UY Scuti

UY Scuti (BD-12 5055) is a red supergiant, and a pulsating variable star in the constellation Scutum. It is one of the largest known stars by radius and is also a variable star, with a maximum brightness of magnitude 8.29 and a minimum of magnitude 10.56.

Nomenclature and history

UY Scuti was first cataloged in 1860, by German astronomers at the Bonn Observatory, during the first sky survey of stars for the Bonner Durchmusterung Stellar Catalogue. It was named BD-12 5055, the 5,055th star between 12°S and 13°S counting from 0h right ascension.

On the next detection of the star in the second survey, it was found to have changed slightly in brightness, suggesting that it was a new variable star. In accordance with the international standard of designation of variable stars, it was called UY Scuti, the 38th variable star of the constellation Scutum (see variable star designation).

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 Sct is a dust-enshrouded red supergiant and is classified as a semiregular variable with an approximate pulsation period of 740 days.

In the summer of 2012, Arroyo-Torres et al. using AMBER interferometry from the Very Large Telescope (VLT) in the Atacama Desert in Chile measured 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).

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.

A hypothetical object travelling at the speed of light would take about seven hours to travel around UY Scuti at maximum, whereas it would take 14.5 seconds to circle the Sun.

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.