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See Inside October 2010

The Biggest Bang Theory: Astronomers Confirm a New Type of Supernova

A new type of supernova is forcing astronomers to rethink the lives of the biggest stars



COURTESY OF NASA/CXC/SAO/ESA/ASU/JPL/CALTECH/UNIVERSITY OF MINNESOTA

When our sun comes to its ending in five billion years or so, it will fade into a quiescent white dwarf. Bigger stars go out with a bang—those with more than 10 times the mass of our sun collapse with enough vigor to spark a supernova, one of the most energetic events in the universe. For decades astronomers have suspected the existence of a type of stellar explosion that is bigger still—a “pair-instability” supernova, with 100 times more energy than an ordinary supernova. In the past year two teams of astronomers have finally found it, redrawing in a stroke the limit of how big things can be in this universe of ours.

All stars balance gravity with pressure. As light elements such as hydrogen fuse in a star’s core, the reactions generate photons that press outward, counteracting the pull of gravity. In larger stars, pressure at the core is high enough to fuse heavier elements such as oxygen and carbon, creating more photons. But in stars bigger than 100 solar masses or so, there’s a hitch. When oxygen ions begin to fuse with one another, the reaction releases photons that are so energetic, they spontaneously trans­mute into electron-positron pairs. With no photons, there’s no outward pressure—and the star begins to collapse.

One of two things can happen next. The collapse can create even more pressure, reigniting enough oxygen to create a burst of energy. This burst is enough to toss off the outer layers of the star but not enough to create a full supernova. The cycle can repeat itself in pulses—astronomers call this case a “pulsational” pair-instability supernova—until the star loses enough mass to end its life in an ordinary supernova. A team led by the California Institute of Technology’s Robert M. Quimby announced it had identified one of these and has submitted a paper for publication.

If the star is really big—and here we’re talking more than 130 solar masses—the collapse happens so fast and gathers so much inertia that even fusing oxygen can’t stop it. So much energy develops in such a little space that eventually the whole thing blows up, leaving no remnant behind. This is “the real deal, the big stuff,” says Avishay Gal-Yam, an astronomer at the Weizmann Institute of Science in Rehovot, Israel, whose team claims in a recent paper in Nature to have discovered the first full-fledged pair-instability supernova (Scientific American is part of Nature Publishing Group).

Before the findings, most astronomers had argued that gigantic stars in nearby galaxies slough off much of their mass before dying out, precluding a pair-instability supernova. These ideas are being reconsidered, now that these biggest of explosions have announced themselves in spectacular fashion. 

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