Cygnus X-1 - Star System

Star System

The compact object and blue supergiant star form a binary system that orbit around their center of mass every 5.599829±0.000016 days. From the perspective of the Earth, the compact object never goes behind the other star; in other words, the system does not eclipse. However, the inclination of the orbital plane to the line of sight from the Earth remains uncertain, with predictions ranging from 27–65°. A 2007 study estimated the inclination is 48.0±6.8°, which would mean that the semi-major axis is about 0.2 AU, or 20% of the distance from the Earth to the Sun. The orbital eccentricity is thought to be only 0.0018±0.002; a nearly circular orbit. Earth's distance to this system is about 1,860 ± 120 parsecs (6,100 ± 390 light-years).

The HDE 226868/Cygnus X-1 system shares a common motion through space with an association of massive stars named Cygnus OB3, which is located at roughly 2,000 parsecs from the Sun. This implies that HDE 226868, Cygnus X-1 and this OB association may have formed at the same time and location. If so, then the age of the system is about 5±1.5 Ma. The motion of HDE 226868 with respect to Cygnus OB3 is 9±3 km/s; a typical value for random motion within a stellar association. HDE 226868 is about 60 parsecs from the center of the association, and could have reached that separation in about 7±2 Ma—which roughly agrees with estimated age of the association.

With a galactic latitude of 4 degrees and galactic longitude 71 degrees, this system lies inward along the same Orion Spur in which the Sun is located within the Milky Way, near where the spur approaches the Sagittarius Arm. Cygnus X-1 has been described as belonging to the Sagittarius Arm, though the structure of the Milky Way is not well established.