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Supernova Brightens the Horizon

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Supernova Brightens the Horizon NEWS AND VIEWS Supernova brightens the horizon Astronomy, as opportunistic a discipline as can be imagined, has been electrified by the lucky appearance of a supernova in a nearby galaxy. The spirit of 1987 is reborn. HuNTING supernovae requires the patience of find out which star had exploded. The So SN l993J has now appeared over a bird watcher in the Sahara. Although one or supernova's field is crowded, but researchers almost the full extent of the electromagnetic two may be discovered in any week, most are are pretty satisfied that they have found the spectrum, although neutrinos have not been in distant galaxies and so are hard to observe. progenitor; ultraviolet observations in a few seen (unsurprisingly given the 3.2 mega­ The circulars ('telegrams') of the Inter­ weeks should confirm that (or otherwise). parsec distance to M81 ). One day, gravi­ national Astronomical Union (now distrib­ Arguments about the progenitorofSN 1987 A tational waves from supernovae may be uted electronically)are full ofannouncements rumbled on for much longer. detected, but the appropriate antennae have of discoveries of supernovae in anonymous As most theories require, the progenitor yet to be built- the last frontier in obser­ or obscure galaxies. Despite their enormous was a red supergiant star, the typical inflated vational astronomy has yet to be crossed. intrinsic luminosity, outshining the Sun a state ofstars that have exhausted the hydrogen Curiously, the one player missing from the billionfold, they barely register in astro­ in their cores and are burning heavier field is the Hubble Space Telescope: while nomical images because they are so far away. elements. The progenitor was not as red as amateurs and large projects alike have turned Another difficulty is that supernovae, astrophysicists would really like, but neither towards M81, Hubble, beset by technical despite their immense energy - or perhaps was it as abnormal as the progenitor of problems (a computer glitch is hindering because of it- are over almost before they SN 1987 A, which was a blue supergiant. Nor operations) and encumbered by a ponderous start. After countless millions of years of was it as difficult to identify, even though bureaucracy, has failed so far to take unrelieved obscurity, a star will exhaust its M81 is 70 times further away than the Large advantage of this 'target of opportunity'. nuclear fuel, collapse and then explode in a Magellanic Cloud. The condition of a star on Even so, the interest in SN 1993J involves matter of seconds. What we see is merely the the point of exploding is evidently not strictly a certain clutching at straws. What astron­ aftermath as the excess heat from the remnant constrained. omers really need is to see a supernova in the is radiated away over the subsequent years. The popularity ofM81 with astronomers Galaxy. CmTent estimates are that one appears Andy Warhol's 15 minutes offame is more also meant that photographs of this region of every40-50 years. But the last two to be seen than most stars can aspire to. sky were available within hours before and appeared in 1604 and 1572. Before that, But patience pays off. The appearance of after the supernova exploded. At about there were visible supernovae in l 006 and supernova 1987 A (SN 1987 A) a little over midnight on 27 March, there was no sign of l 054. Unless the estimates are wrong, the six years ago, although it was outside the a supernova on an image obtained by Jean­ others must have been hidden by the lanes of Galaxy, showed how. That was the first Claude Merlin, an amateur French dust and gas that criss-cross the galactic disk. nearby supernova that could be seen well astronomer; 9 hours later, it was there on an What accurate observations beyond the since 1604; it was the first opportunity to electronic image obtained by Bill Neely, Galaxy are showing is that the characteristics confirm by observation theories of how stars another amateur, in New Mexico. Only rarely oftype II supernovae are remarkably variable. explode. The detection of neutrinos from the can astronomers pin down the moment of All seem to be 'atypical', which begs the supernova, part of that confirmation, eruption so accurately. SN 1987 A was unique, question: what is typical'? Zwicky's inspired inaugurated the field of neutrino astronomy ofcourse, in that the neutrino pulse, produced guess 60 years ago of the basic mechanism (the solar version excepted) and also served at the precise moment of explosion, gave a has proved to be remarkably accurate - as an unprecedented test of ideas about these precise time zero. right down to the prediction of neutron stars ghostly particles. SN 1993J has nicely conformed to the - but the details are turning out to be far Supernova 1987 A also proved the value prescribed pattern of so-called type ll more exciting. SN 1987 A has yet to reveal its ofthe lA U circulars. Within a day, telescopes supernovae. After the exhausted stellar core, neutron star, for example. all over the Southern Hemisphere and also no longer sustained by nuclear reactions, The other regret has been that nobody has aboard astronomical satellites were trained collapsed and then rebounded, a shock wave yet seen a nearby type I supernova. Far on the Large Magellanic Cloud, with the data was driven out through the stellar envelope, brighter than type lis, these explosions arise broadcast daily to astronomers. Something raising the stellar surface to white heat (I 0'- in white dwarf stars, possibly triggered by ofthat heady atmosphere has been recaptured 10" K) within a few hours. This is what the the accretion of a critical amount of mass. in the past fortnight, with the discovery by astronomers first saw. Over the next day or The preconditions seem so precisely tuned Francisco Garcia, a Spanish amateur so, the star's brilliance increased to the point that many think that all type Is (or more astronomer, of a supernova, SN 1993J (the that it could be seen with a small telescope or specifically type las) have to be alike. For tenth recorded this year), in the spiral galaxy even binoculars (I Oth magnitude). cosmologists, this offers the prospect of M81, in the constellation of the Great Bear The star soon started to fade as the old reliable beacons ('standard candles') to be (Ursa major). stellar surface started to cool. But as the seen far across the Universe, and a means to Except for SN 1987 A, this is the brightest shock wave ran into the tenuous wind that measure the cosmic distance scale and supernova at the Earth in more than 20, had been radiated by the progenitor star in its Hubble's constant, which gives the rate of perhaps 50 years. SN 1993J occurred in a declining years, X-rays were detected on 3 cosmic expansion. Indeed, the catalogues galaxy that, like the Large Magellanic Cloud April by Rosat, the astronomical satellite are stuffed with the parameters of distant (hostto SN 1987 A), is among the most closely launched in 1990, and subsequently by the type I supernovae. The trouble is that none watched in the sky: the 'M' in its name shows new Japanese satellite, ASCA. These betoken has been near enough to give an unequivocal that it is conspicuous enough to have been gas heated to I 0' K or more. As material in calibration of the scale. As one astronomer included in the catalogue compiled by Charles front of the shock wave thinned out, radio remarked ruefully, had SN 1993J been a type Messier more than 200 years ago. waves from the shock could escape, to be I, we would have known within days the One of the immediate benefits was that detected on 5 April at the Mullard Radio answer to the most burning astronomical old astronomical plates could be searched to Astronomy Observatory in Cambridge. question. Roland Pease NATURE · VOL 362 · 15 APRIL 1993 585 © 1993 Nature Publishing Group.
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