The area around SN1987A seen by Herschel, with an enlarged image or the region in the circle from the Hubble Space Telescope. While Herschel sees a small dot, Hubble can see the ring of material surrounding the supernova. Image credit: ESA/NASA-JPL/UCL/STScI
Astronomers have long suspected that exploding stars, or supernovae, are responsible for the formation of cosmic dust. The origin of this dust is important because it plays a crucial role in the formation of stars, particularly billions of years ago when star formation was at its peak. Supernovae in our own galaxy are very rare, but 24 years ago astronomers were trated to one in the Large Magellanic Cloud, a small galaxy around 160,000 light years away.
The star which exploded would originally have been around 20 times the mass of the Sun, but in the later stages of its life it lost around a third of its mass into a giant cloud. Such enveloping clouds are one possible source of dust in the Universe, but can only account for a tiny fraction of what is observed. Eventually, the aging star could no longer support its own weight, and on 23rd February 1987 astronomers saw it brighten dramatically as it collapsed in a violent supernova.
The explosion resulting from a collapsing star is one of the most violent events in the Universe, and this event, called “SN1987a”, was no different. The supernova glowed with the brightness of 100 million Suns for around a month, while most of the material that made up the star was blown outwards at immense speeds. The resulting shockwave energised a ring of material in a disc of gas and dust around the star, and is still travelling outwards at speeds of up to 6000 km/s (13 million miles per hour).
The pulse of light from the supernova lit up a ring of material around a light year across (10 million million km), dozens of times the size of our own Solar System. This material glows in visible and ultraviolet light, as well as in x-rays. Small amounts of dust in the ring were warmed to a temperature of -100 Celsius, and this has previously been seen glowing faintly in infrared light.
A spectrum of SN1987a, showing the emission from the warm dust in the ring, detected by Spitzer (green), and the cold dust now seen by Herschel at much longer wavelengths (blue). The lines are the theoretical emission lines, and the points are the measurements from Spitzer and Herschel. Image credit: ESA/NASA-JPL/Caltech/UCL
Twenty three years after the initial explosion the Herschel Space Observatory observed the supernova remnant. As well as the warm dust in the glowing ring, the new measurements have shown that there is dust in the centre of the remnant with a temperature of below -250 Celsius, just 20 degrees above absolute zero. There is much more of this cold dust than the warm dust previously seen – in fact, enough to form more than 200,000 Earths. The wavelengths observed by Herschel are crucial to detecting this dust, as its very low temperature means that it is not detectable at other wavelengths.
“We didn’t expect to see SN1987A when we planned the survey,” explains Margaret Meixner, from the Space Telescope Science Institute in Baltimore, USA, and who leads the HERITAGE survey for which these observations were taken. “Based on our existing knowledge of dust in supernovae, we could not have anticipated that Herschel would have detected this source. It has definitely been one of the biggest surprises of our project,” she adds.
The dust was formed from material that was thrown away from the star in the initial explosion. If similar amounts are created in all such explosions, this could explain the origin of much of the dust seen in the Large Magellanic Cloud. Most of the dust found by Herschel in the form of carbon and silicates, not dissimilar to sand or quartz, though there are smaller amounts of iron present as well. The dust is probably being heated by a range of processes, either by x-rays emitted by the glowing ring, or by the radioactive decay of heavy elements, such as titanium, which were created in the star’s final moments.
The observations of SN1978a are crucial for understanding the remnants of supernova that Herschel is observing in our own Galaxy, which exploded hundreds or thousands of years ago. But this latest result also has much further reaching consequences. “With objects like SN1987a, we can investigate details that are almost impossible to discern in supernovae inside more distant galaxies. This helps us improve our understanding of these stellar explosions, which we can then apply to the broader context of galaxy evolution,” explained Mikako Matsuura, University College London, who led the research, which is published in the 7th July edition of Science Express.
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