25 years since SN1987A was discovered

A particularly exciting aspect of astronomy is ‘discovery’. Sitting in front of a computer, looking at telescopic data, you suddenly realise you are the first person ever to have witnessed some astronomical event. But frustratingly there is often noone else around to share your discovery with!

The bright star in the centre of the right-hand image exploded to become SN1987A
The bright star in the centre of the right-hand image exploded to become SN1987A, clearly visible in the left-hand image. Copyright Australian Astronomical Observatory, photo by David Malin.

Ian Shelton must have felt that surge of excitement 25-years ago on February 24 1987, when developing photographic plates of the southern sky. One, of a region near the Large Magellanic Cloud, showed a bright star where no bright star should be. Stepping outside he looked up and became the first person in almost 400 years to see a supernova with his unaided eye.

A supernova is the death of a massive star, one that begins life with more than about eight times the mass of the Sun. For most of their lives stars exist in a delicate balance with their enormous mass trying to collapse in upon itself while light (or radiation ‘pressure’) pushes out to support the star. When the star’s fuel runs out it’s on a one-way trip to implosion.
The collapse happens inside-out, the central regions go first leaving the outer envelope unsupported. As the outer layers come crashing down a shock wave is generated which blows those outer layers right off the star. All we see, however, is a star which brightens suddenly then dims over several months, as if someone is playing with a dimmer switch.

But of course this supernova didn’t really occur in 1987. The star, Sanduleak -69º 202, was actually in the Large Magellanic Cloud and that is about 160,000 light-years away. What Ian saw was just the delayed ‘movie’ of an explosion that really happened 160,000 years ago. That movie of the exploding star is still travelling, at the speed of light, through our galaxy. It passed by Earth in 1987 and recently passed the bright star Vega. In a year or so it is due to reach the northern star Chi Draconis, in the northern constellation, the Dragon. Chi Draconis is on the opposite side of the celestial sphere to SN1987A.

A classic image of SN1987A taken with the Hubble Space Telescope in 1994
A classic image of SN1987A taken with the Hubble Space Telescope in 1994. The rings are gas shed from the progenitor star and then illuminated by that star’s explosion. Credit: Dr. Christopher Burrows, ESA/STScI and NASA.

SN1987A taught us a lot about exploding stars. We used to think only red supergiant stars (like Betelgeuse in Orion) could go supernova, but this one was a blue star; the detection of neutrinos two hours before the visible light that Ian saw helped us understand what really powers the explosion; and such a bright supernova in an age of radio-telescopes, digital cameras and spacecraft has provided a wealth of information that still keeps astronomers busy.

One significant change between 1987 and now is hidden in the name: SN1987A. The ‘A’ means it was the first supernova discovered that year, in a time of photographic film and eyes at telescopes. In 2012 (up to Feb 23) already 33 have been reported – many of these found by automated telescopes with digital cameras for eyes and a computer algorithm making the discovery.


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