By Ron Cowen
Take a dense, collapsed star, set a helium-rich star in orbit around it, and you’ve got the
ingredients for a series of explosions, each lasting about 10 seconds and releasing as much
energy as the sun does in an entire year.
Such eruptions can happen several times a day on the surface of a neutron star—the superdense
cinder left behind when a massive star jettisons its outer layers and collapses. Now,
astronomers have glimpsed a much rarer, longer-lived neutron-star explosion that unleashed
100 times as much energy.
In the more common explosions, a neutron star, as heavy as the sun but only 20 kilometers
wide, snatches gas from a nearby companion rich in helium. As the stolen helium piles up on
the neutron star’s surface, helium nuclei fuse explosively, briefly hurling a torrent of X rays
into space.
Last year, astronomers using the Rossi X ray Timing Explorer satellite examined a neutron
star and its partner that lie some 20,000 light-years from Earth. Scientists had previously
observed short-lived outbursts from this two-star system, known as 4U 1820-30. But during
the new study, the system emitted a flare that lasted for 3 hours and spewed as much energy
as the sun does in a century.
Tod E. Strohmayer of NASA’s Goddard Space Flight Center in Greenbelt, Md., and his colleagues
suggest that the flare stemmed from the neutron star’s short-lived helium explosions.
When three helium nuclei fuse and ignite, they produce a carbon nucleus. Over time, this carbon
ash may build up under layers of new helium captured from the neutron star’s companion.
When enough carbon nuclei accumulate, they would squeeze together to make even heavier
elements, liberating a tremendous amount of energy in the process—about 1,000 times as
much as a helium explosion produces. These rare explosions would last for hours because the
energy generated by the carbon, buried some 100 meters beneath the neutron star’s surface,
would take time to leak out, Strohmayer notes.
He estimates that the explosion observed by Rossi required about a billion trillion kilograms
of carbon heated to a billion kelvins. Theorists calculate that the neutron star seizes
matter at a million billion kilograms per second. At that rate, it would take 1 to 2 years to
accumulate enough carbon to generate the explosion, Strohmayer reported last month at a
meeting of the American Astronomical Society in Honolulu.
Carbon explosions are “the best bet” to explain the observations, says Edward F. Brown of
the University of Chicago. However, his calculations suggest that a neutron star’s surface can’t
reach temperatures as high as 1 billion kelvins. To ignite carbon at a slightly more modest
temperature, the star would require a billion times as much of the material, and it would take
a century to stockpile that much carbon, Brown notes.
Deeming it unlikely that the Rossi craft happened to record a once-in-a-century explosion,
Brown says he’s trying to determine if the neutron star could reach temperatures higher than
he had at first calculated or if the star could have pulled material from its partner even more
rapidly than Strohmayer assumed.
