How Hot Can Stars Get?

Stars, like our own Sol, burn hydrogen in nuclear fission reactions to create helium and other higher elements, all while giving off nearly incomprehensible amounts of energy in the process. Some stars never get very hot. Those are called brown dwarf stars. Some stars, of "average" size (from 1-8 times our sun's mass), stop burning hydrogen fuel after a while, and the resulting absence of heat and energy causes the matter of the star to shrink into itself, creating what is called a white dwarf, usually around the size of our Earth.
The transition between regular star and white dwarf causes some of the hottest temperatures recorded in the Universe, higher than 100,000 Kelvin (179,000 degrees Fahrenheit!!). These are astounding temperatures. Well, it seems that astronomers have found the hottest White Dwarf yet.
A team of German and American astronomers have found one of the hottest known stars in the Universe, and it is a ridiculously hot white dwarf (named KPD 0005+5106), whose temperature was measured at over 200,000 Kelvin at it's surface! Wow!

It is so hot that its photosphere exhibits emission lines in the ultraviolet spectrum, a phenomenon that has never been seen before. These emission features stem from extremely ionized calcium (nine-fold ionized, i.e., CaX), which is the highest ionization stage of a chemical element ever discovered in a photospheric stellar spectrum. ( Science Daily )

The amount of energy required to ionize calcium to such a degree is just mind-boggling. To think that it is so "hot" that it releases it's energy mainly in the Ultraviolet spectrum is also nuts. The "photosphere" of any astronomical object is region from which externally received light originates. Our own Sol's photosphere is around 5,000-6,000 degrees Kelvin. Below is an image in which the faintest stars you see are the white dwarfs.