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) | 8.44[2] |
| Characteristics | |
| Spectral type | B6Ia+[3] |
| U−B color index | +0.37[4] |
| B−V color index | +1.41[4] |
| J−K color index | 0.599 |
| Variable type | alpha cygni[5] |
| Astrometry | |
| Radial velocity (Rv) | -4.00[6] km/s |
| Proper motion (μ) | RA: -0.71 ± 1.20[1] mas/yr Dec.: +0.01 ± 0.74[1] mas/yr |
| Parallax (π) | 2.52 ± 1.10[1] mas |
| Distance | approx. 1,300 ly (approx. 400 pc) |
| Absolute magnitude (MV) | −8.5[3] |
| Details | |
| Luminosity | 220,000[7] L☉ |
| Surface gravity (log g) | 1.5[7] cgs |
| Temperature | 12,000[7] K |
| Other designations | |
| Database references | |
| SIMBAD | data |
HD 168625 (V4030 Sgr) is a blue hypergiant 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.
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,[8] at a distance to the Sun of 2.2 kiloparsecs (7.200 light years),[9] 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.[10]
Physical characteristics
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.[9] It is losing mass through a very strong stellar wind at a rate of roughly 1.46×10−6 solar masses per year[11] and observations realized in 2012 with the help of the VLT show it's actually a binary star.[12]
However, its most notable characteristic is the presence of a nebula surrounding it that was discovered in 1994[13] and that has been studied with the help of several instruments and observatories and telescopes that include among others the Hubble Space Telescope[10] and the VLT.[11]
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,[10] 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.[14] 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.[14]
References
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