How is it possible for white dwarf stars to have lower luminosity than the sun even though the sun is cooler than white dwarfs?

2 Answers

Size of a white dwarf is small.

Explanation:

So luminosity is also low.

Nov 20, 2017

A white dwarf may be hot but it is not generating energy, it is losing it.

Explanation:

A white dwarf is the remains of a star after fusion reactions have finished. The core collapse which creates a white dwarf makes it initially very hot. It can be over #100,000K#.

White dwarfs emit radiation while they cool down over a very long time period. They are very small and have very low luminosity.

A star like the Sun on the other hand is constantly producing energy from fusion reactions in the core. Although the surface temperature may only be #6,000K#, the core is at #14,000,000K# and constantly producing energy. Hence the Sun is much larger and much more luminous.

Red giant stars are even cooler at less than #4,000K#. They still have fusion reactions taking place in or around the core. They are very large and very much more luminous than the Sun.