A supernova remnant near the centre of the Milky Way has turned out to be the youngest known in our galaxy, plugging a puzzling gap in the astronomical record.
Known as G1.9+0.3, the remnant lies about 28,000 light years away. It was first identified as a ring-like supernova remnant in the early 1980s. Now, observations by NASA鈥檚 Chandra X-ray Observatory and the Very Large Array in New Mexico have shown that the diameter of the glowing gas shell has expanded by 16 per cent over the past 22 years.
If the speed of expansion is roughly constant, then the remnant is only about 140 years old, in the Earth鈥檚 time frame, making it the youngest in the Milky Way. Previously, the most recent supernova was thought to have occurred around the year 1680, creating the ghostly remnant Cassiopeia A.
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The latest supernova would not have been visible to 19th-century astronomers because it occurred in dense gas and dust near the galactic centre. 鈥淭he best telescopes at that time would not have been able to collect enough light to see it,鈥� says Stephen Reynolds of North Carolina State University in Raleigh, who led the Chandra study and revealed the results this week. 鈥淏ut the remnant shines in radio waves and X-rays, so X-ray and radio telescopes can see it.鈥�
The discovery of the young remnant helps plug a mysterious gap in our galaxy鈥檚 supernova history. Measurements of supernova rates in other galaxies suggest that about three supernovae should rattle the Milky Way every century. If so, our galaxy should contain roughly 10 remnants younger than Cassiopeia A. Many of the 鈥渕issing鈥� remnants may never turn up, Reynolds suspects.
Some stars might explode and leave no trace, perhaps because the star somehow creates a bubble of empty space around it before exploding. The shock wave from the blast would then expand in such a low-density gas that its glow would be invisible.
