HD 168625 (V4030 Sagittarii) is a 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 MKK ...

star and candidate luminous blue variable located in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

of Sagittarius easy to see with

amateur An amateur () is generally considered a person who pursues an avocation independent from their source of income. Amateurs and their pursuits are also described as popular, informal, autodidacticism, self-taught, user-generated, do it yourself, DI ...

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 observ ...

s. It forms a visual pair with the also blue hypergiant (and luminous blue variable)

HD 168607 HD 168607 (V4029 Sagittarii) is a blue hypergiant and luminous blue variable (LBV) star located in the constellation of Sagittarius, easy to see with amateur telescopes. It forms a pair with HD 168625, also a blue hypergiant and poss ...

and is located to the south-east of M17, the

Omega Nebula The Omega Nebula, also known as the Swan Nebula, Checkmark Nebula, Lobster Nebula, and the Horseshoe Nebula (catalogued as Messier 17 or M17 or NGC 6618) is an H II region in the constellation Sagittarius. It was discovered by Philippe Loys d ...

Distance

The distance of HD 168625 and its association with the Omega Nebula and HD 168607 is in doubt; while some authors think both stars are physically associated and belong to the stellar association Serpens OB1, at a distance to the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

of , or for both per

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, espec ...

about , a 2002 study estimates this star is farther, at about and unrelated to the other two objects.

Physical characteristics

Assuming a distance of 2.2 kiloparsecs, the star would be 220,000 times brighter than the

, having a surface temperature of 12,000 K. At that distance it can be calculated to be losing mass through a fierce stellar wind at roughly per year however this is to be muted somewhat as work realized in 2012 from the VLT reveal a binary star system – a companion exists around 4.5

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

s fainter than the primary.

Nebula

The most notable characteristic of HD 168625 is the presence of a

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 ...

surrounding it that was discovered in 1994 and that has been studied with the help of several instruments and observatories and telescopes that include among others the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...

and the VLT. Said studies show that HD 168625 is actually surrounded by two nebulae: an inner one that has an elliptical shape and a very complex structure that includes arcs and filaments, and a much larger outer one discovered with the help of the Spitzer Space Telescope that has a bipolar shape and that looks like a clone of the one surrounding

Sanduleak -69° 202 Sanduleak -69 202 (''Sk -69 202'', also known as '' GSC 09162-00821'') was a magnitude 12 blue supergiant star, located on the outskirts of the Tarantula Nebula in the Large Magellanic Cloud. It is notable as the progenitor of supernova 1 ...

, the progenitor of the

supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...

1987A in the Large Magellanic Cloud. This suggests Sanduleak −69° 202 was also a luminous blue variable as well as the possibility of HD 168625 exploding as a

Type II supernova A Type II supernova (plural: ''supernovae'' or ''supernovas'') results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, but no more than 40 to 50 times, the mass of the Sun () to undergo th ...

in the near future. East-north-east of the star and nebula is HD 168701 (HIP 90001), an eclipsing binary of beta Lyrae type. It is at about six times the angular separation of

viewed from the solar system and is the third very bright point to the south-east of the nebula. Its parallax of 0.7106 ± 0.0451 mas implies it is about away.

References

{{DEFAULTSORT:HD 168625 B-type hypergiants Luminous blue variables Sagittarius (constellation) 168625 Sagittarii, V4030 089963 Durchmusterung objects Alpha Cygni variables