Supernova 1987A: astronomers' luck.

Supernova 1978A: Astronomers's Luck

"It's like Christmas,' says astronomerStanford Woosley of the University of California at Santa Cruz, speaking of the supernova in the Large Magellanic Cloud, officially named supernova 1987A (SN: 2/28/87, p.132).

The flow of information from this, thenearest supernova visible to us since 1604, continues to roll in, each piece of it a welcome gift to astronomers who specialize in these violent explosions of stars. Apparently the supernova exploded on Feb. 23. It was first noticed on Feb. 24 by Ian Shelton of the University of Toronto, who was working at the Carnegie Institution of Washington's Las Campanas observatory in Chile, confirmed by nearby Cerro Tololo Interamerican Observatory and seen the same day by an amateur astronomer, Albert Jones of Nelson, New Zealand.

By the first weekend after the discovery,the supernova's brightness in visible light had apparently peaked at a magnitude between 4 and 4.5, characteristic of a dimmer, type II supernova, rather than the type I originally predicted. On the weekend also came a report of bursts of neutrinos from the supernova. And on March 2, Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who is director of the Central Bureau for Astronomical Telegrams, told SCIENCE NEWS that astronomers observing with the International Ultraviolet Explorer (IUE) satellite had evidence "just today' indicating that the star that exploded was not the one most observers had thought it was--the blue giant star Sanduleak -69|202--but a close companion to it.

Nevertheless, according to Woosley, ifthe supernova was the blue giant star, the timing of the neutrino bursts, which Carlo Castagnoli of the Istituto di Cosmogeofisica in Turin, Italy, reported from the neutrino observatory under Mt. Blanc, fits well into a theory of supernovas that Woosley and some colleagues have been developing for years.

In this model a supernova starts withthe collapse of the core of a star. This collapse generates a shock wave that moves through the outer layers of the star, and when the shock reaches the surface, the explosion of bright light and expulsion of matter begin. Neutrinos, however, come from the core collapse, and the difference between their arrival time and that of the first bright light should reveal the time it took for the shock wave to traverse the star.

The neutrino bursts were recorded atMt. Blanc at 2:58 a.m., universal time, on Feb. 23--or, in decimal fractions of a day, as astronomers like to time things, on Feb. 23.124. It happens that on Feb. 23.442, R.H. McNaught of the Siding Spring Observatory in Australia took a picture of the blue giant, which would then have been just at the point of explosion. The difference between those two times is about 30,000 seconds, too quick for the shock wave to traverse a red giant--the sort of star expected to produce a type II supernova --but right for a blue giant.

However, the IUE data of March 2 wereshowing a spectrum characteristic of a B3 blue giant, which seems to indicate that that star was still there, and that the supernova would then have to be some close companion to it. If that's so, it changes everything, Woosley says.

Other avenues of observation remainto be heard from. This supernova should emit powerful gamma rays, and they should last for years, says Woosley. Bursts of gravitational waves might have come from the supernova, but so far there is no report of such an observation.

Photo: Arrow points to supernova 1987A.