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The youngest supernova in our Galaxy has been discovered by tracking the rapid expansion of its remains using the Very Large Array (VLA) and the Chandra X-ray observatory. The stellar explosion occurred in the mid to late 19th century but, until now, the remnants of the exploded star had been concealed behind a thick veil of gas and dust. Using X-ray and radio telescopes, astronomers have been able to penetrate this veil to reveal the presence of the 140 year old supernova remnant (SNR). There are about 250 known SNRs in our Galaxy, and until now the youngest of those - Cassiopeia A - was thought to be 340 years old. "If the supernova rate estimates are correct, there should be the remnants of about 10 supernova explosions in the Milky Way that are younger than Cassiopeia A," said David Green of the University of Cambridge in the UK, who led the VLA study. "It's great to finally track one of them down, but there should also be some even younger objects out there, waiting to be found.”
The scientists compared an X-ray image of supernova remnant G1.9+0.3 made using the Chandra satellite in 2007 with a radio image made with the VLA in 1985, and found that it had expanded considerably. To confirm the observation the team were recently granted further time on the VLA and found that G1.9+0.3 had expanded by a surprising 16% in the intervening 22 years. “It is almost certainly the case that it was expanding faster in the past and it has since been slowing down,” says Stephen Reynolds of North Carolina State University, “Which means the object is much younger than we originally thought.” When the SNR was first identified in 1985, the astronomers estimated its age as between 400 and 1,000 years. G1.9+0.3 is located 26,000 light years away in the galactic centre, which corresponds to a shock wave moving out at about 14,000 kilometres per second – by far the fastest ever seen in such a young supernova remnant. “The density of the surrounding material must be very low in order for it to have attained this velocity after just 100 years,” says Reynolds. “No other object in the Galaxy has properties like this.” The discovery addresses a puzzling lack of recent supernovae in the Milky Way, allowing astronomers to examine the remnant of a supernova at a stage that has never been observed before and providing insight into how some stars explode and what happens in the aftermath. Since it is expanding so rapidly, the research team plan to follow G1.9+0.3’s evolution over the coming years.
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Radio images of the newly identified young supernova remnant G1.9+0.3, as obtained using the Very Large Array, showing its expansion between 1985 and 2008. The colour scheme, blue-green-yellow-red, represents increasing radio intensity. Image: D. Green.