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. The atmosphere of VX Sgr shows molecular water layers and SiO masers in the atmosphere, typical of an
OH/IR star. The SiO masers have been used to derive a distance of . 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. VX Sagittarii's pulsations alternate between a longer, active phase and a shorter, quiescent phase.
Interferometric observations from 2018 to 2025, carried with the
GRAVITY instrument aboard the
Very Large Telescope, showed that during a quiescent phase, the star was pulsating in the fundamental mode with a mean radius of and an amplitude of about , while in the quiescent phase, it was pulsating in the first
overtone with a mean radius of and an amplitude of only . In September 2019, the star increased to its maxium radius, at . During 2020 and 2021, the star experienced a great dimming event, at the end of the active pulsation phase. This event happened due to extreme mass loss from the star and was characterized by the expansion of
carbon monoxide and
water atmospheric layers up to in August 2021, appearence of
Balmer emission in the
spectrum from mid-2020 to mid-2021, and
Brγ hydrogen lines in the spectrum in March 2020. This is also the first detection of Brγ lines in a single-
system red supergiant. One paper from May 2018 suggests that VX Sagitarii may be a
hypergiant. This would make it one of the very rare red hypergiant stars. A 2021 paper, however, concluded 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 may be 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. A 2026 publication found that it is very unlikely that VX Sagittarii is a super-AGB star, but instead a supergiant, as deduced from its high luminosity, large radius, and pulsational properties, which are only consistent with those of red supergiants. == See also ==