July 21, 2011 | 25
On February 23rd 1987, the journey of some light that had been travelling for 168,000 years came to an end. Astronomer Ian Shelton at the Las Campanas Observatory in Chile was observing the night sky as usual when he saw something out of the ordinary. Not long after Shelton reported the discovery, an astronomer in New Zealand noticed the same thing.1 The light they saw was the swan song of a dying star that we now know as supernova 1987A. “1987”, because that was the year of its discovery, and “A” because it was the first supernova seen that year.
Supernova 1987A occurred on the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a dwarf galaxy not far from our own Milky Way. It lies 168,000 light years from Earth.2
Three hours before Shelton saw supernova 1987A, advanced notice of it reached Earth in the form of a burst of neutrinos seen at three different neutrino observatories. Neutrinos are difficult to detect because they are electrically neutral, and most pass through matter completely undetected.3 What counts as a “burst” of neutrinos may not seem like much. Before supernova 1987A there were 11, 8 and 5 detected by Kamiokande, IMB and Baksan respectively in a 13 second period. Compared with the normal background level in these detectors, 24 hits in such a short time period was big news.
Supernova 1987A was a Type II supernova. The sequence of events leading up to its explosion began when the star ran out of hydrogen to fuse, and so started making and fusing heavier and heavier elements until it had a core of iron.
When the core became massive enough, it imploded. Neutrons and neutrinos were created, but the collapse was stopped by something known as neutron degeneracy — in effect, the inability to push the neutrons any closer together. This was enough to push the implosion back out, creating a shock wave that expanded out from the star’s core. Material surrounding the core of the star was thrown off by the shock wave.
The neutrinos that arrived at Earth before the light from supernova 1987A got here because of their slippery nature. They were created as the core collapsed and were able to storm through the outer layers of the dying star, dispersing some of the energy and getting a head start on the light.
Supernova 1987A was so bright that it was visible to the naked eye, the first to be so since the invention of the telescope. In the 24 years since it was discovered, SN 1987A has given astronomers a lot to think about.
Now everyone’s favourite supernova is at it again, helping astronomers at ESA’s Herschel Space Observatory figure out where cosmic dust comes from. Scientists working with Herschel have discovered that supernovae may be the culprits when it comes to the question of what filled the early universe with dust. Dr Mikako Matsuura, the lead author of the Science paper that announced that results, and her colleagues found that supernova 1987A ejected dust with a total mass between 0.4 and 0.7 times that of the Sun.
Cosmic dust turns into stars, then planets and eventually, sometimes, people. Everything in our solar system started out as cosmic dust. When the Sun grows old enough and stops fusing the hydrogen in its core, it too will throw out dust into the universe, and that dust will go on to form new stars.
For decades, astronomers have wondered where all of the dust in the early universe came from. Early on, stars like the Sun had not been around for long enough to make the amounts of dust seen. But supernovae, typically larger stars with shorter lifetimes, had been. The paper published in Science (but also available on the arXiv) provides evidence to support the hypothesis that it is in fact supernovae that got the early universe so dusty.
Herschel can see cold objects that emit little heat because it detects the longest wavelengths of light in the infrared part of the spectrum. It wasn’t specifically looking for supernova 1987A, but happened upon it during a survey of the Large Magellanic Cloud. Astronomers worked out that the glow coming from the remnant was provided by lots and lots of dust — around 10,000 times more than previous estimates that were made 600 days after the explosion. The temperature of the dust is about −250ºC, which is 20 times colder than previous estimates and not far above absolute zero.
Previous estimates were made with less sensitive instruments than Herschel. Matsuura and her colleagues have suggested that this new-found extra dust could have existed at day 600 after the explosion, but was not seen because it was beyond the reach of instruments available at the time. Another possibility is that this dust “reservoir” has grown over the twenty years between day 600 and now (the new measurements were taken on day 8467 and 8564 after the explosion, if you were wondering). Either way, supernovae now seem to be a viable source for all the dust seen in young galaxies in the early universe.
Matsuura M, Dwek E, Meixner M, Otsuka M, Babler B, Barlow MJ, Roman-Duval J, Engelbracht C, Sandstrom K, Lakicevic M, van Loon JT, Sonneborn G, Clayton GC, Long KS, Lundqvist P, Nozawa T, Gordon KD, Hony S, Okumura K, Misselt KA, Montiel E, & Sauvage M (2011). Herschel Detects a Massive Dust Reservoir in Supernova 1987A. Science (New York, N.Y.) PMID: 21737700
Post title courtesy of Jarvis Cocker.
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