A Wonder in the Southern Sky

By MICHAEL D. LEMONICK

While scanning some routine sky photographs at Las Campanas Observatory in Chile last week, Astronomer Ian Shelton felt a surge of excitement. In an exposure he had taken just hours before with one of the observatory's small telescopes was a bright spot that had not appeared in older pictures. Stepping out into the clear mountain air of the Chilean coastal range, the University of Toronto scientist reverted to a technique now used only rarely by professional stargazers: he looked up at the sky. There, in the fuzzy patch of light known as the Large Magellanic Cloud, was the spot. Says Shelton: "For more than three hours, I tried several logical explanations. It took me a long time to actually accept that what I had just seen was a supernova."

But a supernova it was, a massive star dying in an explosion so violent that for a few weeks it will outshine hundreds of millions of stars put together. Its home, the Large Magellanic Cloud, is a satellite galaxy, or island of stars, that lies just beyond the fringes of our Milky Way galaxy, some 170,000 light-years from earth. (A light-year, the distance light travels in a year, is roughly 6 trillion miles.)

At that distance, close by astronomical standards, it could be one of the brightest stars in the sky when it peaks in intensity, perhaps as early as next week. (While it is clearly visible in the Southern Hemisphere, even Hawaii is too far north for much of a view.) The star will be the brightest supernova observed since 1604 and the only one visible to the naked eye since 1885. Says University of Chicago Astronomer W. David Arnett: "This is probably the most important thing that's happened in astronomy since 1604. It finally gives us a way of testing ideas about how stars and galaxies work and how abundances of heavy elements are created." The reason for the superlatives being expressed by Arnett and other astronomers is that this is $ the first supernova close enough to the earth to be scrutinized in fine detail by modern astronomical techniques.

Realizing the importance of his discovery, Shelton moved quickly to contact the International Astronomical Union's telegram service in Cambridge, Mass., the world's clearinghouse for announcements of new comets, asteroids and other transient astronomical phenomena. Shelton was the first to report the supernova, but, according to Service Director Brian Marsden, a New Zealand amateur astronomer named Albert Jones also spotted it that night. By the end of the day the service had sent telegrams announcing the supernova, officially designated 1987A, to some 150 institutions around the world.

By now, says Astronomer Stan Woosley of the University of California at Santa Cruz, "everyone with anything to look with is looking at it." Every optical telescope in the Southern Hemisphere is trained on 1987A; a newly launched Japanese satellite is scanning it for X rays emitted by the supernova's hot gases; the Solar Max satellite is looking for the gamma rays characteristic of very energetic explosions; and another spacecraft, the International Ultraviolet Explorer, has already made observations of the explosion's ultraviolet radiation. These indicate that the star's atmosphere, which astronomers have determined is exploding outward at a speed of about 36 million m.p.h., is already cooling. But the supernova is believed to be getting still brighter. By week's end it was as bright as 100 million suns, and soon it could match the brilliance of a billion stars.

Within a couple of weeks, says Harvard's Robert Kirshner, the temperature of 1987A's expanding shell should drop from its current 10,000 degrees C to roughly 6,000 degrees C, about the same temperature as our sun's surface. During the explosion, though, internal temperatures climbed to billions of degrees, and elements like silicon, sulfur and platinum, synthesized by the star, began spewing out over a vast region of space, where they will form clouds of gas and dust that can coalesce into new stars and planets. Indeed, most of the elements abundant on earth today, except hydrogen, were cooked up in some star that became a supernova. Says Woosley: "The calcium in our bones, the iron in hemoglobin and the oxygen we all breathe came from explosions like this one."

The first question astronomers asked: What kind of supernova is 1987A? There are two main types. A Type II explosion occurs when a massive star exhausts its nuclear fuel and collapses under its own weight. When its matter, falling in from all directions, meets at its center, a shock wave bounces back out in a tremendous explosion that blows apart the star's outermost layers. A Type I explosion occurs when a gravitationally powerful white dwarf star that is part of a binary star system draws gas from its nearby companion. When it accumulates too much, reaching a critical mass about 1.4 times as great as our sun's, it blows up.

Astronomers think 1987A is probably a Type II. But a star called SK 202-69, visible in older photographs in almost exactly the supernova's position and therefore possibly the star that exploded, is a blue supergiant, rather than the red supergiant predicted by theory. Astronomers are still looking for evidence of a previously undetected red supergiant in the same area. "There's a lot of confusion," says Arnett. "We have had so little data on supernovae. We have pieces of the puzzle, but they don't all fit together the same way."

The chance that they might finally be able to pinpoint a specific progenitor star, which could finally confirm or recast their theories about how supernovas explode, has astronomers beside themselves with excitement. "It's like Christmas," says Woosley. "We've been waiting for this for 383 years." Agrees Kirshner: "Everyone in the field has been calling each other up, partly for scientific reasons and partly for sheer pleasure. It's like when someone has a baby -- it's a great event, and you just want to talk."

PIX

CREDIT: TIME diagram by Paul J. Pugliese

CAPTION: DEATH OF A STAR

1. A large star runs out of fuel, and the core collapses under its own weight. 2. The resulting shock wave blows off outer layers of gas. 3. A neutron star or a black hole is all that remains.

DESCRIPTION: Three color illustrations.

With reporting by J. Madeleine Nash/Chicago and Fernando Paulsen/Santiago