Space is big. You know that. But when we talk about a big star in space, we aren't just talking about "large." We are talking about objects so physically massive they defy the basic logic of how matter should hold together. For a long time, if you asked an astronomer what the biggest thing with a heartbeat of nuclear fusion was, they’d point you toward the constellation Scutum. Specifically, they’d point to UY Scuti.
It's a red supergiant. A monster.
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If you replaced our Sun with UY Scuti, the surface of the star would extend past the orbit of Jupiter. It might even swallow Saturn. Think about that for a second. Earth, Mars, and the entire asteroid belt wouldn't just be "hot"—they would be deep inside the belly of a star. You’d be standing on a planet that had been turned into a marshmallow in a bonfire.
The Problem With Measuring a Big Star in Space
Honestly, measuring these things is a nightmare. It’s not like you can fly a tape measure out there. We rely on parallax and luminosity, but red supergiants are notoriously flaky. They pulsate. They breathe. UY Scuti changes its brightness over a period of roughly 740 days because its outer layers are literally expanding and contracting.
When a star is that big, its gravity at the surface is incredibly weak. Because the surface is so far from the core, the gas is barely held on. It creates this "fuzzy" atmosphere. Astronomers like those at the European Southern Observatory (ESO) have to figure out where the star actually ends and where the shed cocoons of gas and dust begin. This is why you’ll see different numbers in different papers. One year it’s 1,700 times the radius of the Sun; the next, a new study using Gaia mission data suggests it might be smaller, or perhaps further away than we thought.
It’s Not Just About UY Scuti Anymore
For years, UY Scuti held the crown. It was the undisputed heavyweight champion of the "big star in space" category. But space is a competitive place.
Enter Stephenson 2-18.
Located about 20,000 light-years away, this star is part of a massive young cluster. Some estimates put its radius at 2,150 times that of the Sun. If that's true, it’s so large that light—the fastest thing in the universe—would take nearly nine hours just to travel around its circumference. For comparison, light circles the Earth’s equator in about 0.13 seconds.
But here is the catch: Stephenson 2-18 might be cheating. Some astrophysicists argue that stars shouldn't technically be able to get that big. The Humphreys-Davidson limit suggests there’s a ceiling on how luminous and large a star can get before it simply blows itself apart. When we see stars that seem to break these rules, it usually means our distance measurements are slightly off, or we're looking at a star in a very brief, very unstable phase of its life.
The Life of a Giant Is Short and Violent
Big stars are the rock stars of the cosmos. They live fast and die young.
Our Sun is middle-aged at about 4.6 billion years. It’ll probably live another 5 billion. A big star in space like UY Scuti? It might only last a few million years. Because they are so massive, the pressure in their cores is unimaginable. They burn through their hydrogen fuel like a semi-truck with a leaking gas tank. Once the hydrogen is gone, they start fusing heavier and heavier elements—helium, carbon, neon, oxygen, silicon.
Eventually, they hit iron.
Iron is a dead end. Fusing iron doesn't produce energy; it consumes it. The moment iron starts to build up in the core of a red supergiant, the game is over. The outward pressure stops. Gravity, which has been waiting for this moment for millions of years, finally wins. The star collapses in on itself in a fraction of a second and then bounces back in a Type II Supernova.
Why Size Doesn't Equal Weight
This is the part that usually trips people up. Just because UY Scuti is 1,700 times wider than the Sun doesn't mean it’s 1,700 times heavier. In fact, it's only about 7 to 10 times the mass of the Sun.
Wait. Read that again.
It is thousands of times larger in volume but only about ten times heavier. That means it is incredibly "airy." Its density is lower than the air you're breathing right now. It's basically a glowing, radioactive vacuum. Most of the mass is concentrated in a tiny, hyper-dense core, while the rest of the star is just a bloated, tenuous shell of gas.
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Contrast that with something like R136a1.
R136a1 is a Wolf-Rayet star. It isn't the biggest star in terms of volume—it’s actually quite small compared to the supergiants—but it is the most massive star we know of. It’s about 200 to 300 times the mass of the Sun. It’s dense, blue, and screaming with radiation. If UY Scuti is a giant hot air balloon, R136a1 is a cannonball.
The Most Famous Giant: Betelgeuse
We can't talk about a big star in space without mentioning the one everyone knows: Betelgeuse.
Positioned in the "shoulder" of Orion, Betelgeuse is a red supergiant that gave everyone a heart attack back in 2019 and 2020. It started dimming. Fast. People thought, "This is it! It's going supernova!"
It didn't.
It turns out the star basically "coughed." It ejected a massive clump of plasma that cooled into dust, blocking our view of the star’s light. This is common for these giants. They are unstable. They are messy. They are literally falling apart as we watch them.
How to Find These Giants Yourself
You don't need a multi-billion dollar telescope to see some of these.
- Betelgeuse: Easy. Look for Orion in the winter sky (Northern Hemisphere). It’s the reddish star on the top left.
- Antares: The "Heart of the Scorpion." It’s another red supergiant, roughly 700 times the size of the Sun. You can see it in the constellation Scorpius during the summer.
- Mu Cephei: Also known as Herschel's "Garnet Star." It is one of the deepest red stars visible to the naked eye and is a massive supergiant in its own right.
What This Means for Us
Studying the big star in space isn't just about collecting records for the Guinness World Books. These stars are the factories of the universe. Every bit of oxygen you breathe, the calcium in your teeth, and the iron in your blood was forged inside the heart of a massive star that eventually blew up.
When we look at UY Scuti or Stephenson 2-18, we are looking at the future ancestors of new solar systems. Their deaths provide the raw materials for everything else.
Actionable Steps for Amateur Observers
If you're interested in keeping track of these celestial giants, stop just looking at static photos. The universe is dynamic.
- Download a Star Map App: Use something like Stellarium or SkyGuide. Search for "Scutum" to find where UY Scuti lives, though you'll need a decent telescope and a very dark sky to actually spot it since it's obscured by dust.
- Follow the AAVSO: The American Association of Variable Star Observers tracks the brightness of stars like Betelgeuse. You can actually see the data points of the dimming and brightening in real-time.
- Watch the Gaia Data Releases: The Gaia satellite is currently mapping the Milky Way with unprecedented precision. Every time a new "Data Release" (DR) comes out, the rankings of the "biggest stars" usually shift as our distance measurements get better.
- Check Local Astronomy Clubs: Most have "Star Parties." Ask someone to point out a red supergiant. Seeing the distinct orange-red hue of a dying giant through an eyepiece is a lot different than seeing it on a phone screen.
The "biggest star" title will likely change again. As our technology improves, we find more obscured clusters and better ways to peer through the galactic dust. But for now, UY Scuti and its cousin Stephenson 2-18 remain the gold standard for just how absurdly large the universe can get.