VX Sagittarii is an extreme asymptotic giant branch star located more than 1.5

kiloparsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

away from the Sun in the constellation of Sagittarius. It is a

pulsating variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...

with an unusually large

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

range. It is also one of the largest stars discovered so far, with a radius varying between . It is the most luminous known

AGB star The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) l ...

, at

bolometric magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it ...

–8.6, which is even brighter than the theoretical limit at –8.0.

Observations

The star is classed as a cool

semiregular variable In astronomy, a semiregular variable star, a type of variable star, is a giant or supergiant of intermediate and late (cooler) spectral type showing considerable periodicity in its light changes, accompanied or sometimes interrupted by various irre ...

of type SRc with a pulsational period of 732 days. The variations sometimes have an amplitude comparable to a

long period variable The descriptive term long-period variable star refers to various groups of cool luminous pulsating variable stars. It is frequently abbreviated to LPV. Types of variation The General Catalogue of Variable Stars does not define a long-period vari ...

, 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 In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

is Ia indicating a bright supergiant. The

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...

shows

emission line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to identi ...

s indicating that the star is losing mass through a strong stellar wind. The annual parallax of VX Sagittarii has been measured extremely accurately using

VLBI Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or in space. T ...

and found to be , 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 apparently highly 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 highly extended, irregular, and variable during the pulsations of the star, but the

bolometric luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a ...

varies much 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 __notoc__ An OH/IR star is an asymptotic giant branch (AGB) or a red supergiant or hypergiant (RSG or RHG) star that shows strong OH maser emission and is unusually bright at near-infrared wavelengths. In the very late stages of AGB evolution, ...

. The masers have been used to derive an accurate 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 Mira (), designation Omicron Ceti (ο Ceti, abbreviated Omicron Cet, ο Cet), is a red-giant star estimated to be 200–400 light-years from the Sun in the constellation Cetus. ο Ceti is a binary stellar system, consisting of a vari ...

variables, but a supergiant luminosity and size. More recent papers state 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.

Observations

Stellar characteristics

References

Further reading