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.
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.