Crust on a star

Science News, May 13, 2006 by Ron Cowen

By analyzing X rays generated by the rumblings of a neutron star 40,000 light-years from Earth, astronomers have estimated the thickness of the dense star's crust.

In December 2004, spacecraft including NASA's Rossi X-ray Timing Explorer detected the brightest X-ray flash ever seen from beyond the solar system (SN: 2/26/05, p. 132). The radiation came from an explosion on the neutron star SGR 1806-20.

Intensity fluctuations within the flash were probably generated by seismic vibrations traveling along different paths within the star's iron crust, according to astronomers led by Tod Strohmayer of NASA's Goddard Space Night Center in Greenbelt, Md. X rays of some frequencies would have come from seismic waves on twisting paths around the star's circumference, while other X rays were probably generated by vibrations on a more direct path through the crust.

If a previous estimate is correct that the neutron star is about 20 kilometers across, then the crust is about 1.6 km thick, the team announced last month at a meeting of the American Physical Society in Dallas. That thickness supports theoretical models in which neutron stars consist of ordinary matter. A star made of exotic subatomic particles would form a much thicker crust, according to some models.