X-ray burster
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X-ray bursters are a class of binary stars which have periodic outbursts luminous in X-rays. They contain a neutron star and an accreting companion.
When a star in a binary fills its Roche lobe (either due to being very large or very close to its companion), it begins to lose matter, which streams into an accretion disk surrounding the other star. In an X-ray burster, this hydrogen-rich material accretes onto the surface of the neutron star as a layer of degenerate gas. After enough of this material accumulates, fusion ignites in the degenerate gas. Since fusion is a runaway process in a degenerate medium, the result is explosive. The resulting spike in X-ray luminosity is called an X-ray burst. Most X-ray bursters have irregular periods on the order of a few hours.
Luminous X-ray bursts can be considered standard candles, since the mass of neutron star determines the luminosity of the burst. Therefore, comparing the observed X-ray flux to the predicted value yields relatively accurate distances. Observations of X-ray bursts allow also the determination of the radius of the neutron star.