An intense source of X-rays in the constellation Cygnus, discovered in 1966. It has since been the subject of much study and speculation, being regarded as a candidate black hole. The source has been identified as a binary star system, the primary of which, designated HDE 226868, is a hot supergiant of spectral type O or B. The best evidence from the binary orbit of HDE 226868 around its unseen companion suggests that the invisible star has a mass significantly higher than the upper limit for neutron stars. The conclusion is that the invisible star can only be a black hole. The X-ray energy is generated by matter from the primary streaming on to the compact companion.

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.

Pertaining to the Moon. Cynthia is an alternative name for the Roman goddess of the Moon who is more usually called Diana. The name is derived from Mount Cynthus, her birthplace according to the myths.

Pertaining to Venus; used as an alternative to Venusian. Cythera is the Ionian island on which the goddess Venus first set foot, according to mythology.