What is the Biggest Star in the Universe?

by Fraser Cain on May 13, 2013

This article was originally published in 2008, but I’ve updated it and added this cool video.

My six-year old daughter is a question asking machine. We were driving home from school a couple of days ago, and she was grilling me about the nature of the Universe. One of her zingers was this, “What’s the Biggest Star in the Universe”? I had an easy answer, the Universe is a big place, and there’s no way we can possibly know what the biggest star is. But that’s not a real answer. So she refined the question. What’s the biggest star that we know of?

Of course, I was stuck in the car, and without access to the Internet. But now I’m back at home, doing some research, and I thought I’d share the answer with the rest of you too.

Before we jump straight to the answer, let’s take a look at our own Sun for a sense of scale. Our familiar star is a mighty 1.4 million km across (870,000 miles). That’s such a huge number that it’s hard to get a sense of scale. The Sun accounts for 99.9% of all the matter in our Solar System. In fact, you could fit one million planet Earths inside the Sun.

Astronomers use the terms “solar radius” and “solar mass” to compare large and smaller stars, so we’ll do the same. A solar radius is 690,000 km (432,000 miles) and 1 solar mass is 2 x 1030 kilograms (4.3 x 1030 pounds). That’s 2,000,000,000,000,000,000,000,000,000,000 kg.

One huge, famous star in our galaxy is the monster Eta Carinae, located approximately 7,500 light years away, and weighing in at 120 solar masses. It’s a million times as bright as the Sun. Most stars blow with a solar wind, losing mass over time. But Eta Carinae is so large that it casts off 500 times the mass of the Earth every year. With so much mass lost, it’s very difficult for astronomers to accurately measure where the star ends, and its stellar wind begins.

So the best answer astronomers have right now is that Eta Carinae’s radius is 250 times the size of the Sun. And as star size estimates go, that’s pretty accurate.

And one interesting side note: Eta Carinae should explode pretty soon as one of the most spectacular supernovae humans have ever seen.

But the most massive star in the Universe is thought to be R136a1, located in the Large Magellanic Cloud. There’s some controversy, but it might contain as much as 265 times the mass of the Sun. And this is a puzzle for astronomers, since the largest theoretical stars were thought to be about 150 solar masses, formed in the early Universe, when stars were made of hydrogen and helium left over from the Big Bang. The answer to this contradiction is that R136a1 was probably formed when several massive stars merged together. Needless to say, R136a1 is set to detonate as a hypernova, any day now.

In terms of large stars, let’s look at Betelgeuse, the familiar star located in the shoulder of Orion. This red supergiant star has a radius of 950-1200 times the size of the Sun, and would engulf the orbit of Jupiter if placed in our Solar System.

But that’s nothing. The largest known star is VY Canis Majoris; a red hypergiant star in the Canis Major constellation, located about 5,000 light-years from Earth. University of Minnesota professor Roberta Humphreys recently calculated its upper size at more than 1,540 times the size of the Sun. Placed in our Solar System, its surface would extend out past the orbit of Saturn.

That’s the biggest star that we know of, but the Milky way probably has dozens of stars that are even larger, obscured by gas and dust so we can’t see them.

But let’s see if we can work out the original question, what’s the biggest star in the Universe? Obviously, it’s impossible for us to actually find it – the Universe is a big place, and there’s no way we can peer into every corner.
Pistol Star, another star thought to be one of the largest. Image credit: Hubble
But according to theorists, how big can stars get?

I contacted Roberta Humphreys from the University of Minnesota, the researcher who calculated the size of VY Canis Majoris, and posed this question to her. She noted that the largest stars are the coolest. So even though Eta Carinae is the most luminous star we know of, it’s extremely hot – 25,000 Kelvin – and so only a mere 250 solar radii.

The largest stars will be the cool supergiants. For example, VY Canis Majoris is only 3,500 Kelvin. A really big star would be even cooler. At 3,000 Kelvin, a cool supergiant would be 2,600 times the size of the Sun.

That, she believes, is the largest possible star.

Finally, here’s a great animation that shows the size of various objects in space, starting with our tiny planet and finally getting to VV Cephei A. I guess they didn’t have the new info on VY Canis Majoris to include it in the animation.

We have written many articles about stars for Universe Today. Here’s a list of the 10 brightest stars.

Want to learn more about the birth and death of stars? We did a two part podcast at Astronomy Cast. Here’s part 1, Where Stars Come From, and here’s part 2, How Stars Die.

NASA: Eta Carinae
NASA: What is the biggest star we know?


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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