UY Scuti

DSS2 image of red supergiant star UY Scuti (brightest star in the image), surrounded by a dense starfield.
Observation data Epoch J2000      Equinox J2000
Constellation	Scutum
Right ascension	18h 27m 36.5334s[1]
Declination	−12° 27′ 58.866″[1]
Apparent magnitude (V)	8.29 - 10.56[2]
Characteristics
Evolutionary stage	Red supergiant[3]
Spectral type	M2-M4Ia-Iab[2]
U−B color index	+3.29[4]
B−V color index	+3.00[5]
Variable type	SRc[6]
Astrometry
Radial velocity (Rv)	18.33±0.82[7] km/s
Proper motion (μ)	RA: 1.3[8] mas/yr Dec.: −1.6[8] mas/yr
Parallax (π)	0.5166 ± 0.0494 mas[9]
Distance	5,871+534 −446[10] ly (1,800+164 −137[10] pc)
Absolute magnitude (MV)	−6.2[11]
Details
Radius	909[12] R☉
Luminosity	124,000[12] L☉
Temperature	3,550[12] K
Other designations
UY Sct, BD−12°5055, IRC −10422, RAFGL 2162, HV 3805
Database references
SIMBAD	data

UY Scuti (BD-12°5055) is a red supergiant star in the constellation Scutum. It is considered to be possibly one of the largest known stars and is also a pulsating variable star, with a maximum brightness of magnitude 8.29 and a minimum of magnitude 10.56. Its radius has been given various estimates with high uncertainty, including 1,708 ± 192 solar radii (1,190 ± 134 million kilometres; 7.94 ± 0.893 astronomical units), thus a volume nearly 5 billion times that of the Sun, as based on the modelling spectrum by the Very Large Telescope (VLT),^[4][13][11] and 755 solar radii (525 million kilometres; 3.51 astronomical units), thus a volume over 430 million times that of the Sun, as based on parallax measurements by the Gaia DR2 database.^[14] It is approximately 1.8 kiloparsecs (5,900 light-years) from Earth as measured by the Gaia EDR3 database.^[10] These estimates imply that if it were placed at the center of the Solar System, its photosphere would approach, or possibly extend beyond, 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.^[16] 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.^[17]

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.^[18]

Characteristics

UY Scuti is a dust-enshrouded bright red supergiant^[19] and is classified as a semiregular variable with an approximate pulsation period of 740 days.^[6][20][21] Based on an old radius data 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.^[22]

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:^[4] 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,^[13] 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_☉ (1.188×10⁹ ± 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 Scuti.^[11] 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).^[4]

Direct measurements of the parallax of UY Scuti published in the Gaia Data Release 2 give a parallax of 0.6433±0.1059 mas,^[7] implying a closer distance of approximately 1.5 kiloparsecs (4,900 ly),^[23] and consequently much lower luminosity and radius values of around 86,300–87,100 L_☉ and 755 R_☉ respectively.^[24] However, the Gaia parallax might be unreliable, at least until further observations, due to a very high level of astrometric noise.^[7]

The distance of UY Scuti has been re-measured, based on noiseless data from Gaia EDR3, giving a much closer distance 1,800+164
−137 pc away from Earth.^[10]

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_☉.^[4] Mass is being lost at 5.8×10⁻⁵ M_☉ per year, leading to an extensive and complex circumstellar environment of gas and dust.^[25]

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.^[26]

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.^[27]