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The post Explosions visible from across the Universe appeared first on Australian Science.
]]>December 9th, 2011. NASA’s Swift telescope detected a sudden spike of gamma rays from somewhere within the constellation of Phoenix, visible in the Southern hemisphere. Gamma ray bursts (GRBs for short) like these are normally subdivided into two types, short bursts and long bursts, which have different causes. Short bursts last for just a couple of seconds, while long bursts can last for several minutes. But this particular burst, dubbed GRB 111209A (pictured above), was very different. One of a unique, rare breed of extra long lived GRBs, and by far the longest ever observed – lasting an unprecedented 7 hours!
Ever since then, astronomers have been picking at the data which were recorded, trying to work out exactly what kind of unusual circumstances would cause such a goliath GRB. One team, a collaboration between Bruce Gendre now at the French National Center for Scientific Research, and David Coward and Eric Howell at the University of Western Australia, managed to get to what may be the bottom of the puzzle. In short, these extra long GRBs are caused by the deaths of the Universe’s most massive stars.
It’s a sobering fact that all stars die. Some burn slowly for billions of years. The very smallest may burn for trillions. But the most massive stars burn out rapidly before dying in titanic explosions which we know as supernovae. It seems that the most massive of the massive, exceptionally rare, die in a unique type of GRB.
So far, only three of these ultra-long GRBs have been found, appearing so bright that some astronomers initially believed that they came from inside our own galaxy. As it happens, this wasn’t the case. Analysing GRB 111209A, Gendre and his colleagues hypothesised the source of it to be an extremely hot, massive star.
A furiously burning blue supergiant (possibly a hypergiant), hundreds of times the diameter of the Sun. When a star like this dies, its core collapses into a black hole, which then proceeds to start devouring the star from the inside out. As that black hole hungrily eats, its mammoth gravitational forces brutally tearing matter apart at the subatomic level, it generates an immense amount of energy which causes two jets of material to shoot out from its north and south poles. These are blasted outwards out with such ferocity that they actually punch two holes in the doomed star.
Read that again. A star which is hundred of times as massive as the sun, and these jets are powerful enough to blast two holes in it! The amount of energy required to accomplish that is truly difficult to comprehend.
The stars which die in these titanic events still require a specific set of circumstances. These stars are apparently enriched in elements heavier than helium (astronomers refer to these as “metals”, even though chemists would disagree on a few counts). More of these heavier elements cause a star’s stellar wind to increase, causing its rotation speed to slow as it loses material. But faster spinning stars seem to be more likely to cause these long bursts, which led some to wonder if the massive star behind GRB 111209A was also cannibalising another nearby star – a process which could possibly cause its rotation to speed up.
The exact cause is still a matter of puzzles and hypotheses. We won’t know more until we observe more of these ultra-long GRBs, and seeing as we’ve only ever spotted 3 of them, it may remain a puzzle for a while yet. But a tantalising glimpse like this of one of the Universe’s most extreme events gives us a good reason to keep watching and waiting. When we do finally understand, it will almost certainly have been worth waiting for.
Image credits:
Top – NASA/Swift/B. Gendre (ASDC/INAF-OAR/ARTEMIS)
Bottom – CNRS/ARTEMIS – Céline Lavalade
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The post Explosions visible from across the Universe appeared first on Australian Science.
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The post A Supernova Post-Mortem in Radio Waves appeared first on Australian Science.
]]>Named SN 1987A, this was the death of a massive star in the Tarantula nebula. Distant enough to not even be in our own galaxy, but in the Large Magellanic Cloud – one of the Milky Way’s smaller satellite galaxies, the discovery was reported independently by Albert Jones in New Zealand. This began decades of fascinating observations for astronomers, as many began to watch this supernova expand over the years, in real time.
Supernova which are close enough to see with the naked eye are rare beasts. This was, and still is, the only one close enough and visible enough to see properly with modern telescopes, giving us some of the best information we’ve ever had about how an exploding supernova interacts with the dusty interstellar clouds which surround it.
The latest observations of this literally awesome event come courtesy of a team of astronomers working in Australia and Hong Kong, led by Giovanna Zanardo at the International Centre for Radio Astronomy Research (ICRAR). Using CSIRO’s Australia Telescope Compact Array in New South Wales, the researchers have published the highest resolution images of the stellar explosion’s aftermath ever taken.
High resolution images are wonderful in astronomy. The higher the resolution, the more you can learn about what you’re seeing. Zanardo and her colleages compared their observations with other images and data taken at optical and x-ray wavelengths. On doing so, they gained some fresh insight into exactly what happens shortly after a star explodes.
In the centre of the explosion, stellar ground zero, they discovered a pulsar wind nebula. This is a pocket of intensely hot material emitted by a neutron star*, the last remains of the exploding star’s core, proving that SN 1987A did not create a black hole.
Referred to in technical jargon as a “compact source”, a neutron star is a tiny ball of incredibly dense material. With a mass up to over 3 times the mass of the Sun, these bizarre little objects truly are compact. An average neutron star has a radius of just 12 km, which is comparable with the size of Sydney. Yes, you read that correctly. The mass of a star compressed into something with a size similar to a large city.
This discovery actually answers a long standing puzzle about SN 1987 A. Supernovae like SN 1987A are normally expected to form neutron stars, because of the near-unimaginable pressures which occur inside an exploding star. But for several years, despite looking carefully, no astronomers could find any trace of a neutron star amid the stellar debris. But the star which caused this supernova would not have been massive enough to collapse into a black hole, leading theoreticians to try and devise explanations for why there was no neutron star to be seen.
If Zanardo’s team are right, and they have indeed found a pulsar wind nebula inside the shattered remnants of this dead star, then it has to be generated by something. Unless I’m mistaken, this may be some of the most convincing evidence yet for the missing neutron star!
Discovering all of this, however, was far from easy. Radio images at centimetre wavelengths are difficult to capture with detail. Exceptionally good weather conditions are needed. Zanardo explains, “For this telescope, these [observations] are usually only possible during cooler winter conditions, but even then the humidity and low elevation of the site makes things very challenging.
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The post A Supernova Post-Mortem in Radio Waves appeared first on Australian Science.
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