UY Scuti

UY Scuti (BD-12°5055) is an extreme red hypergiant or 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 , thus a volume nearly 5billion times that of the Sun. It is approximately  from Earth. If placed at the center of the Solar System, its photosphere would at least engulf 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.

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 a radius of , this pulsation would be an overtone of the fundamental pulsation period, or it may be a fundamental mode corresponding to a smaller radius.

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 , with distances adopted from earlier publications.  UY Scuti was found to be the largest and the most luminous of the three stars measured, at  based on an angular diameter of  and an assumed distance of  (kpc) (about ) which was originally derived in 1970 based on the modelling of the spectrum of UY Scuti.  The luminosity is then calculated to be  at an effective temperature of 3,365 ± 134 K, giving an initial mass of  (possibly up to  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 the parallax of UY Scuti published in the Gaia Data Release 2 give a parallax of , implying a closer distance of approximately , and consequently much lower luminosity and radius values of around  and  respectively. However, the Gaia parallax might be unreliable, at least until further observations, due to a very high level of astrometric noise.  Gaia Early Data Release 3 has published a parallax of  for this star, again with a large value for astrometric noise, rated at a significance of 122 where anything over 2 is "probably significant".

UY Scuti has no known companion star and so its mass is uncertain. However, it is expected on theoretical grounds to be between . Mass is being lost at  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.

See also
 Stephenson 2 DFK 1
 AH Scorpii
 KW Sagittarii

References

Scutum (constellation)
Semiregular variable stars
Scuti, UY
M-type supergiants
Durchmusterung objects
TIC objects
M-type hypergiants
Population I stars