A neutron star is the remnant of an exploded star, compressed to the size of a city. On its surface, a thermonuclear fire sometimes erupts: accumulated gas explodes like a giant hydrogen bomb. After the burst, ash remains — a mixture of heavy elements that rises to the surface and forms a hot atmosphere.
Astrophysicists built models of such atmospheres for four ash compositions: dominated by helium, chromium, iron, or nickel. The calculations included about 5000 spectral lines and a new method for accounting for light scattering by electrons. It turned out that in the transition layer between the transparent and opaque parts of the atmosphere, the force of radiation pressure sharply increases. This sets a limit on the maximum burst brightness, like a cork that prevents steam from escaping too quickly.
The model spectra were compared with observations of two neutron stars: HETE J1900.1-2455 and GRS 1747-312. Both show absorption edges in their spectra. This constrains the composition of their atmospheres and helps understand which nuclear reactions occur deep inside.
The atmosphere of a neutron star is only a few centimeters thick, but it weighs more than the entire Earth.