Insprired by the Super Giant Star thread:
What is the biggest star (size and/or mass and/or temperature) known to science ?
Do black holes count as stars?
Back when I was a kid, this one was touted as the physically largest star ever discovered.
No, not the slightly-more-than-one-AU class F supergiant, but the 20-AU “unknown something” that it orbits every 27.1 years!
Back when I was a kid (and a bit of an astronomy geek), it was thought to be a 2+ billion mile wide star. I see from the description on this site that the big dark mysterious item is no longer necessarily thought of as a star, though.
You can still find a site or two that do describe it as a helluva big star, though.
Actually, no one knows what the biggest star is, even of the known stars. It’s very difficult to know the radius of these giant stars, because the physics of their atmospheres is not well understood. And how do you define their size? Stars like Betelgeuse (in Orion) look like they just kinda fade away as you move outward from the center. In a way, it’s like asking how high up our atmosphere goes on Earth: you have to define what you mean, because our atmosphere just keeps getting thinner with height until it blends with space.
Basically, you can make a list of stars that we know are huge. Eta Carina is a good one. So are some of the others listed here in this thread. Mu Cephei is another. I am not sure just how well known the mass of the Pistol star is; I have heard that the observations didn’t nail it down all that well. It could be a less massive star, but fooling us into thinking it’s more. I need to look into this, because it’s kinda cool (and I am a big geek).
Here’s an Astronomy Picture of the Day about the Pistol Star: