[lazide]

Edit window closed, but here is a more in-depth explanation. It’s decidedly non-obvious [https://www.reddit.com/r/AskPhysics/comments/1arh55g/why_can...]

Part of it is due to the ‘Principle of Etendue’ [https://en.m.wikipedia.org/wiki/Etendue#Conservation]

What confuses people I think (practically) is that the actual high temperature (~4500F) is far beyond the limits of a useful highest temperature in 99% of situations we might want in engineering. A spacecraft hitting that temperature is going to be a molten piece of scrap long before it hits that point.

But the limit does actually exist - it won’t somehow hit 10,000F for instance. That is also why we can’t produce infinitely high temperatures with a huge magnifying glass - the highest we can hit, regardless of how big it is, is still ~ 4500F. Higher temperatures need something like an electric arc furnace, or LASER.