Why is Stephenson 2-18 so large? How does a star as large as Stephenson 2-18 get this big?  It is 2500 times larger than the sun.
Did it start out as a large star with a lot of gas left over that gradually consumed the gas growing larger?  Or did all of the gas collapse at one time?
If it gradually kept consuming the gas cloud, what kept the gas from being blown away from the star when its fusion kicked off?  Shouldn't there have been a massive solar wind blowing the gas away from the star?
 A: Stephenson 2-18 is a red supergiant. It would have started as a very massive main sequence star, and become a supergiant after the end of core hydrogen burning. During shell burning, stars expand as they blow off gases into the envelope. This expansion creates red giants.
That said, Stephenson 2-18 appears to be substantially larger than the maximum theoretical size of a red supergiant. Nonetheless, this is not certain, because it depends on an assumption that it is a member of a visually close cluster, the open cluster Stephenson 2. If it is not a member of the cluster, could be a lot closer, in which case it is not as large as the calculations suggest.
A: Stephenson 2-18 is a Red Hypergaint, it is the largest star in the universe and so far, that is an established fact. It is around 2150 times the size of the sun and makes the sun look like a grain of sand next to it. But with its size brings some uncertainties, since the limit of a Red Hypergaints size is estimated to be 1500 times the size of the sun, Stephenson 2-18 is substantially larger than this limit.
It's size is truly unchallenged, and if it's size is truly confirmed as 2150 suns, it's origins are still quite ambiguous. It's original size is still mysterious, as it's orgins have to make it amazingly huge in its main sequence, but maybe not so. Assuming the suns nature is the same everywhere else, than when the sun reaches its Giant Phase, it's diameter will be 200 times greater than its original size. Than Stephenson 2-18's original size would most likely have to be 200 times smaller than its current observed size, using basic division, than Stephenson 2-18's original size, could mean that it was a blue star 10.75 times the size of the sun. This is only assuming that Stephenson 2-18's location is actually in the Stephenson 2 open cluster, but that's a story for another time.
