UY Scuti is a bright red supergiant or hypergiant pulsating variable star in the constellation
Scutum.
It is currently the largest known star at most likelihood, and is one of the most luminous
and best-known of its kind.
It has the average median radius of 1,708 solar radii, or a diameter of 2,378,704,272
km; thus a volume 4,982,686,912 times that of the Sun.
It is approximately 9,140 light-years from Earth, making it one of the nearest known
hypergiants.
If placed at the center of the Solar System, its photosphere would engulf the orbit of
Jupiter, although the radius is poorly known and may be larger than the orbit of Saturn.
Observations The star's name, UY Scuti, indicates that
it was the 38th variable star discovered in the constellation Scutum.
It is located a few degrees north of the A-type naked-eye star Gamma Scuti.
Although the star is very luminous it is, at its brightest, only 11th magnitude as viewed
from Earth, due to its distance and location in the Zone of Avoidance, within the Cygnus
rift.
It can be seen in large binoculars or a small telescope, but were it not for the dust extinction,
the star would be a 5th magnitude naked eye object.
Characteristics
The star is classified as a semiregular variable with the approximate pulsation period of 740
days.
Visually, the star is about 120,000 times brighter than the Sun in visible light.
However, when combined with its light in infrared it is about 340,000 times brighter than the
Sun making it one of the most luminous cool supergiant stars, and one of the most luminous
stars in the galaxy.
It also has the mass loss rate of 5.8×10−5M☉/yr, one of the highest known for any star.
Although it is an extremely large and luminous star showing broad emission lines in its spectrum,
some cited sources of its spectral type shows that it is of intermediate luminosity at minimum
brightness, making the classification of the star as a hypergiant or a bright supergiant
potentially unclear.
Size
Like other supergiant stars, UY Scuti is surrounded by a large circumstellar disc and like other
large diffuse stars is plagued by phenomena that make them difficult to measure with precision.
It is not entirely spherical, but being surrounded by opaque shells of a highly extended atmosphere,
which, coupled with limb darkening, unstable luminosity, high mass loss rate, and other
factors, make it difficult to determine its true size.
Torres et al gave UY Scuti's radius at 1,708 ± 192 solar radii, making it one of the largest
known stars, but this measurement is based through indirect methods of calculation of
the star's temperature and luminosity.
The definition of the boundary of such stars is based on the Rosseland Radius, the location
at which the optical depth is one.
In the case of UY Scuti, the radius is not clearly defined and may be on a different
value at a different wavelength.
UY Scuti's size alone makes it very intriguing and very complex.
Strictly speaking, the star is a very large and very luminous red supergiant.
The largest estimate of the star's size fall outside the bounds of the stellar evolutionary
theory, that is, beyond the expected size of a star of its temperature and luminosity.
Stars like UY Scuti are extremely rare, occurring, perhaps only one in a cube 1,500 light-years
on a side, or only a few dozen similar stars in our galaxy.
Consequently, they have yet to be studied properly.
Mass UY Scuti's mass is also uncertain, primarily
because it has no visible companion star in which its mass can be measured through orbital
measurements.
Stellar evolutionary models conclude that the initial mass of a star reaching the red
supergiant stage like UY Scuti would be no more than 25 to 50 times the sun, and would
have been lower this time.
Environment Its location within the Scutum-Centaurus Arm
suggests that this star may be a member of the Sagittarius OB5 association, a massive
grouping of stars 7,800 light-years from Earth in the direction of the Galactic Center.
The stellar association is one of the largest known in our galaxy, next only to Cygnus OB2.
Sgr OB5 also includes KW Sagittarii, a red supergiant and another of the largest stars
known, and the Sh 2-15 to Sh 2-20 complex of H II regions, massive nebulae which triggers
intense star formation.
Because of its position on the outskirts of Sgr OB5, the effects of the star on its stellar
environment are limited to regions away from the other hot luminous stars of the association.
Fate UY Scuti's large distance from the Sh 2-15
to Sh 2-20 complex of H II regions, in addition to its location within the Hertzsprung–Russell
diagram and its severe mass loss, shows it to be an old dying star that left the main
sequence a few million years ago.
Its exact future is unclear but will almost certainly involve a supernova explosion.
Stars of its size, luminosity, and mass do not appear to explode while they are red supergiants;
they evolve instead back to higher temperatures as they lose mass.
Current models of stellar evolution do not properly predict the proportion of massive
star types or the progenitors of different types of supernovae, and observations do not
show sufficient mass loss to account for the observed stars.
Depending on the exact rate of mass loss, it will explode as a core collapse supernova
at the yellow hypergiant, luminous blue variable, or Wolf-Rayet stage.
If it cannot lose sufficient mass before exploding, it will leave behind a black hole, otherwise
a neutron star will remain.
See also NML Cygni
VY Canis Majoris KW Sagittarii
AH Scorpii References