# UY Scuti

UY Scuti (BD-12°5055) is a red supergiant star, located 5,900 light-years away in the constellation Scutum. It is also a pulsating variable star, with a maximum brightness of magnitude 8.29 and a minimum of magnitude 10.56, which is too dim for naked-eye visibility. It is considered to be one of the largest known stars, with a radius estimated at 909 solar radii (632 million kilometres; 4.23 astronomical units), thus a volume of 750 million times that of the Sun. This estimate implies if it were placed at the center of the Solar System, its photosphere would extend past the orbit of Mars or even the asteroid belt.

## Nomenclature and history

UY Scuti was first catalogued in 1860 by German astronomers at the Bonn Observatory, who were completing a survey of stars for the Bonner Durchmusterung Stellar Catalogue. It was designated BD-12°5055, the 5,055th star between 12°S and 13°S counting from 0h right ascension.
On detection in the second survey, the star was found to have changed slightly in brightness, suggesting that it was a new variable star. In accordance with the international standard for designation of variable stars, it was called UY Scuti, denoting it as the 38th variable star of the constellation Scutum.
UY Scuti is located a few degrees north of the A-type star Gamma Scuti and northeast of the Eagle Nebula. Although the star is very luminous, it is, at its brightest, only 9th magnitude as viewed from Earth, due to its distance and location in the Zone of Avoidance within the Cygnus rift.

## Characteristics

UY Scuti is a dust-enshrouded bright red supergiant and is classified as a semiregular variable with an approximate pulsation period of 740 days. Based on an old radius of 1,708 R$_☉$, this pulsation would be an overtone of the fundamental pulsation period, or it may be a fundamental mode corresponding to a smaller radius.
In mid 2012, AMBER interferometry with the Very Large Telescope (VLT) in the Atacama Desert in Chile was used to measure the parameters of three red supergiants near the Galactic Center region: UY Scuti, AH Scorpii, and KW Sagittarii. They determined that all three stars are over 1,000 times bigger than the Sun and over 100,000 times more luminous than the Sun. The stars' sizes were calculated using the Rosseland radius, the location at which the optical depth is 2⁄3, with distances adopted from earlier publications. UY Scuti was analyzed to be the largest and the most luminous of the three stars measured, at 1,708 ± 192 R$_☉$ (1.188×10$^9$ ± 134,000,000 km; 7.94 ± 0.89 AU) based on an angular diameter of 5.48±0.10 mas and an assumed distance of 2.9±0.317 kiloparsecs (kpc) (about 9,500±1,030 light-years) which was originally derived in 1970 based on the modelling of the spectrum of UY Scuti. The luminosity is then calculated to be 340,000 L$_☉$ at an effective temperature of 3,365±134 K, giving an initial mass of 25 M$_☉$ (possibly up to 40 M$_☉$ for a non-rotating star).
A 2023 measurement based on the multimessenger monitoring of supernovae, puts the radius at a smaller value of 909 R$_☉$, together with a smaller luminosity of 124,000 L$_☉$ and effective temperature of 3,550 K. Direct measurements of the parallax of UY Scuti published in the Gaia Data Release 2 give a parallax of 0.6433±0.1059 mas, implying a closer distance of approximately 1.5 kiloparsecs (4,900 ly), and consequently much lower luminosity and radius values of around 86,300–87,100 L$_☉$ and 755 R$_☉$ respectively. However, the Gaia parallax might be unreliable, at least until further observations, due to a very high level of astrometric noise.
The distance of UY Scuti has been re-measured by Bailer-Jones et al. in 2021, based on a method that uses the stellar parallax from Gaia EDR3, its color and apparent brightness, giving it a much closer distance of 1,800 pc (5,900 ly).
UY Scuti has no known companion star and so its mass is uncertain. However, it is expected on theoretical grounds to be between 7 and 10 M$_☉$. Mass is being lost at 5.8×10$^{−5}$ M$_☉$ per year, leading to an extensive and complex circumstellar environment of gas and dust.

## Supernova

Based on current models of stellar evolution, UY Scuti has begun to fuse helium and continues to fuse hydrogen in a shell around the core. The location of UY Scuti deep within the Milky Way disc suggests that it is a metal-rich star.
After fusing heavy elements, its core will begin to produce iron, disrupting the balance of gravity and radiation in its core and resulting in a core collapse supernova. It is expected that a star like UY Scuti should evolve back to hotter temperatures to become a yellow hypergiant, luminous blue variable, or a Wolf–Rayet star, creating a strong stellar wind that will eject its outer layers and expose the core, before exploding as a type IIb, IIn, or type Ib/Ic supernova.