UY Scuti

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UY Scuti is a red supergiant star located in the direction of Scutum constellation. With an estimated radius of 1,708 solar radii, it is a previous record holder for the largest star known. UY Scuti is classified as a pulsating variable star and its brightness varies from magnitude 8.29 to 10.56. As it is only 9th magnitude even at its brightest, the star is invisible to the naked eye. It can be seen in 4-inch and larger telescopes.

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UY Scuti, image: Wikimedia Commons/Haktarfone (CC BY-SA 3.0)

With an absolute magnitude of -6.2, UY Scuti is a highly luminous star – about 340,000 times more luminous than the Sun – but its distance (9,458 light years) and location in the region of the sky obscured by the Milky Way, known as the Zone of Avoidance, make it only a 9th magnitude star when seen from Earth.

The star is enshrouded in dust and does not have any visible companions, which would allow astronomers to measure its mass through gravitational interference. The estimated mass of UY Scuti is between 7 and 10 solar masses, and the star is losing mass at a rate of 5.8 x 10-5 solar masses per year, resulting in a large and complex envelope of dust surrounding it. The cloud of gas lost by the star stretches across some 400 astronomical units.


UY Scuti was designated BD-12 5055 by a group of German astronomers at the Bonn Observatory, who were the first to catalogue the star in 1860, while conducting a survey for the Bonner Durchmusterung (BD) astrometric star catalogue. The second survey showed a slight change in the star’s brightness and the star was subsequently given the variable star designation UY Scuti, as the 38th variable star found in Scutum. UY Scuti has a pulsation period of about 740 days.

The star’s true size was calculated after a survey with the Very Large Telescope (VLT) in the Atacama Desert in Chile, conducted in 2012 to measure the parameters of three large stars in the region of the sky near the Galactic Centre. The other two stars were AH Scorpii and KW Sagittarii, also red supergiants with a luminosity over 100,000 times that of the Sun and among the biggest stars known. Following the survey, UY Scuti was named the largest known star, leaving behind VY Canis Majoris, NML Cygni and Betelgeuse, which had all previously carried the title.

A star’s size does not always correlate with its mass. In spite of its enormous size – 7 to 8 astronomical units across – UY Scuti does not even make the list of the most massive stars known. With an estimated mass between 7 and 10 solar masses, it is not even in the same ballpark as the current record holder R136a1 (315 solar masses), located in the Tarantula Nebula in the Large Magellanic Cloud. For reference, R136a1 has an estimated radius of only about 30 solar radii. In terms of mass, UY Scuti also does not come close to the famous Eta Carinae A (between 100 and 200 solar masses), the central star of the Carina Nebula, or to the Peony Star (WR 102ka, 100 solar masses), which illuminates the Peony Nebula.

The margin of error on the measurement of UY Scuti’s size is 192 solar radii, which means that there is a possibility that it may not be as large as WOH G64 (1,540 – 1,730 R), the largest star in the Large Magellanic Cloud.  It certainly does not come close to the current record holders, Stephenson 2-18 (2,150 R) and MY Cephei. (2,061 R) As UY Scuti belongs to the class of variable stars that vary in brightness because they vary in size, the star’s radius will probably change over time.

More recent measurements of the star’s parallax by the Gaia space observatory have yielded significantly lower values for the star’s distance (5,100 light years) and consequently luminosity (86,300–87,100 L) and radius (755 R). However, the Gaia measurements may have been affected by astrometric noise and are not completely reliable.


The size of UY Scuti is about 750 million miles, or almost eight astronomical units. If the star were placed at the centre of our solar system, it would extend far beyond the orbit of Jupiter, closer to the orbit of Saturn.

To illustrate, if the Earth were the size of a ball 20 cm (8 inches) in diameter, Jupiter would be 2.1 m (7 feet) in diameter, the Sun would be 22 m (7 feet) across, and the diameter of UY Scuti would extend for about 38,000 m (125,000 feet). A hypothetical object moving at the speed of light would take only 14.5 seconds to travel around the Sun, while it would take about seven hours to circle around UY Scuti.

Size comparison

UY Scuti compared to the Sun

uy scuti vs sun

UY Scuti compared to the Sun, image: Wikimedia Commons/Philip Park (CC BY-SA 3.0)

UY Scuti compared to VY Canis Majoris

UY Scuti vs VY Canis Majoris

An accurate comparison showing the red supergiants UY Scuti and VY Canis Majoris, image: Wikimedia Commons/Faren29 (CC BY-SA 4.0)

UY Scuti compared to planets and other large stars

UY Scuti compared to other stars, image: Wikimedia Commons/IStoleThePies (CC BY-SA 4.0)


Based on UY Scuti’s known properties, the star fuses hydrogen around its core and has begun to fuse helium. It will eventually become a yellow hypergiant, a Wolf-Rayet star, or a luminous blue variable. The star’s location in the galactic disc indicates that it is metal-rich and, once it runs out of helium, fusing heavy elements will result in the star’s core producing iron and disturbing the balance needed to sustain the core against its own gravity. This will trigger a core collapse and, as a result, the star will violently expel its outer layers and end its life in a supernova explosion. When it does, the X-ray and gamma ray radiation will not affect Earth. For a supernova blast to damage our ozone layer, it would need to occur within 50 light years and UY Scuti lies 5,100 light years away.


UY Scuti lies two degrees north of the white magnitude 4.67 star Gamma Scuti, in the region of the sky just northeast of the famous Eagle Nebula (Messier 16). The star can be observed in a 4-inch or larger telescope in good conditions.  The open cluster NGC 6604 (mag. 6.5) is located in the same area of the sky.

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UY Scuti location, image: Wikisky


UY Scuti is located in the constellation Scutum, the Shield. The constellation was created in the 17th century and originally named Scutum Sobiescianum in honour of the Polish King John III Sobieski and his triumph in the 1683 Battle of Vienna.

scutum stars

Scutum constellation, image: Roberto Mura

Scutum is one of the smallest constellations in the sky (84th in size out of 88), but contains several relatively bright deep sky objects that are popular targets for stargazers. These are the famous Wild Duck Cluster (Messier 11), an exceptionally rich and densely populated open cluster with an apparent magnitude of 5.8, the considerably fainter (mag. 8) open cluster Messier 26, the globular cluster NGC 6712 (mag 8.69), and the planetary nebula IC 1295 (mag. 12.7).

Scutum is also home to the famous variable star Delta Scuti, an F-type giant that serves as a prototype for its own class of variable stars.

None of the stars in Scutum are among the 300 brightest stars in the sky. The 10 brightest stars in the constellation are Alpha Scuti (mag. 3.83), Beta Scuti (mag. 4.22), Zeta Scuti (mag. 4.66), Gamma Scuti (mag. 4.67), Delta Scuti (mag. 4.60 – 4.79), Eta Scuti (mag. 4.83), Epsilon Scuti (mag. 4.88), HD 175156 (mag. 5.08), HD 171391 (mag. 5.12), and R Scuti (mag. 4.2 – 8.6).

UY Scuti

Spectral classM2-M4Ia-Iab
Variable type:Semiregular variable (SRc)
U-B colour index+3.29
B-V colour index+3.00
Apparent magnitude8.29 – 10.56
Absolute magnitude-6.2
Distance9,458 (2,900 parsecs) or 5,100 light years (1,600 parsecs)
Parallax0.6433 ± 0.1059 mas
Radial velocity18.33 ±0.82 km/s
Proper motionRA: 1.3 mas/yr
Dec.: -1.6 mas/yr
Mass7-10 M
Luminosity340,000 L (180,000 – 630,000 L)
Radius1,708 ±192 R
Temperature3,365 ± 134 K
Surface gravity-0.5 cgs
Right ascension18h 27m 36.5334s
Declination-12° 27′ 58.866”
DesignationsUY Scuti, UY Sct, BD-2 5055, HV 3805, IRC-10422, RAFGL 21162, GSC 05698-05176, IRAS 18248-1229, TYC 5698-5176-1, Gaia DR2 4152993273702130432, PPM 234561