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The rp-process (rapid proton-capture process) is a nucleosynthesis process consisting of proton captures (nuclei incorporating incoming protons) sufficiently frequent that the atomic number rises despite the resulting beta decay of positrons. In other words, nuclei are undergoing more proton captures than beta decays. Such a process can build heavier nuclei, but any significant amount of the process requires the right conditions, and it competes with other nucleosynthesis processes that would be occurring simultaneously. It requires hydrogen (for the protons) and a high temperature (109 K) to overcome the Coulomb force. It is thought that it might occur under some conditions at the surface of neutron stars that are accreting and may explain some of the characteristics of neutron-star X-ray bursts. It is also thought to be a possibility within supernovae.