Rare supernova brightens April Fool's Day.

"I thought that the message on my [answering] machine about a new supernova was an April Fool's joke at first," says Robert P. Kirshner. Once reassured that it wasn't, Kirshner, of the Harvard-Smithsonian Center for Astrophysics, spent much of the weekend alerting his fellow astronomers to the explosion of the dying star.

Amateur astronomers first spotted Supernova 19941-the ninth discovered this year - on April 1. It exploded in a spiral galaxy known as M-51, or the Whirlpool Galaxy. The supernova itself lies about 18 million to 20 million light-years away and should soon reach its peak brightness, says Richard A. Sramek of the National Radio Astronomy Observatory (NRAO) in Socorro, N.M. The supernova can be seen through a telescope.

"We get one about this bright every 10 years or so," says Kirshner. There was an equally bright supernova last year, he adds, but "once in a while, you get the once-in-a-decade supernova twice."

Astronomers detected radio signals the next day, an unusual occurrence. Only radio waves from supernova 1987A, which exploded in a neighboring galaxy, have been detected as quickly, "And compared to this one, 1987A was a real wimp," says NRAO astronomer Michael P. Rupen. "You would never have seen anything [from it] at this distance."

All stars continuously shed their skin, releasing particles that form a so-called stellar wind. When a supernova explodes, it flings off its outer layers of gas. To recreate the last moments of a dying star, astronomers look at the light and ultraviolet emissions produced in the heat of the star's explosion and the radio waves created as the star's outer layers slam into its stellar wind.

Because they saw so many different radio waves, astronomers hypothesize that 19941's stellar wind included a narrow, but dense, band of particles. "If you throw a brick through the air, you don't hear much of anything," Rupen says. "But if you throw it into a wall, it goes 'Bang.'" Yet it must have been a narrow band, he adds, because otherwise the radiation couldn't have "eaten away at the curtain to let you see in."

Supernova 19941 resembles a select group of 5 to 10 known supernovas. Astronomers speculate that they formed from the collapse of a large star that had already lost its outer layers of hydrogen and helium.

"We're still guessing on that," Kirshner says, "and it may be that this one is going to give us the chance to sort it all out." One speculation is that supernova 1994I may have formed from the collapse of a blue Wolf-Rayat star that was up to 40 times more massive than our sun.

"Presumably, the fate of a star has much to do with its mass," says Kirshner, "but we don't know what path in its life brings it to this colossal moment of destruction. By sorting out this one's biography, we may be able to sort out some of the factors."