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.[15]
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.[16]
Characteristics
UY Scuti is a dust-enshrouded bright red supergiant[17] and is classified as a semiregular variable with an approximate pulsation period of 740 days.[6][18][19] 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.[20]
In the summer of 2012,
AMBERinterferometry 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,[21] 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),[22] and consequently much lower luminosity and radius values of around 86,300–87,100 L☉ and 755 R☉ respectively.[23] 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]
A measurement based on the multimessenger monitoring of supernovae, puts the radius at 909 R☉ with an estimated luminosity of 124,000 L☉ and effective temperature of 3,550 K.[12]
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 5.8×10−5M☉ per year, leading to an extensive and complex circumstellar environment of gas and dust.[24]
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.[25]
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.[26]
^Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0.
Bibcode:
2002yCat.2237....0D.
^Prager, R. (1927). "Katalog und Ephemeriden veraenderlicher Sterne fuer 1927". Kleine Veroeffentlichungen der Universitaetssternwarte zu Berlin Babelsberg. 1: 1.i.
Bibcode:
1927KVeBB...1....1P.
^Whiting, Wendy A. (1978). "Observations of Three Variable Stars in Scutum". The Journal of the American Association of Variable Star Observers. 7 (2): 71.
Bibcode:
1978JAVSO...7...71W.