Here's why:
* Hot: White dwarfs are incredibly hot, with surface temperatures ranging from 8,000 to 40,000 Kelvin. This is due to their incredibly dense core, which is primarily composed of carbon and oxygen.
* Not very luminous: Despite their high temperature, white dwarfs are relatively small and compact. This means they have a limited surface area to radiate light, making them less luminous than larger stars.
In summary: White dwarfs are hot because of their dense core but not very luminous because of their small size.
