It is the only star that we can see an actual disk and not just a pinpoint of light. It is a Red Giant and is roughly around 765 million miles in diameter.
Or, to think of it another way, you could fit 1.3 million Earths inside our Sun. You could fit around 700 million Suns inside Betelgeuse. Along with their great size, stars have great mass. .
Astronomer Spiderman says - with great mass comes great variability.