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Why is Stephenson 2-18 so large?

Physics Asked on October 19, 2020

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?

One Answer

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.

Answered by Charles Francis on October 19, 2020

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