen fuel and undergo explosive episodes for reasons that are still unclear.

Outbursts of SN 2009ip in 2009 and 2010 saw hydrogen-rich gas speeding outwards at about 600km/s. A third outburst was detected in July 2012. Now a team led by Jon Mauerhan from the University of Arizona reports that the third blast was a genuine supernova in which the star blew itself to smithereens.

Supernova That Cried WolfThe team detected material blasting out from the explosion at about 10,000 km/s. “This was our first indication that SN 2009ip had finally exploded,” says Mauerhan. Its brightness peaked after several weeks in typical supernova fashion.

The finding challenges stellar evolution theories that predict that LBV stars shouldn’t blow up completely. It also suggests that Eta Carinae, a star system in our own Galaxy containing an LBV, could stage a supernova bright enough to be visible in daylight at any time.

What caused this final supernova? Either iron was produced in the core of the star, shutting off nuclear fusion, or a process known as ‘pair production’, in which energy is catastrophically converted into pairs of positrons and electrons, ran away with itself.

SupernovaLight from the explosion itself may hold the key, as it usually comes from the radioactive decay of unstable elements produced in the supernova. The speed and structure of the star’s fade from prominence might be the critical clue.

Supernova that cried wolf

But studying the decay is not going to be an easy task. This supernova started off faint, but then brightened as fast-moving material caught up with debris from previous outbursts. This will have to be carefully accounted for before a clear picture emerges.

In the end, the observations reveal a complicated story about a complicated event, but it’s by studying such unusual objects that real astronomical knowledge is won.

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