How does a white dwarf star glow for billions of years if all of its fuel is depleted?


Apparently when our sun has lived out its entire life, it will pulse and fade away to a white dwarf. It will do this because it will have used up all of its fuel. That being said, since the Hydrogen Fusion is what creates sunlight with Photons, what is it that keeps the white dwarf it will be glowing? What creates the light then?

In: Physics

They aren’t producing any more energy, but they still have a bunch of it left from when they were a big muscly star with lots of fuel. They radiate light and heat over time, but eventually fade

It’s all blackbody radiation from the enormous amount of residual heat. As the star cools, it will eventually dim and then go dark, glowing only in infrared. It really does take that long. In fact, the universe is too young to even have an black dwarfs in existence yet.

Any atoms emit radiation depending on on it’s temperature. Most stuff here on earth is in a temperature range that emit in infrared. If you look at your oven heating element it’s gonna be reddish, a fire is yellow at the center when the temperature is higer, but orange to red on the outside, a plasma torch will glow white. This is where the glow from a white dwarf come from. There is a LOT of residual heat from the star concentrated on the white dwarf, the temperature will be high enough that it will glow white.

Now in heat transfer in 3 ways, convection, conduction and radiation. There isn’t a fluid around the white dwarf for convection, space is empty so nothing for conduction, so the only way the heat of the dwarf excape into space is by thermal radiation, which take a lot of time so they will glow for a very long time. In 2012 some white dwarf were found at temperature below 3 900 K or 3 600 celsius or 6 500 fahrentheit and those were 11 to 12 billion years old.

> since the Hydrogen Fusion is what creates sunlight with Photons,

That is not quite true. The fusion is the sun hot. the light is just created by black body radiation on the surface of the sun. It grows because it is hit just like all other stuff. Why it is hot is irrelevant for the fact that it glows.

Heat up a piece of iron with a torch and it glows. Remove the torch and it continues to glow until it has cooled down. The same it true for the sun. An object as large as the sun take tremendous time to cool down because the surface is very small compared to volume.

If you look at the hydrogen fusion reaction it will directly release photons but the energy is so high so they are gamma rays, not visible light. Gamma rays released in the core will not reach the surface but hit atoms in the sun and heat them up.

It is really really hot, and takes a really really long time to cool off.

Space is a vacuum, or near enough to one. There is no other matter to transfer the star’s heat to, so the only way it loses its heat is by emitting it as light, slowly leaking away its accumulated energy over billions of years.

Also, the more it cools, the less light it gives, extending its life even further.

Think how much energy you need to send a rocket from Earth to space.

Now think what happens if I had the rocket in space and dropped it to Earth. How much energy would it release? The answer is the same amount as if you needed to throw it up to where the rocket is now. We call the energy that is contained in the rocket before you drop it – potential energy.

Now imagine doing the dropping for a whole star to the size of a white dwarf. That’s a whole lot of potential energy being released. This is what heats up a white dwarf.