UY Scuti – 7 Facts About the Biggest(?) Monster Star

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Written By: Zane Landers
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UY Scuti star, one of the largest known stars, was first observed in 1860 by German astronomers at the Bonn Observatory during a survey of the brightest stars in the night sky. At the time of its discovery, it was named BD-12 5055. UY Scuti is located in the constellation Scutum and is thus best seen in the summer for Northern Hemisphere observers or winter in the Southern Hemisphere. Its precise coordinates in the sky are 18h 27m 36.5334s right ascension and -12° 27′ 58.866″ declination. Being only 12.5 degrees from the celestial equator, UY Scuti can be seen from pretty much anywhere on Earth except extremely close to the North Pole.

UY Scuti is approximately 9,500 light-years away from Earth. If we were to travel to UY Scuti at the speed of light, it would take us 9,500 years to reach it.

In addition to its official designation, UY Scuti is also known by several alternative names, including BD-12 5055, the name it was given upon its discovery, and IRAS 18248-1229, a designation it received from the Infrared Astronomical Satellite (IRAS) mission.

Here are some interesting facts about UY Scuti:

1. UY Scuti is a Red Supergiant

UY Scuti is a red supergiant star, boasting a size that is, according to the latest estimates, approximately 1700 times the radius of our Sun. Despite its enormous size, it is less than 40 times the mass of our Sun, though exactly how massive it is is still unclear. This discrepancy is due to the fact that red supergiants are extremely bloated stars, with much of their mass spread out over a large volume. 

Red supergiants like UY Scuti are formed from main sequence stars (albeit those with much higher mass than our Sun, probably a blue supergiant of class B or O) that have exhausted the hydrogen fuel in their cores. As a result, the core of the star contracts and heats up, igniting helium fusion and causing the outer layers of the star to expand.

The temperature of UY Scuti’s “surface”, or its photosphere, is relatively cool for a star due to its large size and low density. This is why it glows red, similar to the way a cooler metal will glow red when heated. As for the possibility of planets orbiting UY Scuti, it’s theoretically possible. However, the star’s intense heat and radiation, along with its strong stellar winds, would make life’s sustainability highly unlikely.

If UY Scuti were to go supernova, as it will sometime in the next few million years, it would not directly affect Earth due to its considerable distance from us. However, it would be a spectacular event to observe, potentially visible even in daylight.

2. It’s a Variable Star

UY Scuti is classified as a variable star due to its fluctuating brightness as seen from Earth. This variability is a result of its pulsations, a common phenomenon with red giant and supergiant stars where the star’s puffy outer layer expands and contracts, leading to periodic changes in its luminosity. The star’s absolute magnitude, a measure of its intrinsic brightness, is quite remarkable. However, its ranking in terms of visual brightness is not at the top due to its cooler temperature and the fact that a significant portion of its light is emitted in the infrared spectrum, which is invisible to the human eye.

With a variable magnitude between 8.29 and 10.56, UY Scuti is usually visible in 35mm or larger aperture binoculars and is always observable with even a small telescope in light-polluted conditions. At such a high brightness, a telescope of 8 inches or greater in aperture should show its crimson-orange coloration. You can also easily measure its variability by eye, though UY Scuti takes a rather long 740 days to pulsate from bright to dim and back to maximum brightness again. The American Association of Variable Star Observers collects data submitted by amateurs, so if you’re interested in attempting to make accurate brightness measurements of the star, you can actually contribute to the scientific research of UY Scuti from your backyard, too.

3. UY Scuti could be much brighter

UY Scuti resides in the Zone of Avoidance in the Milky Way – that is, the part of the sky nearest to the galactic center, and specifically behind the large Cygnus Rift of dust and gas. This material dims UY Scuti from our vantage point by quite a bit. The exact amount of dimming is unknown, and this affects numerous other measurements since, without an accurate brightness estimate, it is harder to infer other physical properties of the star.

UY Scuti seen from Rutherfurd Observatory
UY Scuti seen from Rutherfurd Observatory. Credit – Haktarfone

4. UY Scuti may not be the largest

Since stars are so far away, the angular diameter of most of them renders them pinpoints with even the largest ground-based telescopes. Large red supergiants like Betelgeuse, Antares, and a few others, as well as closer (albeit smaller) stars, can have their angular diameters measured via interferometry or simply huge apertures. UY Scuti has a large enough angular diameter that it indeed could have its disk resolved, but astronomical observatories have more important things to do than resolve every resolvable red supergiant in the sky, so nobody has measured UY Scuti. Furthermore, its distance was a subject of debate until relatively recently, partly due to the dust obstructing it.

The Gaia space telescope’s preliminary estimates seem to suggest UY Scuti is in fact twice as close as was previously measured, which means that with current estimates as to its angular diameter, it would in fact not be the largest star, and by quite a large margin. However, until Gaia’s mission is complete and someone measures UY Scuti’s exact angular diameter, we won’t quite know. Even with older estimates, the error for UY Scuti’s size is about 192 solar radii on either side, meaning that even at its previously-guessed position, its status was debatable.

Additionally, red supergiants like UY Scuti can physically pulsate in size and have ill-defined surface/atmosphere boundaries, as they are extremely diffuse.

The issue of exact distance, angular size, and what actually defines a red supergiant’s size plagues other stars as well. The red supergiants VY Canis Majoris, VV Cephei, Mu Cephei, and others all currently contest UY Scuti’s position as the largest known star, and it is possible that some other red supergiant is in fact bigger than them all due to measurement errors. It will take a long time to find out, as measuring the sizes of red giants and supergiants is again not high on the list of priorities on astronomers’ lists.

5. It’s not the most massive, and we don’t know its mass

UY Scuti has no known orbiting companions, so there is no way to measure its mass directly. The only other way to estimate its mass is to know its luminosity and size, and since its exact distance is unknown and its size is hotly contested, this is not possible either. It is basically guaranteed that UY Scuti is less than 40 solar masses, probably under 20, and it is possible that it is as little as 7 to 10 solar masses.

Because they’re so puffy and not dense in the slightest, red supergiants come nowhere near close to becoming the most massive stars. That title falls to blue supergiants and hypergiants, particularly those of the class known as Wolf-Rayet stars. The most massive star of all is R136A1, a Wolf-Rayet star in the Large Magellanic Cloud.

6. UY Scuti is Dying

Red supergiant stars typically evolve from O- and B-class main sequence stars. While massive, these stars burn through their hydrogen fuel extremely quickly—typically within 30 million years—and begin to burn helium, swelling into red supergiants. Red supergiants are unstable, often pulsating, and quickly changing to various different colors and spectral classes. Some, in the case of UY Scuti, actually begin to shed their outer layers while still burning fuel. Eventually, UY Scuti will explode in a relatively bright supernova, probably in under a million years from now. It will likely leave a black hole or neutron star behind, though some supernovae are so violent that they can result in no byproducts at all.

7. You could fit all of the inner planets and asteroids inside it

Assuming its (likely false) maximum possible size, you could nearly fit the entire solar system out to Saturn (about 8 astronomical units) within UY Scuti. Even with more conservative estimates, everything out to Jupiter (about 5 astronomical units)! Compare this to the Sun’s future maximum size when it becomes a red giant—a mere 1 astronomical unit or so—less than 20% of the size and 0.8% of the volume! Even Betelguese is smaller, at only 4 astronomical units—80% of the size and a little over half the volume of UY Scuti.

To give you an idea of just how big that is, if you haven’t understood already, UY Scuti is big enough to fit over 1 and a half billion Earths inside.

Zane Landers

An amateur astronomer and telescope maker from Connecticut who has been featured on TIME magazineNational GeographicLa Vanguardia, and Clarin, The Guardian, The Arizona Daily Star, and Astronomy Technology Today and had won the Stellafane 1st and 3rd place Junior Awards in the 2018 Convention. Zane has owned over 425 telescopes, of which around 400 he has actually gotten to take out under the stars. These range from the stuff we review on TelescopicWatch to homemade or antique telescopes; the oldest he has owned or worked on so far was an Emil Busch refractor made shortly before the outbreak of World War I. Many of these are telescopes that he repaired or built.

4 thoughts on “UY Scuti – 7 Facts About the Biggest(?) Monster Star”

  1. Uy scuti is the biggest star we have found but maybe not the biggest in out universe, but we do know its mass just not pitch perfect in its mass.

  2. Case in point, VY Canis Majoris is only 3,500 Kelvin, and a really big star would be even cooler. At 3,000 Kelvin, Humphreys estimates that cool supergiant would be as big as 2,600 times the size of the Sun.

    Mu Cephei (circled) as can be seen in binoculars. The bright star on the right is Alpha Cephei (Alderamin).

    Based on current data, VV Cephei A is at least 1,050 times larger than the Sun. If placed at the centre of the solar system, the star would extend to the orbit of Jupiter and possibly beyond.


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