Question: Why do stars get so hot in space and what temperature could they reach?

Hugh Osborn answered on 21 Nov 2014:

Stars get so hot because, at their cores, they are extreme nuclear reactors. Our sun fuses 600 tonnes of hydrogen into helium *every second*! The energy from that has to escape, so rushes out through the surface, heating it up. The surface temperature, though, is not just about how much energy the star is producing. Imagine a star giving out the same energy as the sun, but that was much larger. That large size would create a much larger surface area for the energy to escape, so the energy *per square meter* would be less. That is how we measure temperature, so such a star would be colder than the sun (and actually much redder than the sun too, as Red-Hot is cooler than White-Hot).

So the hottest stars are the ones with lots of energy being created that are also really compact. White dwarfs, which are tiny stars as small as the Earth, are the best bet. Some of these objects (which are formed after a star has run out of fuel for nuclear fusion) can be more than 125,000 degrees on the surface! MUCH hotter than our’s Sun’s disappointing 6000 degrees.