me wrote:
Hi

Sirius B, a white dwarf, has a surface temperature of around 25,000 degrees
C. Sirius A, a blue/white main sequence star has a surface temperature of
around 12,000 degrees C. Usually the bluer a star is, the hotter it is. Just
wondered why it's not the case here.



Essentially, you have answered your own question, assuming you know what a white
dwarf is supposed to be. A white dwarf is supposed to be the core of a now
dead star, a low mass type like our Sun. As the core of a star, they simply
retain the heat from those nuclear fusion processes that were proceeding prior
to the end of the star's life. Now, heat stored within them, they simply
radiate that heat, lowering their temperature eventually to absolute zero over
10s and possibly 100s of billions of years.

And, since they are not generating their own energy but simply radiating stored
energy, comparisons of their color to that of stars is comparing apples to oranges.