From SPACE.com, 28 sept 2000 .. J Craig Wheeler
I have been working in the field of
We have known since the 30's that supernovae light up in distant galaxies, representing enough energy release to tear a star asunder. There are two very broad categories -- those that represent the thermonuclear explosion of a white dwarf and those that represent the collapse inward and subsequent explosion outward of an aged stellar core.
Images
Gamma rays are imbedded in stellar winds emitted by exploding stars.
Chandra's image of supernova remnant G21.5-0.9. At both radio and x-ray wavelengths, this relic of a supernova blast appears as a round patch in the sky.
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Hubble image of Supernova 1987-A shows bright features as of February 2, 2000 (left) compared with the early days of the developing ring in 1997. |
The first is like a stick of dynamite and leaves nothing behind. The second involves the formation of a compact star, a
neutron star or a black hole. Interestingly, both of these processes appear to eject the same amount of kinetic energy -- the energy of mass motion -- into space. For the thermonuclear process that is essentially all the energy. For the collapse process, a hundred times more energy is liberated in total, but that extra comes out in ephemeral neutrinos, not in the exploding energy of ordinary matter.The hypernovae have brought in a surprising new perspective. The term "hypernova" has a confused etymological history and means different things to different astrophysicists. Bohdan Paczynski of Princeton University used it to refer to the very bright events associated with the newly discovered optical counterparts to cosmic
gamma-ray bursts. These are much brighter than supernovae. Then supernova researchers used it to describe Supernova 1998-bw. This event is strongly suspected to have been the source of a gamma-ray burst, but one very nearby in gamma-ray burst terms, only 120 million light-years, not the billions of light-years we have learned are typical.The association of the supernova with the gamma-ray burst is still controversial. With the supernova community being ardent believers since the supernova was so weird, and some members of the gamma-ray burst community retaining suspicions since a burst this close would have to be of unusually low energy. The supernova is, undoubtedly, odd. It was exceptionally bright for a supernova. It was a very powerful source of radio radiation requiring expansion of a shock wave at nearly the speed of light, and its spectral features revealed very high velocities compared to "normal" supernovae.
Several groups of researchers made models of this event. Making the standard assumption that the supernova blew up equally vigorously in all directions, these models suggested that Supernova 1998-bw had an expansion energy of matter that was over 10 times that of normal supernovae. That became a new operational definition of a "hypernova."
A handful of other supernovae have been identified that also seem to require very high energy if we assume the explosions exerted equal force in all directions, by dint of being very bright events with very high expansion speeds. These were also only recently discovered in the last two or three years. Where were these very bright, very noticeable, supernovae for the first 30 years of my career?
This is a rich area with many lines of thought to be explored. A critical one is whether the hypernovae, or any supernova for that matter, actually does expand outward equally in all directions. The answer is that normal supernovae, at least those associated with collapsing stars, do not. They tend to blow matter out faster along two opposing directions rather than in other directions. They might also tend to be brighter in that direction.
It is possible that the "hypernovae" are just "normal" core-collapse supernovae, but seen from a special angle? The majority opinion is that hypernovae really are different. The question then arises as to how and why. Do they represent black-hole, rather than neutron-star formation? Do they represent neutron-star formation of a special kind, say with an extra-strong magnetic field or extra-fast rotation? Are they related to gamma-ray bursts of some kind?
These are the sorts of issues that keep a scientist in constant overdrive.
Dr. J. Craig Wheeler is the author of Cosmic Catastrophes: Supernovae, Gamma-Ray Bursts, and Adventures in Hyperspace, and is the Samuel T. and Fern Yanagisawa Regents Professor of Astronomy, at The University of Texas at Austin. His course "Astronomy Bizarre" specializes in the weirder aspects of space science for non-science majors.