New Scientist | Astrophile | David Shiga | 2012 Feb 24
Object type: Exploding star
Location: 160,000 light years from Earth, in the Large Magellanic Cloud
"Once in a lifetime" barely begins to describe it – astronomers had literally been waiting centuries for such a spectacle. The supernova that blazed forth on 23 February 1987 was the brightest since the one Johannes Kepler spotted in 1604.
The explosion happened just an astronomical stone's throw away, in a satellite galaxy of the Milky Way, which is why it was so bright it could be seen with the naked eye. No supernova so nearby had been seen since the invention of the telescope.
"There was a lot of excitement," recalls Roger Chevalier of the University of Virginia in Charlottesville. "People started to call each other up, saying 'This is it!'"
But even before telescopes detected any brightening, experiments in Japan and the US were hit by neutrinos generated during the star's death. In fact, SN 1987A remains the only supernova to explode close enough for its neutrinos to be detected on Earth.
So does the fact that the explosion's neutrinos arrived faster than its light support the claim made last year that the subatomic particles might break the cosmic speed limit set by Einstein's special theory of relativity? Surprisingly, no.
Birthday surprise
The neutrinos showed up just a few hours before the supernova appeared in telescopes. That slight time difference was expected, since neutrinos hardly interact with normal matter. They could just fly unimpeded through the stellar shrapnel, while the photons were waylaid by the exploding star stuff. Even setting off slower than light, the standoffish neutrinos could have made the trip faster.
If, on the other hand, the supernova's neutrinos had been moving as fast as those reported by researchers on the OPERA experiment last year, they would have arrived at Earth years before the light from the explosion.
That, clearly, was not the case. So perhaps it was the ultimate birthday present for SN 1987A when the OPERA team said on Thursday – the 25th anniversary of the supernova's appearance – that the faster-than-light claim might have been down to an instrumental glitch.
Morbid curiosity
Aside from boosting evidence for relativity, the supernova also shed light on a vexing problem for astronomers: what do stars look like just before they die? Supernovae usually pop off so far away that only their host galaxy can be made out – not individual stars.
SN 1987A, being in a neighbouring galaxy called the Large Magellanic Cloud, was different. Astronomers scoured archival images taken of the same region of sky and were able to spot a star at exactly the same spot. The only problem was – the star wasn't what they were expecting at all.
Stars expand and cool as they age, so researchers thought they would find a bloated, tepid red giant. Instead they found a much hotter, blue star.
Stellar union
One possible explanation is that about 20,000 years before going supernova, the doomed star merged with a stellar partner. It shed its cool outer layers in the process – forming rings that are still visible today – and transformed into a hot blue star.
Astronomers have recently identified other, more distant supernovae that resemble SN 1987A and were likely also produced by blue stars. Studying their properties could help confirm or rule out the binary merger explanation.
And we're still learning from SN 1987A itself as its expanding debris cloud continues to evolve. Recent observations show that the debris is starting to collide with the main ring of material ejected when the stars merged.
The collision is heating up the expanding supernova debris, making it glow, according to Hubble Space Telescope observations reported last year. By studying this glow, we can learn more about the innards of the star that went supernova, perhaps gaining new insights into its structure and composition.
"It's a rare opportunity," says Chevalier. "Every year we find lots of supernovae, but they're typically at least a factor of 100 farther away. Having something this close by is an extremely special event."
The Type II supernovae 2006V and 2006au: two SN 1987A-like events - F. Taddia et al
- Astronomy & Astrophysics 537 A140 (Jan 2012) DOI: 10.1051/0004-6361/201118091
arXiv.org > astro-ph > arXiv:1111.2509 > 10 Nov 2011
- Nature 474(7352) 484 (23 Jun 2011) DOI: 10.1038/nature10090
arXiv.org > astro-ph > arXiv:1106.2300 > 12 Jun 2011
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