In Distant Galaxy : Scientists Find Dim New Type of Supernova
A new type of supernova, or exploding star, that is much dimmer than any supernova previously observed has been discovered in a distant galaxy by astronomers at the University of California, Berkeley, and Caltech.
Alexei Filippenko of UC Berkeley and Wallace L. W. Sargent of Caltech said the new type of supernova seems to have come from a star that lost most of its outer layer of hydrogen and helium before it exploded. They said the discovery should give theorists new insight into the layered structure of dying stars and into how the heavy elements that are produced in a star’s nuclear furnace, such as iron, are dispersed into space and recycled into planets.
It may also explain the origin of the well-known star, Cassiopeia A, which is the strongest source of radio emissions outside the solar system, Filippenko and Sargent said.
Their discovery is reported in today’s issue of the British journal, Nature.
‘Burn Out Quickly’
“Two different types of supernovae were previously known to exist,” Filippenko said in a telephone interview Wednesday. “Type II supernovae occur in regions of vigorous star formation, where there are lots of young, massive, hot stars that burn out quickly.”
A Type II supernova is thought to occur when the huge star--which may have 40 times the mass of the sun--burns so much of its fuel into iron that the overburdened core collapses. The collapse occurs in less than a second, and the resulting shock wave explodes the star.
During the explosion, the star becomes brighter than an entire galaxy. In a month, the supernova gives off as much light as the sun emits in a billion years.
“Type I supernovae appear in old stellar formations, where there is little dust and no massive stars, because they have burned out a long time ago,” Filippenko said.
The supernovae are thought to occur when a small, dying white dwarf star--with a mass 8 to 15 times that of the sun--is inundated by material from a larger companion star. They are perhaps one-third as bright as Type II supernovae because they are smaller.
The new object was discovered by accident in February, when Filippenko and Sargent were studying light from the center of a nearby galaxy called NGC 4618. The light looked like it was from a Type II supernova, but it did not show characteristic hydrogen and helium.
“We were puzzled,” Filippenko said.
When the pair took a picture of the complete galaxy and compared it to pictures taken two months earlier, he added, “we saw that the object wasn’t there before.”
Normal Brightness
This and other evidence made them sure that they were looking at a supernova, even though a supernova of normal brightness had not been observed during the previous two months. Their results, however, reminded them of another celestial mystery object, Cassiopeia A.
Cassiopeia A was discovered in 1953. Its radio emissions indicate that it is the remnant of a supernova that would have exploded around 1665. None of the astronomers of that period reported observing a supernova, however.
A possible explanation for the phenomenon was first offered in 1976 by Roger A. Chevalier of the University of Virginia. He suggested that Cassiopeia was the remnant of a Type II supernova that had somehow lost its outer layers before it exploded.
The massive stars that become Type II supernovae have a layered structure much like an onion. Iron, the final product of the nuclear fusion that gives a star its heat and light, is located in the center of the star. Other, lighter elements that are intermediates in the fusion process surround the iron in successive layers.
From the center out, the layers are iron, silicon, magnesium, oxygen, carbon, helium and hydrogen. Chevalier calculated that if the outer hydrogen and helium layers were somehow removed, the star would still be massive enough to explode, but that it would be small enough for the light from the explosion to be relatively dim.
