Portal:Radiation astronomy/X-ray astronomy article/7

RHESSI observes 2.2 MeV line emission from a solar flare. The solar flare at Active Region 10039 on July 23, 2002 exhibits many exceptional high-energy phenomena including the 2.223 MeV neutron capture line and the 511 keV electron-positron (antimatter) annihilation line. The RHESSI low-energy channels (12-25 keV) are represented in red and appear predominantly in coronal loops. The high-energy flux appears as blue at the footpoints of the coronal loops. Violet is used to indicate the location and relative intensity of the 2.2 MeV emission. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio.

Stellar surface fusion is one of the few direct views physicists have of thermonuclear fusion. Nuclear fusion usually occurs within a star as a part of stellar nucleosynthesis. However, an accreting star can undergo surface nuclear burning when the accretion rate exceeds a certain limit. The stellar luminosity then becomes dominated by hydrogen burning. The energy liberated by hydrogen burning exceeds that due to accretion by an order of magnitude or more, depending on the mass of the star. Steady hydrogen burning on the stellar surface processes hydrogen into helium at the rate of accretion. Surface fusion occurs above a star's photosphere to a limited extent as found in studies of near coronal and corona activity.

Based on the ³He-flare flux from the Sun's surface and Surveyor 3 samples (implanted ¹⁵N and ¹⁴C in lunar material) from the surface of the Moon, the level of nuclear fusion occurring in the solar atmosphere is approximately at least two to three orders of magnitude greater than that estimated from solar flares such as those of August 1972.