The Largest Star We Know – UY Scuti Size, Scale and Facts

UY ScutiThe next time you point your telescopes to the sky, look for the Eagle Nebula. Turn your telescope a little bit to the northwest and you’ll see a bright, reddish spot in the sky. That is not any ordinary star – it is the largest star we have ever seen.

It is UY Scuti.

How far away is UY Scuti?

9500 light years away, UY Scuti is in a dense star field in the so-called Zone of Avoidance in space – the area of space obscured by the Milky Way’s disk. The star is so gigantic, and so luminous, that even with the impending stars and space dust, you can see it with binoculars on a dark night.

UY Scuti diameter – The largest star in the galaxy is a beautiful and unusual one. It is hard to say exactly how large UY Scuti is – it is really, really far from us and it is a variable star, which means it changes its luminosity. That makes it difficult to judge it’s size accurately – UY Scuti pulsates once every 740 days. By our best estimates, the star is about 7 AU across.

So what does that mean?
To get a clearer perspective, let’s do a quick thought experiment. If you dropped UY Scuti in the middle of our solar system, where the sun is supposed to be, what would happen?

Whenever you compare UY Scuti vs Sun, our beloved home star is totally dwarfed by the former. This thing has 5 billion times the volume of the sun. The star would completely engulf the solar system all the way till Saturn. It might just consume Saturn as well. It would cause a giant supernova, of course, as the Sun would make UY Scuti’s core unstable.

In comparison to UY Scuti, the celestial bodies we are around are like little playthings. We once thought Betelgeuse was the largest star we could seem – it’s about a 1000 timesmthe size of our sun. But UY Scuti soared past with a size 1.7 times bigger than Betelgeuse. UY Scuti is only slightly larger than the second largest star we know, VY Canis Majoris. Unfortunately for Betelgeuse, it has been bumped to third place. The sun, and our home planet of Earth have long disappeared into an infinitesimal point on the scale.

Here’s a quick comparison of sizes between our solar system’s bodies, and the largest stars we know.

These comparisons are all well and good, but let’s get an actual, conceivable picture in our heads for it, shall we?

UY Scuti vs Earth – Let’s suppose the Earth is the size of a football (soccer, America). In this scenario, the sun would be 22 metres wide, about the size of a seven story building.

And UY Scuti would be 38 km wide – four times the height of Mt Everest.

UY Scuti vs Sun
UY Scuti vs Sun

Phew! Look at that.

The size of the star is expressed by its sheer visibility – that side of the Universe is called the Zone of Avoidance for a reason – our telescopes have trouble telling apart stars and distant galaxies in the rich mix of gas, stellar bodies and a myriad of other celestial objects clouding the Milky Way.

UY Scuti facts – Would life be able to sustain itself around a star that size?

Let’s do another fun thought experiment.

If one were to imagine the habitable zone around a star the size of UY Scuti, what would it be like?

The habitable zone – the orbital zone with the highest probability of life – is a tricky thing whose influence can vary depending upon several factors. By our best calculations, the habitable zone around UY Scuti would be between 700 to 1300 AU.

That is an insanely large distance. The number in kilometres is simply incomprehensible – it’s about 149,597,870,700 km, if you were to take an average.

That is 1/100th of a light year, and it would take light from UY Scuti 30 to 40 days to reach the zone. If a habitable planet is at a safe distance of say, 923 AU from the star, the year on said planet would be 9612 Earth years long. That’s almost 2500 hundred years of winter! And 2500 year of summer. Entire generations would pass that only know one season.  

UY Scuti might actually have an existing planetary system in that zone, but if it does, it won’t last very long. Currently, UY Scuti is fusing helium in its core. This means, that as it runs out of helium, the star will begin to fuse heavier elements like lithium, carbon, oxygen, neon and silicon. In a million years or so (not very long on astronomical timelines) the star will begin to produce iron. At this time, it’s core would collapse in a stunningly destructive supernova which will be quite a show for us. (PS: We are in NO DANGER from that supernova)

After the supernova, UY Scuti will most likely turn into either a yellow hypergiant, a blue variable star or even a Wolf-Rayet star. If it does become a Wolf-Rayet star, it will birth many new stars in the wake of its supernova.

UY Scuti location

Even with its monstrous size, UY Scuti’s brightness drops to +9 magnitude and sometimes as low as +11, thanks to the obscurity of the Zone of Avoidance and the 9500 light years between us.

The star is still an easy catch with any 4 inch or larger telescopes on decently dark nights. Find the constellation Scutum. UY Scuti is located about 2° north of 5th-magnitude Gamma (γ) Scuti and 4° southwest of 4th-magnitude Alpha (α) Scuti just below the tail of Aquila.

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