Cygnus OB2 #8A is a double-lined

spectroscopic binary A binary star is a system of two star, stars that are gravity, gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separa ...

located near the centre of the

Cygnus OB2 Cygnus OB2 is an OB association that is home to some of the most massive and most luminous stars known, including suspected Luminous blue variable Cyg OB2 #12. It also includes one of the largest known stars, NML Cygni. The region is em ...

association located 5,500 light years away.

Discovery

Until 1951 Cyg OB2 #8 had been known only as an anonymous catalogue entry in the

Bonner Durchmusterung In astronomy, Durchmusterung or Bonner Durchmusterung (BD) is an astrometric star catalogue of the whole sky, compiled by the Bonn Observatory in Germany from 1859 to 1903. The name comes from ('run-through examination'), a German word used for ...

. Then it was identified as one of several highly luminous hot stars close together in Cygnus. Despite being commonly referred to as Schulte #8A, the number 8 was first published in an earlier paper. Schulte identified the grouping as a massive

stellar association A stellar association is a very loose star cluster, looser than both open clusters and globular clusters. Stellar associations will normally contain from 10 to 100 or more stars. The stars share a common origin, but have become gravitationally u ...

and split star #8 into four components, including #8A.

System

The Cygnus OB2 #8A system contains two massive luminous O class stars in a 21.9 day orbit. The primary is a

supergiant Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spa ...

and the secondary is a

giant star A giant star is a star with substantially larger radius and luminosity than a main sequence, main-sequence (or ''dwarf'') star of the same effective temperature, surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moo ...

. The two stars are not thought to be exchanging mass and their luminosity classes match the

main sequence turnoff The turnoff point for a star refers to the point on the Hertzsprung-Russell diagram where it leaves the main sequence after its main fuel is exhaustedthe main sequence turnoff. By plotting the turnoff points of individual stars in a star cluster ...

in the Cyg OB2 association at around O6. The nearby stars Cyg OB2 #8B, #8C, and #8D, originally thought to be a single star, are all massive and luminous class O stars.

References

External links

* {{Stars of Cygnus Cygnus (constellation) Emission-line stars Astronomical X-ray sources O-type giants BD+40 4227 101425 O-type supergiants Spectroscopic binaries J20331508+4118504