Using European and Japanese/NASA X-ray satellites, astronomers have seen Einstein’s predicted distortion of space-time around three neutron stars, and in doing so they have pioneered a groundbreaking technique for determining the properties of these ultra-dense objects. Neutron stars contain the most dense observable matter in the universe. They cram more than a sun’s worth of material into a city-sized sphere, meaning a few cups of neutron-star stuff would outweigh Mount Everest. Astronomers use these collapsed stars as natural laboratories to study how tightly matter can be crammed under the most extreme pressures that nature can offer. Astronomers studied a spectral line from hot iron atoms that are whirling around in a disk just beyond the neutron star’s surface at 40 percent the speed of light. They found that the iron line is broadened asymmetrically by the gas’s extreme velocity, which smears and distorts the line because of the Doppler effect and beaming effects predicted by Einstein’s special theory of relativity. The warping of space-time by the neutron star’s powerful gravity, an effect of Einstein’s general theory of relativity, shifts the neutron star’s iron line to longer wavelengths.
An artist depicts a disk of hot gas whipping around a neutron star. The gas in the inner part of the disk whirls around the neutron star at about 40 percent the speed of light, so fast that it experiences effects predicted by Einstein's theories of relativity. Superheated iron atoms in this region emit X-rays at a characteristic wavelength, but the spectral feature is highly distorted by the relativistic effects. (Credit: NASA/Dana Berry)
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