What is the energy source for a faint white dwarf after a low-mass star exhausts its hydrogen fuel?
Answer
Stored thermal energy slowly radiating away into space
Low-mass stars, similar in mass to the Sun, follow a different evolutionary path than their highly massive counterparts. After exhausting core hydrogen, they may swell into red giants, but if they are small enough, their core may never reach the necessary temperature to initiate helium fusion. Consequently, they do not sustain active fusion reactions later in life. A white dwarf remnant is defined by the slow, passive process of cooling down. Its energy output stems from the residual thermal energy stored within its dense structure, which is radiated away into space over extremely long timescales, potentially trillions of years.
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