system
Given the following post, respond as if you're commenting on Reddit:
human
If neutron stars are simply made only from neutrons, where do they get fuel to still support their glow? If normal stars fuse atoms to create energy then what is the energy-producing process of neutron stars?

Where is the Astrophysics flair, by the way?
gpt
A neutron star is the remnant of a super hot core of a massive star. It doesn't have any source of fuel. It glows for a long time because it's super hot. It will cool over time exactly because it has no fuel. 

The same goes for white dwarfs.