HD 168625

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

HD 168625 and its nebula in Hydrogen-alpha with MUSE
Observation data Epoch J2000      Equinox J2000
Constellation	Sagittarius
Right ascension	18h 21m 19.54840s[1]
Declination	−16° 22′ 16.0751″[1]
Apparent magnitude (V)	8.30–8.41[2]
Characteristics
Spectral type	B6Ia+[3] (B2—B8[4]) May be B60 due to hypergiant designation
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]
Astrometry
Radial velocity (Rv)	−4.00[7] km/s
Proper motion (μ)	RA: +0.304[1] mas/yr Dec.: −1.978[1] mas/yr
Parallax (π)	0.6542±0.0259 mas[1]
Distance	5,000 ± 200 ly (1,530 ± 60 pc)
Absolute magnitude (MV)	−8.39[4]
Details[4]
Radius	105 R☉
Luminosity	380,000 L☉
Surface gravity (log g)	1.74 cgs
Temperature	14,000 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
SIMBAD	data

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,^[8] at a distance to the Sun of 2.2 kiloparsecs (7,200 ly),^[9] or for both per Gaia Data Release 3 about 1.5 kiloparsecs (4,900 ly),^[1] a 2002 study estimates this star is farther, at about 2.8 kiloparsecs (9,100 ly) and unrelated to the other two objects.^[10]

Physical characteristics

A visual band light curve for V4030 Sagittarii, plotted from ASAS data^[11]

Assuming a distance of 2.2 kiloparsecs, the star would be 220,000 times brighter than the Sun, having a surface temperature of 12,000 K.^[9] At that distance it can be calculated to be losing mass through a fierce stellar wind at roughly 1.46×10⁻⁶ solar masses per year^[12] however this is to be muted somewhat as work realized in 2012 from the VLT reveal a binary star system – ^[13] a companion exists around 4.5 magnitudes fainter than the primary.^[14]

Nebula

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^[10] 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,^[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.^[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]

East-north-east of the star and nebula is HD 168701 (HIP 90001), an eclipsing binary of beta Lyrae type.^[17] It is at about six times the angular separation of HD 168607 viewed from the Solar System and is the third very bright point to the southeast of the nebula. Its parallax of 0.7106 ± 0.0451 mas implies it is about 1,700 parsecs (5,500 ly) away.^[17]