Neutron Star KS 1731-260
Credit: NASA/Chandra/Wijnands et al.

Neutron Star too Cool for School?

Neutron stars are balls of extremely dense material only a few tens of kilometers in radius. Since they are so dense they are of great interest to astronomers, since they allow astronomers to examine the effects of extreme gravity on matter and light. Since neutron stars are forged in titanic stellar explosions, they should be formed hot, but should cool as time passes - the exact manner by which they cool provides astronomers with important information about the inner composition of the neutron star. Complications occur since some neutron stars have gravitationally bound companion stars, and accretion of material from the companion by the neutron star can reheat the neutron star surface. Exactly how this happens is not yet well understood. A recent observation by the Chandra X-ray observatory of a neutron star binary system known as KS 1731-260 showed that this system (the blue dot at the upper left in the image above), which had been bright in X-rays since its discovery in 1989, had greatly decreased in brightness. Most surprising to astronomers, the temperature of the neutron star as measured by Chandra was "only" 3 million degrees, about a factor of 3 lower than expected. Astronomers suspect that the neutron star was very cold for thousands of years due to the absence of accretion, and that its current relatively cool temperature signals that accretion onto the neutron star surface has only recently started.

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Page Author: Dr. Michael F. Corcoran
Last modified January 21, 2002