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An article in today's issue of Nature, led by Josefin Larsson (University of Stockholm) reports that the remains of the supernova that exploded in the Large Magellanic Cloud over 24 years ago, supernova 1987A, are continually increasing in brightness.
Since the day of the explosion (23rd February 1987) the remnant generated by the explosion slowly faded, but already by 2001 this tendency seemed to invert, and the structure continued to brighten in successive years, until today, when the characteristic ring (about a light year across), which surrounds the site of the explosion, is as bright as it's ever been.
The reason why the remnant is now brightening, rather than continuing to fade, is rather simple, even though it has taken approximately twenty years of study with the Hubble Space Telescope to confirm.
Thousands of years before exploding, the progenitor star of a supernova like 1987A reaches a phase of instability that throws off large quantities of surface material. When the star later explodes, the material expelled eventually catches up with the slower ejecta from the previous phase, compressing it violently.
Before the meeting of the two ejecta components, the supernova shines essentially due to the energy released by the radioactive decay of heavy nuclei produced in the explosion, and this energy reduces as the years go by. However, when the fast supernova ejecta meets the slower material thrown off the star in earlier phases, a new energy source is available.
This causes, amongst other things, X-ray emission from the compressed gas.
The interaction of these X-ray photons (absorption/re-emission) with gas in the remnant then causes the emission of ultraviolet and visible photons. The research of Larsson's team has also been the first to measure the temperature of the internal part of the ring. The study suggests that the increase in luminosity of the remnant will continue for many years to come.
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