Valery F. Suleimanov

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.

A waterfall that doesn't heat up from the fall but freezes — that's roughly how gas behaves as it flows onto a neutron star in an X-ray pulsar when the star shines dimly. Usually, infalling matter is heated to a white-hot glow, but new work shows the opposite.

The light of a distant lighthouse does not go out all at once: first it flares brightly, then slowly fades into darkness. That is how the X-ray pulsar 4U 0115+63 behaves — a neutron star that periodically brightens when gas from a companion ordinary star falls onto it.