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HD 168625 (V4030 Sagittarii) is a blue hypergiant star and candidate luminous blue variable located in the constellation of Sagittarius easy to see with amateur telescopes. It 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.

HD 168625
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension  18h 21m 19.548s[1]
Declination −16° 22′ 16.0572″[1]
Apparent magnitude (V) 8.30 - 8.41[2]
Spectral type B6Ia+[3] (B2 - B8[4])
U−B color index +0.37[5]
B−V color index +1.41[5]
J−K color index 0.599
Variable type α Cygni[6]
Radial velocity (Rv)-4.00[7] km/s
Proper motion (μ) RA: -0.71 ± 1.20[1] mas/yr
Dec.: +0.01 ± 0.74[1] mas/yr
Parallax (π)0.6212 ± 0.0640[8] mas
Distanceapprox. 5,300 ly
(approx. 1,600 pc)
Absolute magnitude (MV)−8.39[4]
Luminosity380,000 L
Surface gravity (log g)1.74 cgs
Temperature14,000 K
Rotational velocity (v sin i)60 km/s
Other designations
V4030 Sgr, HD 168625, BD -16°4830, SAO 161375, HIP 89963, AAVSO 1815-168
Database references


The distance of HD 168625 and its association with the Omega Nebula and HD 168607 is dubious; while some authors think both stars are physically associated and belong to the stellar association Serpens OB1,[9] at a distance to the Sun of 2.2 kiloparsecs (7.200 light years),[10] others think HD 168625 is farther, at a distance estimated to be 2.8 kiloparsecs (kpc) (9,100 light years) and unrelated to the former objects.[11] The parallax published in Gaia Data Release 2 indicates a distance close to 1,600 pc, consistent with HD 168607.[8]

Physical characteristicsEdit

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.[10] It is losing mass through a very strong stellar wind at a rate of roughly 1.46×10−6 solar masses per year[12] and observations realized in 2012 with the help of the VLT show it's actually a binary star,[13] with the companion being around 4.5 magnitudes fainter than the primary.[14]


HD 168625 is the left star of the pair below the Omega Nebula. The other is the hypergiant HD 168607.

The most notable characteristic of HD 168625 is the presence of a nebula surrounding it that was discovered in 1994[15] and that has been studied with the help of several instruments and observatories and telescopes that include among others the Hubble Space Telescope[11] and the VLT.[12]

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,[11] 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.[16] 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.[16]


  1. ^ a b c d Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  2. ^ Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  3. ^ Chentsov, E. L.; Ermakov, S. V.; Klochkova, V. G.; Panchuk, V. E.; Bjorkman, K. S.; Miroshnichenko, A. S. (2003). "An atlas of spectra of B6-A2 hypergiants and supergiants from 4800 to 6700Å". Astronomy and Astrophysics. 397 (3): 1035–1042. Bibcode:2003A&A...397.1035C. doi:10.1051/0004-6361:20021430.
  4. ^ a b c Mahy, L.; Hutsemékers, D.; Royer, P.; Waelkens, C. (2016). "Tracing back the evolution of the candidate LBV HD168625". Astronomy & Astrophysics. arXiv:1608.01087. Bibcode:2016A&A...594A..94M. doi:10.1051/0004-6361/201628584.
  5. ^ a b Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D.
  6. ^ Sterken, C.; Arentoft, T.; Duerbeck, H. W.; Brogt, E. (1999). "Light variations of the blue hypergiants HD 168607 and HD 168625 (1973-1999)". Astronomy and Astrophysics. 349: 532. Bibcode:1999A&A...349..532S.
  7. ^ Kharchenko, N.V.; Scholz, R.-D.; Piskunov, A.E.; Röser, S.; Schilbach, E. (2007). "Astrophysical supplements to the ASCC-2.5: Ia. Radial velocities of ∼55000 stars and mean radial velocities of 516 Galactic open clusters and associations". Astronomische Nachrichten. 328 (9): 889–896. arXiv:0705.0878. Bibcode:2007AN....328..889K. doi:10.1002/asna.200710776.
  8. ^ a b Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  9. ^ 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.
  10. ^ 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.
  11. ^ 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.
  12. ^ 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.
  13. ^ Martayan, C.; Lobel, A.; Baade, D.; Blomme, R.; Frémat, Y.; 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.
  14. ^ Martayan, C.; Lobel, A.; Baade, D.; Mehner, A.; Rivinius, T.; Boffin, Henry M. J.; Ronny, B.; 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. (2016). "Luminous blue variables: An imaging perspective on their binarity and near environment" (PDF). Astronomy & Astrophysics. 587. arXiv:1601.03542. Bibcode:2016A&A...587A.115M. doi:10.1051/0004-6361/201526578. A115.
  15. ^ 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.
  16. ^ 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.