16 Feb 2000
During its 15-billion-year or so lifetime, our Milky Way galaxy has been blasted with anywhere from 100 million to a billion supernova explosions. These "have progressively enriched it with the oxygen we breathe, the iron in our cars, the calcium in our bones and the silicon in the rocks at our feet," University of Arizona astronomy Professor Adam S. Burrows says in a cover story published in today's (February 17) Nature.
Supernovae explode with forces of incomprehensible magnitude. If you write it in numbers, a supernova explodes with something like the force of 25 hundred trillion trillion Megatonnes of TNT . Or, you can think of it as the force of the simultaneous explosion of 25 hundred trillion trillion nuclear weapons.
"It's not of terrestrial magnitude," Burrows said in an interview earlier this week. "It's its own scale. It would be like taking hundreds of Earths and converting that mass to energy. It would be energy comparable to the total amount of energy the sun radiates in 10 billion years, although the supernova explosion is the energy of motion rather than light. And the supernova explosion happens not over billions of years but in a matter of seconds."
Burrows and others who mathematically simulate these explosions with powerful computing show that these events unfold not only with amazing magnitude and speed but also with incredible beauty. For a look, check the images and quick-time movies on Burrow's web site.
Supernova explosions influence the birth of stars, are the source of the energetic cosmic-rays that irradiate us on Earth, and, collectively, by their prodigious energy and momentum during the birth of galaxies in the infant universe, may have helped shape the galaxies themselves, Burrows writes in Nature.
However, two recent developments have landed supernovae on astrophysics' center stage, Burrows says. One is the role of supernovae in measuring the geometry of the universe. The other is a growing appreciation that supernovae are implicated in the decades-old mystery of the origin of cosmic ray bursts.
One kind of supernovae, a sub-type classified Type 1a, is arguably astronomy's most accurate probe of the scale and geometry of the universe. They are useful as the cosmologist's "standard candle," or events of observable, known brightness that can be used as yardsticks in measuring cosmological distances. They are used in studying the Hubble constant, or the rate at which the universe is expanding.
Recently, as astronomers have begun studying Type 1a supernovae at very great distances, they believe they are beginning to see the curvature of the universe.
"What they believe their data says -- and I'm not entirely convinced yet -- is that the universe is not only expanding, but accelerating," Burrows says. "It can't accelerate with the normal material we know dominating its gravity -- not just 'baryon' matter, the stuff of which stars and people are made -- and not even just 'dark matter,' the unseen material suspected perhaps to be in the halo of our galaxy and close to the mass of our galaxy."
If the new data are right and the universe is accelerating as it expands, something else -- a very big something else -- is going on, Burrows says. It might be something to do with what Einstein called the cosmological constant. It might be associated with some sort of 'vacuum energy' of the universe, something that isn't particulate, that isn't dark matter, that doesn't radiate but is still matter.
"We don't know how to approach this yet. Some people say it could be a cosmological constant. Or, it could be something that slowly varies over a long, long time," Burrows says. "But the upshot is that a new component of the universe may have been discovered. If these data hold up, supernovae will have made a major contribution in the discovery of something very, very fundamental in the universe."
Another kind of supernovae might answer a question so long-standing it turned hair gray: What is the source of gamma ray bursts, events sometimes more energetic than even supernova explosions?
It will take a lot more study to know for sure. But there is growing evidence that gamma ray bursts are somehow triggered in the collapse of massive star cores into black holes, a type of supernova explosion that is very different from typical supernovae. There's a delay in the explosion of these rarer supernovae, but they then perhaps explode more vigorously, firing jets of very light debris particles at the speed of light with very great energies as they go. Burrows reviews the evidence in the Nature article.
It would take only a small fraction of supernovae to produce all the gamma ray bursts we see, Burrows says. In our own Milky Way, a supernova explodes every 100 years. A supernova explosion once every million years could explain the entire population of gamma ray bursts peppered throughout the universe, Burrows says.
Computer simulations of supernovae visually describe events in a supernova explosion. This simulation is one of many to be found in Burrows' "Supernova Cornucopia" at his web site.