Stars radiate energy in many wavelengths. Visible light, the wavelength we see with the naked eye, is only one small part of the electromagnetic spectrum. Toward the short-wavelength end, beyond visible blue light and invisible ultraviolet light, are X-rays.

The discovery of X-ray stars had to wait until the space age. The ozone layer of the earth's atmosphere is a shield that prevents X-rays from space from reaching the surface. That is fortunate for us, since exposure to X-rays is dangerous. The only way for scientists to detect celestial X-rays is to get above the ozone layer, and that is precisely what American physicist Herbert Friedman (1916-) did in the late 1940s. Using captured German V-2 rockets from World War II, Friedman launched detectors above the atmosphere and discovered that the Sun emits X-rays.

Following in Friedman's footsteps was Riccardo Giacconi (1931-). He and his colleagues...