VX Sagittarii

VX Sagittarii is an asymptotic giant branch star located more than 1.5 kiloparsec away from the Sun in the constellation of Sagittarius. It is a pulsating variable star with an unusually large magnitude range. It is one of the largest stars discovered, with a radius varying between 1350 and 1940 solar radius. It is the most luminous known AGB star, at bolometric magnitude –8.6, which is brighter than the theoretical limit at –8.0.

Observations
The star is classed as a cool semiregular variable of type SRc with a pulsational period of 732 days. The variations sometimes have an amplitude comparable to a long period variable, at other times they are much smaller. The spectral type varies between M4e around visual maximum and M9.8e at minimum light, and the luminosity class is Ia indicating a bright supergiant. The spectrum shows emission lines indicating that the star is losing mass through a strong stellar wind.

The annual parallax of VX Sagittarii has been measured as $6.47$, indicating a distance of about 5,100 light years. This is compatible with the distance to Sagittarius OB1, the stellar association that VX Sagittarii is thought to belong to. Its radial velocity and proper motions are also consistent with other members of the association.

Stellar characteristics
The effective temperature of VX Sagittarii is variable from around 2,400 K at visual minimum to around 3,300 K near maximum. Such low temperatures are comparable to the very coolest AGB stars and unprecedented for a massive supergiant. Its atmosphere is extended, irregular, and variable during the pulsations of the star, but the bolometric luminosity varies less than the visual brightness and is calculated to be about. At an effective temperature of 3,300 K, the radius is expected to be somewhere between and. Older studies frequently calculated higher luminosities.

The atmosphere of VX Sgr shows molecular water layers and SiO masers in the atmosphere, typical of an OH/IR star. The masers have been used to derive a distance of 1,590 parsecs. The spectrum also indicates strong VO and CN. In many respects the atmosphere is similar to low mass AGB stars such as Mira variables, but with a supergiant's luminosity and size.

Another paper from May 2018 suggests that VX Sagitarii may be a hypergiant. This would make it one of the very rare red hypergiant stars. However, a 2021 paper concludes that VX Sagittarii is a massive AGB star, rather than a red supergiant or hypergiant. Because it displays rubidium in its spectrum and has a high mass loss and luminosity, it is possible that it is a type of AGB star known as a super-AGB star, a type of star with masses in between low-mass stars and high-mass stars.