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

UY Scuti is a red supergiant star in the constellation Scutum. With a variable magnitude between 8.29 and 10.56, it is frequently visible in binoculars and 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 back to maximum brightness again.

Here are some interesting facts about UY Scuti:

1. 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 by quite a bit. The exact amount of dimming is unknown, and this affects numerous other measurements.

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

2. It 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 don’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 the 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.

3. 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 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 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.

4. It’s 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.

5. 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 Betelgeuse 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.

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