English: The XMM-Newton spectral signature from superheated iron atoms at the inner edge of the accretion disk orbiting the neutron star in Serpens X-1. Normally, the line would be a symmetrical peak, but it exhibits the classic features of distortion due to relativistic effects. The extremely fast motion of the iron-rich gas causes the line to spread out. The entire line has been shifted to longer wavelengths (left, red) because of the neutron star's powerful gravity. The line is brighter toward shorter wavelengths (right, blue) because Einstein's special theory of relativity predicts that a high-speed source beamed toward Earth will appear brighter than the same source moving away from Earth.