HD 168625

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HD 168625
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension 18h 21m 19.548s
Declination −16° 22′ 16.0572″
Apparent magnitude (V) 8.44
Radial velocity (Rv) -4 km/s
Proper motion (μ) RA: -0.71 mas/yr
Dec.: 0.01 mas/yr
Parallax (π) 2.52 ± 1.10 mas
Distance approx. 1,300 ly
(approx. 400 pc)
Absolute magnitude (MV) −7.85[1]
Spectral type B6Ia+[1]
U−B color index 0.57
B−V color index 1.24
J−K color index 0.599
Variable type alpha cygni
Luminosity 220,000[1] L
Surface gravity (log g) 1.5[1] cgs
Temperature 12,000[1] K
Database references
Other designations

HD 168625 is a blue hypergiant and a variable star located in the constellation of Sagittarius easy to see with amateur telescopes. Forms a visual pair with the also blue hypergiant (and luminous blue variable) HD 168607 and is located to the south-east of M17, the Omega Nebula.

Its distance and association with that nebula and the mentioned star is dubious; while some authors think both stars are physically associated and belong to the stellar association Serpens OB1,[2] at a distance to the Sun of 2.2 kiloparsecs (7.200 light years),[3] others think HD 168625 is farther, at a distance estimated to be 2.8 kiloparsecs (9,100 light years) and unrelated to the former objects.[4]

Physical characteristics[edit]

Assuming a distance of 2.2 kiloparsecs, HD 168625 is 220,000 times brighter than the Sun, having a surface temperature of 12,000 °K.[3] It is losing mass through a very strong stellar wind at a rate of roughly 1.46×10−6 solar masses per year[5] and observations realized in 2012 with the help of the VLT show it's actually a binary star.[6]

However, its most notable characteristic is the presence of a nebula surrounding it that was discovered in 1994[7] and that has been studied with the help of several instruments and observatories and telescopes that include among others the Hubble Space Telescope[4] and the VLT.[5]

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,[4] 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, the progenitor of the supernova 1987A in the Large Magellanic Cloud.[8] 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 in the near future.[8]


  1. ^ a b c d e Bibcode1992A&A...264...88V
  2. ^ Chentsov, E.L.; Gorda, E.S. (2004). "Spatial Closeness of the White Hypergiants HD 168607 and HD 168625". Astronomy Letters 30 (7): 145–180. Bibcode:2004AstL...30..461C. doi:10.1134/1.1774398. 
  3. ^ a b Nazé, Y.; Rauw, G.; Hutsemékers, D. (2012). "The first X-ray survey of Galactic luminous blue variables". Astronomy & Astrophysics 538: A47. arXiv:1111.6375. Bibcode:2012A&A...538A..47N. doi:10.1051/0004-6361/201118040. A47. 
  4. ^ a b c Pasquali, A.; Nota, A.; Smith, L.J.; Akiyama, S.; Messineo, M.; Clampin, M. (2002). "Multiwavelength Study of the Nebula Associated with the Galactic LBV Candidate HD 168625". The Astronomical Journal 124 (3): 1625–1635. arXiv:astro-ph/0207613. Bibcode:2002AJ....124.1625P. doi:10.1086/341820. 
  5. ^ a b Umana, G.; Buemi, C.S.; Trigilio, C.; Leto, P.; Hora, J.L. (2010). "Spitzer, Very Large Telescope, and Very Large Array Observations of the Galactic Luminous Blue Variable Candidate HD 168625". The Astrophysical Journal 718 (2): 1036–1045. Bibcode:2010ApJ...718.1036U. doi:10.1088/0004-637X/718/2/1036. 
  6. ^ Martayan, C.; Lobel, A.; Baade, D.; Blomme, R.; Frémat, et al.; Lebouquin, J.-B.; Selman, F.; Girard, J.; Mérand, A.; Montagnier, G.; Patru, F.; Mawet, D.; Martins, F.; Rivinius, Th.; Štefl, S.; Zorec, J.; Semaan, T.; Mehner, A.; Kervella, P.; Sana, H.; Schödel, R. (2012). "On the Binarity of LBV Stars". ASP Conference Proceedings 464: 293. Bibcode:2012ASPC..464..293M. 
  7. ^ Hutsemekers, D.; vanDrom, E.; Gosset, E.; Melnick, J. (1994). "A dusty nebula around the luminous blue variable candidate HD 168625". Astronomy and Astrophysics 2904: 906–914. Bibcode:1994A&A...290..906H. 
  8. ^ a b Smith, Nathan (2007). "Discovery of a Nearby Twin of SN 1987A's Nebula around the Luminous Blue Variable HD 168625: Was Sk -69 202 an LBV?". The Astronomical Journal 133 (3): 1034–1040. arXiv:astro-ph/0611544. Bibcode:2007AJ....133.1034S. doi:10.1086/510838.