V915 Scorpii

V915 Scorpii (HR 6392, HD 155603) is a hypergiant and semiregular variable star, located 1718 pc away in the constellation Scorpius. Its apparent magnitude varies between 6.22 and 6.64, being heavily diminshed by 2.93 magnitudes due to interstellar extinction.

Surroundings
V915 Scorpii is surrounded by the sparse OB association Moffat 2. It is also surrounded by an envelope of dust and gas, producing a significant infrared excess.

V915 Scorpii has been classified as a triple star. 15" away is the Wolf-Rayet star WR 85, one of the most luminous stars known, but still visually four magnitudes fainter than V915 Scorpii. Component C is a 10th magnitude K class star 17" away. There is also a 14th magnitude star 22" away. Photometry and space motions suggest that only V915 Scorpii and WR 85 lie at the same distance, while the other two stars are foreground objects. Assumptions about the brightness of each star suggest a distance of $5,600$, and a projected separation of $1,718$.

Four arc minutes distant are two other assumed members of the association, a 10th magnitude B0 giant and an 11th magnitude OB star. Fitting the association members to a main sequence gives a highly uncertain distance of 1.8 kpc. A kinematical distance has been calculated for the bubble around WR 85 at 2.8 kpc. The distance to V915 Scorpii derived assuming minimal interstellar extinction is $2,600 pc$. However, the star is considerably reddened and this results in a distance of $0.2 pc$. Analysis of WR 85 as a luminous hydrogen-rich star gives a distance of $7,300 pc$. Bailer-Jones et al. 2018 estimates a distance of 1,720 pc. The parallax for WR 85 is considerably reliable and suggests a distance of around $2,630 pc$.

Variability
V915 Scorpii is classified as a semiregular variable, with an apparent magnitude that varies between 6.22 and 6.64. Any period associated with the variation is longer than 600 days.

The variability of V915 Scorpii was first announced by the Konkoly Observatory in 1979. It was given the variable-star designation V915 Scorpii in 1978.

Properties
V915 Scorpii has a spectral classification of K0 Ia-0, meaning that it is a massive, hypergiant star. It has a mass estimated at 8.5 or 14.7 times the mass of the Sun, while its radius is of, thus giving a volume equivalent to about 330 million times that of the Sun. V915 Scorpii is radiating a luminosity equivalent to 74,000 or 185,000 times the solar luminosity and has a surface effective temperature of 4,600K, giving it the typical orange color of a K-type star.

Despite being a K-type star, it is seen as red in the sky, with a B-V color index of 2.21. This happens because of massive interstellar reddenning between Earth and the star, its apparent brightness is diminished by 2.93 magnitudes due to interstellar dust.

The spectral type of V915 Scorpii was determined to be G5Ia in 1954, G5Ia-0 in 1973, G8Ia in 1977, K0Ia in 1982, and K0Ia-0 in 1989, all indicative of a luminous supergiant or hypergiant.

A 2001 paper published an angular diameter of 0.0032 arcseconds for this star.