HD 168607

HD 168607 (V4029 Sagittarii) is a blue hypergiant and luminous blue variable (LBV) star located in the constellation of Sagittarius, easy to see with amateur telescopes. It forms a pair with HD 168625, also a blue hypergiant and possible luminous blue variable, that can be seen at the south-east of M17, the Omega Nebula.

Physical properties
HD 168607 was estimated to be about as far away as is the Omega Nebula (2.2 kiloparsecs, 7,200 light years, from the Sun) and no respective measurements have been found that discount physical association with HD 168625. Assuming this distance is correct, this star is 240,000 times brighter than the Sun with a surface temperature of $9,300 K$. The Gaia Data Release 2 parallax of $0.644 mas$ implies a closer distance of about $1,500 pc$.

The apparent magnitude of this star or star system was observed to vary by 0.25 to 0.30 magnitudes with a period of 64 days when it was first identified as an α Cygni variable. Unlike its neighbour HD 168625, no nebula has been found around this star. It is classified in the General Catalogue of Variable Stars as a luminous blue variable or S Doradus variable with the variable star designation V4029 Sagittarii and a maximum and minimum visual magnitude of 8.12 and 8.29 respectively. Although it is suspected of being in, or about to enter, an S Doradus phase, no outbursts have been observed. A magnitude variation between 8.05 and 8.41 is reported from a broader range of observations.

HD 168607 is thought to have had a mass between when it first formed on the main sequence, but now much less. Analysis of its period and photospheric abundances suggest that it has evolved through a red supergiant stage and has now expelled its outer atmosphere and increased its temperature again.