Out with a bang

The discovery of a Wolf-Rayet supernova rebuts the idea that the biggest stars go quietly.

A long time ago, a faraway star threw up its insides and ended its days in a colossal explosion. The first light to hold the record of this supernova reached Earth about this time last year. Just a few hours later, quick-thinking astronomers were able to point a telescope at the hole in the sky where the star had been. The resulting images help to resolve a key question in stellar physics. And they might raise more questions about the fate of Earth.

Supernovae are one of the most stunning events in the night sky; the explosions are so well known for their violence that the term has even entered common parlance. Yet supernovae are rare, and so, therefore, are direct observations of the circumstances immediately before and after them.

As astronomers describe on page 471 of this issue, being able to focus on the immediate aftermath of a supernova has shed new light on why some stars go bang with such force. In this case, the emissions spectra sent out by the dying star show that it was a Wolf–Rayet star, massive bodies that shed their mass rapidly in strong stellar winds.

The finding is significant because, although astronomers assumed that Wolf–Rayet stars would go supernova, there was no direct evidence that they did. In fact, in the absence of observations of such supernovae, a rival theory was gaining ground: that they might end their lives not in a bang but with a whimper. As John Eldridge explains in an accompanying News & Views article on page 431: “Until this event, there was growing evidence that such stars were likely to have dim or unobservable deaths.”

Wolf–Rayet stars are more than 20 times more massive than our Sun and are very breezy places: their fierce stellar winds can reach more than 1,000 kilometres per second. They are also rare, so if the name rings a bell then it could be because you have heard of a particular specimen: WR 104, a binary star about 2,450 parsecs (8,000 light years) from Earth that shot to fame in 2008 when astronomers warned that we could be in the firing line if it exploded. If you are concerned by this (and you probably needn’t be), then the finding that Wolf-Rayet stars do go supernova will do little to ease your anxiety.

A mere supernova would not threaten us at that distance, but some very massive stars explode as two powerful beams of lethal radiation known as γ-ray bursts. Depending on which way WR 104 is pointing — and the jury remains out on that — one of those bursts could head our way.

There are plenty of ifs and buts there — evidence suggests, for instance, that WR 104 has the wrong environment for γ-ray bursts — but, technically, the odds of such an event just shortened, very slightly. All Wolf–Rayet stars will go bang, the paper proposes, WR 104 included. The question is when — it could be next week, or thousands of years hence. Or it may already have happened.

Related links

Related links in Nature Research

A cosmic history 2014-May-07

Astrophysics: Lopsided stellar death 2014-Feb-19

Asymmetries in core-collapse supernovae from maps of radioactive 44Ti in Cassiopeia A 2014-Feb-19

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