NGC 6910 is an

open cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...

in the constellation Cygnus. It was discovered by William Herschel on October 17, 1786. The cluster was also observed by

John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint and did botanical wor ...

on September 18, 1828. It is a poor cluster with prominent central concentration and Trumpler class I2p. NGC 6910 is the core cluster of the

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 ...

Cygnus OB9.

Details

NGC 6910 is located half a degree east-north east of

Gamma Cygni Gamma Cygni (γ Cygni, abbreviated Gamma Cyg, γ Cyg), officially named Sadr , is a star in the northern constellation of Cygnus, forming the intersection of an asterism of five stars called the Northern Cross. Based upon parallax mea ...

, also known as Sadr. It may be physically related with the

nebula A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regio ...

IC 1318 Year 1318 ( MCCCXVIII) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar. Events January–December * March – King Birger of Sweden is deposed, and forced to flee to Denmark (alter ...

(also known as the Gamma Cygni Nebula) as it lies at a similar distance, behind the galactic Great Rift. Cygnus OB9 is located within the

Orion Arm The Orion Arm is a minor spiral arm of the Milky Way Galaxy that is across and approximately in length, containing the Solar System, including Earth. It is also referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion ...

of the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

. Cygnus OB9's dimensions in the sky are 2.5 degrees by 1.5 degrees, which corresponds at its distance to 175×105 light years across. It includes many OB stars, along with supergiant stars, like the red supergiant

RW Cygni RW Cygni is a semiregular variable star in the constellation Cygnus, about a degree east of 2nd magnitude γ Cygni. Its apparent magnitude varies between 8.05 and 9.70 and its spectral type between M3 and M4. Distance The Gaia Data Rel ...

. Gamma Cygni is a foreground star, lying at a distance of approximately 1,500 light years. Because it lies behind a number of molecular clouds, the light from NGC 6910 is dimmed by more than one magnitude. The core radius of the cluster is 0.8 parsec (2.6 light years), while the tidal radius is 4.2 parsecs (13.7 light years) and represents the average outer limit of NGC 6910, beyond which a star is unlikely to remain gravitationally bound to the cluster core. 125 stars, probable members of the cluster, are located within the central part of the cluster, and 280 probable members are located within the angular radius of the cluster. The brightest apparent member of NGC 6910 is a

blue supergiant A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They have luminosity class I and spectral class B9 or earlier. Blue supergiants are found towards the top left of the Hertzsprung–Russell diagram, above an ...

HD 194279 (V2118 Cygni) with spectral type B2 and apparent magnitude 7.0. It is located at the southeast edge of the cluster. It is a

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...

with P Cygni profile. A mag 8.1 star lies at the northwest edge of the cluster and a mag 8.5 O6 giant star ( HD 229196 , also known as V2245 Cygni) lies a bit southwest from the line connecting the two brightest stars. Other members of the cluster include two 10.3 mag stars, one being an O9.5 and the other a B0.5

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 ...

stars. The star NGC 6910 37 is categorised based on its emission as a

Be star Be stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as ''classical Be stars'', is a non-supergiant B star whose spectrum has, or had at some time, one or more Balmer e ...

and NGC 6910 14 and NGC 6910 25 are

beta Cephei variable Beta Cephei variables, also known as Beta Canis Majoris stars, are variable stars that exhibit small rapid variations in their brightness due to pulsations of the stars' surfaces, thought due to the unusual properties of iron at temperatures of 200, ...

s. In total there are four beta Cephei variables in the cluster, a rather high number. Their presence has been attributed to the higher

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...

of the cluster.

Observing NGC 6910

Although visible via

binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...

, the cluster is too compact to resolve in low power instruments. Individual stars can be seen with 15×100 binoculars. When seen in low magnification through a 5-inch

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 ...

, the cluster looks like a crescent between the two brighter stars, while more powerful magnification reveals more stars forming streams extending to either side of the cluster.

References

External links

{{Cygnus (constellation) 6910 Cygnus (constellation) Open clusters Orion–Cygnus Arm Discoveries by William Herschel