An X-ray burst is a sudden, powerful surge of X-ray emission from an X-ray binary. Such bursts start without warning, rise rapidly, then decline gradually, lasting in total only about 20 seconds. Like novae, bursts involve mass transfer in close binary systems, but instead of the compact object being a white dwarf, as in the case of a nova, it is a neutron star. Material escaping from the ordinary companion star falls onto the neutron star. Most of this material consists of hydrogen, which becomes compressed against the hot surface of the star by the star's powerful surface gravity. In fact, temperatures and pressures in this accreting layer are so high that the arriving hydrogen is promptly converted into helium by the hydrogen-burning process. Constant hydrogen burning soon produces a layer of helium that covers the entire neutron star. Finally, when the helium laver is about 1 meter thick, helium burning ignites explosively, and we observe a sudden burst of X-rays. In other words, whereas explosive hydrogen burning on a white dwarf produces a nova, explosive helium burning on a neutron star produces a burster. In both cases, the burning is explosive because the fuel is so strongly compressed against the star's surface that it is degenerate, like the star itself. As we saw with the helium flash inside red giants, ignition of a degenerate thermonuclear fuel always involves a sudden thermal runaway because the usual "safety valve" between temperature and pressure is not operating.