A source of X-rays in the constellation Cygnus. It is a close binary system in which the orbital period is 4.8 hours. The primary star has a mass about the same as the Sun’s. Spectra reveal it has the characteristics of a Wolf-Rayet star. The secondary is a pulsed source of gamma-rays with a period of 12.6 milliseconds. This star is likely to be a pulsar whose rate of rotation has been increased through interaction within the binary system. Cosmic rays have been detected at ground level on Earth from the direction of Cyg X-3. This suggests that Cyg X-3 is emitting the most energetic photons known from any astronomical source, and demonstrates that binary X-rays sources are important generators of high-energy cosmic rays. See also: X-ray astronomy.