HD 168625


HD 168625 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 HD 168607 and is located to the south-east of M17, the Omega Nebula.

Distance

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, at a distance to the Sun of, others think HD 168625 is farther, at a distance estimated to be and unrelated to the former objects. The parallax published in Gaia Data Release 2 indicates a distance close to, consistent with HD 168607.

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

Nebula

The most notable characteristic of HD 168625 is the presence of a nebula 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 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, the progenitor of the supernova 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 in the near future.