UY Scuti

**UY Scuti** (**BD-12°5055**) is a red supergiant star in the constellation Scutum. It is considered one of the largest known stars by radius and is also a pulsating variable star, with a maximum brightness of magnitude 8.29 and a minimum of magnitude 10.56. It has an estimated radius of 755 solar radii; thus a volume nearly 430 million times that of the Sun. It is approximately 2.9 kiloparsecs (9,500 light-years) from Earth. If placed at the center of the Solar System, its photosphere would almost reach the orbit of Jupiter.

## 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.

The star is classified as a semiregular variable with an approximate pulsation period of 740 days. It has the total bolometric power of 340,000 L☉.

Despite its large size, UY Scuti is not classified as a hypergiant. There is an MKK luminosity class 0 (zero) for hypergiants, but this is rarely seen in published spectral classifications. More commonly, hypergiants are classed as Ia-0, Ia+, or even just Iae based solely on the observed spectra and red supergiants rarely receive these extra spectral classifications. A high luminosity and large size are insufficient for it to be defined as a hypergiant. That requires the detection of the spectral signatures of atmospheric instability and high mass loss.

UY Scuti was initially reported as 9.0 intensity, then was before downgrade it to 8.1, and then to a 7.6 intensity is correct.

## Size

In the summer of 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 found to be the largest and the most luminous of the three stars measured, at 1,708 ± 192 *R*☉ (1.188×109 ± 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 Sct. 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 hypothetical object travelling at the speed of light would be observed to take about seven hours to travel along UY Scuti's great circle whereas it would take 14.5 seconds to circle the Sun.

Direct measurements of parallax of UY Sct by the Gaia Data Release 2 have recently given a parallax of 0.6433 ± 0.1059 mas, yielding a much lower distance of approximately 1.55 kiloparsecs (5,100 ly), and consequently much lower luminosity and radius values of around 86,300–87,100 L☉ and 755 R☉ respectively.

UY Scuti's mass is uncertain, primarily because it has no visible companion star by which its mass can be measured through gravitational interference. However, it is expected 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 stars 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.