In a Twitter discussion with Ned Nikolov, I discovered that some people believe that the Sun is powered by Galactic electric currents. I don’t even understand how this is meant to work, but that doesn’t really matter since it’s clearly nonsense. A star like the Sun (in fact all stars on the main sequence) are powered by nuclear fusion in their cores; specifically, the fusion of hydrogen into helium.
There are a number of reasons why we know this. One basic issue is that if the Sun were simply radiating gravitational potential energy into space it would have a lifetime of only a few tens of millions of years. We know, however, that the Solar System is about 4.5 billion years old, so this can clearly not be the source of the Sun’s energy. Whatever powers the Sun has to be able to do so for billions of years, and the only known source that can do so is nuclear fusion.
Another factor that I wanted to highlight here (partly because I’ve just been teaching this in one of my classes) is that stars spend most of their lives on what we call the main sequence. While on the main sequence their properties (radius, temperature, luminosity) will remain quite constant. For a star like the Sun, this phase will last for just over 10 billion years. It’s quite interesting (I think) to consider how this is possible.
One reason is that the energy generated in the core of a star doesn’t stream straight out into space; it can takes 10s of thousands of years to go from the core to the surface via a combination of radiative diffusion and convection. A consequence of this is that a star then settles into a state of hydrostatic equilibrium. A self-gravitating system in hydrostatic equilibrium satisifies what is known as the Virial Theorem (which I also discussed in this post).
The Virial Theorem is essentially that twice the total thermal energy in the star plus the gravitational potential energy is zero. Since the total energy is the sum of the total thermal energy and the gravitational potential energy, being in Virial equilibrium means that the total energy is also half the gravitational potential energy, or minus the total thermal energy (i.e., the total energy is negative and has a magnitude equal to the thermal energy). What this means is that if the total energy in the star goes up (becomes less negative) the thermal energy goes down (it cools) and the gravitational potential energy goes up (becomes less negative which can only happen if the star expands). The opposite happens if the total energy in the star were to go down.
So, why does this mean that stars can sustain a roughly stable state for billions of years? Well, the rate at which nuclear fusion generates energy is strongly temperature dependent; it increases dramatically with increasing temperature. Given that the star is in Virial equilibrium, if the fusion rate were to increase, this would increase the total energy in the star and it would expand and cool, and the fusion rate would go back down. If the fusion rate were to go down, this would reduce the total energy in the star and it would contract and heat up, and the fusion rate would go back up.
Therefore, the rate at which energy is generated by nuclear fusion can be kept very constant. That the energy then leaks out slowly means that the star can be kept in a state of hydrostatic equilibrium, with the surface temperature and radius (and hence overall luminosity) also kept constant.
Hence, that stars settle into a state of hydrostatic equilibrium, which results in them satsifying the Virial Theorem, and because the energy generation via nuclear fusion is strongly temperature dependent, leads to stars being able to maintain a roughly constant state for billions of years. I have no idea of how Galactic electric currents are supposed to power stars, but I have no real interest in trying to work out how this is meant to work, and I’m pretty sure it wouldn’t be able to explain the basic properties of stars.