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Dying Stars Give Life to the Universe : Cosmos: One recent supernova, located far beyond the Milky Way and about 12 million light years from Earth, can be seen with powerful telescopes.

July 18, 1993|DONALD J. FREDERICK | NATIONAL GEOGRAPHIC

Some stars die cataclysmic deaths: They release a flood of energy, create the elements of life and produce spectacular light shows.

Shining more brilliantly than a hundred million suns, an exploding star, known as a supernova, can unleash energy comparable to one second's output of all the stars and galaxies of the observable universe.

Such supernovas occur about once every 100 years in Earth's or nearby galaxies, according to Harvard University astronomer Robert P. Kirshner. "In more distant galaxies, we can see them too. There are searches around the world that turn up about 50 of these every year," he said.

A particularly bright supernova, 1993J, was recently discovered in the spiral-shaped galaxy M-81, far beyond the Milky Way, Earth's home galaxy. Invisible to ordinary observers, the exploding star, which lies only about 12 million light years from Earth, can be seen with powerful telescopes.

"This is the brightest one since 1987, and the brightest in the Northern Hemisphere since 1937," said Kirshner. It began as a "red supergiant," a star that is perhaps as big as Earth's solar system.

The last supernova close enough to be seen without a telescope occurred in 1987 in the Large Magellanic Cloud, the nearest galaxy to the Milky Way. It was seen only in the Southern Hemisphere. Previously, a supernova had not been seen by the naked eye since 1604, when German astronomer Johannes Kepler reported a sighting.

An exploding star lives on even though it dies. Supernovas not only spew out the matter that will eventually form other stars, but also the elements of life itself.

"The calcium in your bones and iron in your blood, were literally inside these stars that exploded in our galaxy, before the sun was formed," Kirshner said. "In a sense the stars helped form us."

The star's elemental ashes--silicon, oxygen, neon, carbon, and helium--mix with interstellar gas and enrich the gas with heavy elements such as calcium and iron.

Recent findings indicate that a pocket of interstellar gas created by a supernova surrounds the solar system, extending for several hundred light years in all directions.

Scientists got their first evidence of the gas from low-energy X-rays that radiated from areas of seemingly empty space, including the pocket that envelops the solar system. It is believed that the X-rays come from clouds of invisible ionized gas produced long ago by cataclysmic supernova explosions.

Analyzing data gathered by instruments on the space shuttle Endeavor in January, a group of University of Wisconsin scientists now believe that the X-rays are coming from hot gas, which is thinly dispersed and poses no harm to Earth.

"It's difficult to think of another way to get hot gas except from shock waves, and the easiest way I think of to get one of those is a supernova," said astrophysicist Richard J. Edgar of the University of Wisconsin. "So I would say it's a good bet now that there was a supernova that went off somewhere in the last million years or so, a date relatively recent in astronomical terms."

A supernova typically leaves behind a corpse--a spinning neutron star, also known as a pulsar. These tiny stellar embers, whose diameters are each about 15 miles, are made up of neutrons, atomic particles that can survive the crushing gravity of a supernova collapse. The neutrons are so densely packed together that a thimbleful would weigh more than all the cars on Earth.

Recent observations have shown that these supernova survivors can be sent hurtling through space at incredible speeds. A neutron star created about a million years ago has been detected streaking through the Milky Way, at 500 to 620 miles a second--a speed at which it could travel from Earth to the moon in less than six minutes.

Most neutron stars stay in the galaxies in which they are created. But this one has so much velocity that it will break free from the gravitational pull of the Milky Way and sail into intergalactic space.

"That won't happen soon," said Jim Cordes, the Cornell University astronomer who led the team that discovered the neutron star. "It will take 20 million years for the pulsar to cross this galaxy and 100 million years before it is even halfway between Earth and the next galaxy in its path."

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