Want to feel really small?

Meet UY Scuti, the largest star in the universe

by Brendan Hesse

Looking up at the sky inspires deep moments of introspection and curiosity. It’s easy to feel small under a starry night sky, but in order to begin to grasp just how small we truly are, we must know what our relative size is compared to the larger celestial bodies of the Galaxy — and what makes a better point of comparison than a star? Enter UY Scuti, a bright red supergiant variable star that resides within the Scutum constellation and is currently believed to be the largest star in the Milky Way galaxy.

German astronomers originally discovered UY Scuti at the Bonn Observatory in 1860, but it wasn’t until astronomers observed UY Scuti through the Very Large Telescope in Chile’s Atacama Desert in 2012 that the star’s true size became well documented. Following this discovery, UY Scuti was officially named the largest known star in the galaxy, surpassing previous record holders such as Betelgeuse, VY Canis Majoris, and NML Cygni.

While there are stars that are brighter and denser than UY Scuti, it has the largest overall size of any star currently known, with a radius of 1,708 ± 192 R☉. That figure amounts to somewhere between 1,054,378,000 and 1,321,450,000 miles in size, which is about 1,700 times larger than our Sun’s radius and 21 billion times the volume. Wrapping one’s head around such number can be difficult, so let’s break this down a bit.

Consider the Following.

From our point of view, Earth is pretty big compared to the size of a single human. But compared to even some of our close neighbors, our home is incredibly tiny. Let’s imagine for a moment that the Earth were an 8-inch diameter ball. At that scale, the Sun would be about 73 feet in diameter, which is a few feet more than the height of the White House. Our enormous, distant friend, UY Scuti, would be 125,000 feet in diameter, which is just a smidge under 24 miles. Remember that crazy Redbull-sponsored stunt a few years ago performed by Felix Baumgartner, who jumped from the outermost limit of Earth’s atmosphere back to the surface? That was 24 miles in the air. Now imagine a sphere that large. That’s how big UY Scuti is.

So, what would happen if UY Scuti were to swap positions with our Sun? The Star would engulf the the entirety of Jupiter’s orbit, swallowing the Sun, the first five planets of our solar system, and the asteroid belt without so much as a belch. Some speculate the star may be even larger, enough to surpass the orbit of Saturn. Not only that, but the UY Scuti’s gravity would gobble up the larger planet and distant planetoids of our solar system like a interstellar Pac-Man, and whatever remained unconsumed would take thousands of years to complete a single orbital rotation around the luminescent giant.

This video from Youtuber J Dude simulates what such an event would look like:

Hopefully, it’s beginning to sink in just how enormous UY Scuti is. We haven’t even touched the distance between us and the supermassive star — approximately 2.9 kiloparsecs, or 9,500 light years — but that’s a topic for another day.

It must be stressed that we’re constantly exploring, observing, and discovering new things about our galaxy and the trillions of celestial bodies that populate it. It stands to reason that we could uncover yet another, even larger star in the future. Our perspective, on a universal scale, is infinitesimally small. Even if UY Scuti is the largest star in the Milky Way, there could be enormous other stars in far away galaxies that dwarf those of our own, somewhere within this indescribably vast universe. Alas, we will may never know.