Cygnus OB2 #12 is an extremely bright blue

hypergiant A hypergiant (luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term ''hypergiant'' is defined as luminosity class 0 (zero) in the MK ...

with an absolute

bolometric magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it w ...

(all electromagnetic radiation) of −10.9, among the most luminous stars known in the

galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

. This makes the star nearly two million times more luminous than the Sun, although the estimate was less than half when the star was first discovered. It is now known to be a binary, with the companion approximately a tenth as bright. A very approximate initial estimate of the orbit gives the total system mass as and the period as 30 years.

Cygnus OB2

Cygnus OB2 #12 is generally assumed to be a member of the Cyg OB2 Association, a cluster of young massive stars about 5,740 light years away in Cygnus, and resides in a region of the Milky Way from which visible light is heavily absorbed by interstellar dust when viewed from Earth. The dust causes the star to be strongly reddened despite being an intrinsically hot and blue star, hence it has been extensively studied in the infra-red. Were it not for the dust

extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

, the star would have a visual magnitude about 1.5, nearly as bright as

Deneb Deneb () is a first-magnitude star in the constellation of Cygnus, the swan. Deneb is one of the vertices of the asterism known as the Summer Triangle and the "head" of the Northern Cross. It is the brightest star in Cygnus and the ...

(Alpha Cygni), but because of the dust the observed visual magnitude is 11.4 so that it requires binoculars or a small

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...

to be seen. There are several fainter stars around Cygnus OB2 #12, thought to comprise a small cluster. Two stars are only resolved by speckle interferometry. One is thought to be a

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

B star in a 100–200 year orbit. It is 2.3 magnitudes fainter than the primary star and 0.063" away. The other is 4.8 magnitudes fainter and about an arc-second distant. No

spectroscopic Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wav ...

companion could be detected in the

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...

. However, analysis of

x-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

emission suggests that there may be a hot luminous companion in a close orbit, with their colliding winds creating the x-rays.

Distance

The

Gaia Data Release 2 The ''Gaia'' catalogues are star catalogues created using the results obtained by ''Gaia'' space telescope. The catalogues are released in stages that will contain increasing amounts of information; the early releases also miss some stars, especia ...

parallax for Cygnus OB2 #12 indicated a distance around , which was inconsistent with the distance of other Cygnus OB2 members. However, the result included a significant amount of noise suggesting that it may have been inaccurate. Earlier measures of the parallax were too imprecise to be meaningful. If the closer distance were accepted then the star would be a more conventional blue supergiant with a luminosity around , a radius of , and a current mass of . The later

Gaia Data Release 3 The ''Gaia'' catalogues are star catalogues created using the results obtained by ''Gaia'' space telescope. The catalogues are released in stages that will contain increasing amounts of information; the early releases also miss some stars, especia ...

is more precise and indicates a distance around , more consistent with estimates of the distance to Cygnus OB2, although there is still a significant amount of astrometric noise.

Properties

Cygnus OB2 #12 is a candidate

luminous blue variable Luminous blue variables (LBVs) are massive evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of the Larg ...

(LBV). Its position in the

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, luminosity, and spectrum all classify it as an LBV. It shows brightness variations of a few tenths of a magnitude, but these do not seem to be associated with colour changes that would be expected from an LBV. The spectral type has varied slightly since its discovery, but not to the extent that would be normal for an LBV. If the lower Gaia DR2 distance were correct then the properties of Cygnus OB2 #12 would mean it would be expected to pulsate at low amplitude with a period of a few days.

References

External links

Online Astronomy: The Brightest Stars

* http://jumk.de/astronomie/big-stars/cygnus-ob2-12.shtml {{Stars of Cygnus Cygnus (constellation) 101365 Luminous blue variables B-type hypergiants J20324096+4114291 Suspected variables