How Could the Sun Be Destroyed?

It recently came up in conversation about how (hypothetically) one might be able to destroy the sun. The ideas included...


1. Stop fusion

If we were somehow able to make the sun inert (i.e. stop fusion), then it would seem to have no way to replenish the energy radiated away. Thus, it would cool, the pressure would fall, and the sun would begin to contract. However, this contraction is itself a source of energy -- in fact, they used to think that this was what powered the sun. This means that the sun will live for a while even after burning ceases...about 10 million years.

2. Disrupt it with a black hole

A very low mass black hole would have no noticeable effect on the sun, while a very large one (of order the sun's mass) would gravitationally disrupt it -- but we couldn't create such a beast artificially. In the intermediate range, a black hole at the center of the sun might even make it live longer. The reason for this is that the lifetime of the sun depends on the efficiency of its energy source; that is, the more effectively it can turn matter into energy, the longer it can keep itself up from the pull of gravity. Other than matter-antimatter annihilation, accretion onto a black hole is the most efficient source of energy that we know of, converting of order 10% of the rest mass of its fuel into energy. If the star reached a stable equilibrium with the black hole at its center, then slow accretion onto the black hole could maintain the star for a very long time.

There a lot of "ifs" in this one, however. It's not clear what kind of equilibrium (if any) the star would reach with a sizable black hole at its center. The efficiency of an accreting black hole is also extremely uncertain. Finally, depending on the initial mass of the black hole, it may be a problem even getting it to the center without seriously disturbing the star's structure.

3. Shoot it with a laser beam

Shooting it with a laser beam wouldn't put a hole in it, but it would heat it up. Unfortunately (or fortunately), to have a noticable impact, you'd need an extremely powerful laser beam, powerful enough to provide an energy comparable to the gravitational binding energy I quoted above.

4. Add Mass

If you add enough of anything to the sun, you can shorten its lifespan. However, to shorten the lifespan to anything less than a million years, you would have to add on the order of 100 times the sun's current mass.

5. Change its chemical makeup

For example, one person suggested that you could accelerate the nuclear reactions by introducing vast quantities of some mediator isotope, but that wouldn't destroy the sun, it would just cause it to expand and reach a new equilibrium. If the reactions increased the energy production very quickly, one might be able to blow the sun apart before it could equilibrate, but I'm pretty sure there aren't any known isotopes that could do this.

Decreasing the hydrogen content is another option, but since the sun is made mostly of hydrogen, this would be practically equivalent to pulling it apart, piece by piece. The gravitational binding energy of the sun is

$E \sim \frac{GM^2}{R} \sim 10^{48}~ergs$

so we won't be fulfilling these energy requirements anytime soon.

6. Other

There are a variety of timescales on which changes in the sun can take place. Some of the most important ones are:

Nuclear timescale - The approximate time it takes for the sun to burn the available nuclear fuel. This timescale is obviously relevant for the evolution of the sun, since this is the sun's energy source. In general, you'll see major changes in its position on the Hertzsprung-Russel diagram on the nuclear timescale. For hydrogen burning in the sun, it comes out to about 10 billion years.

Kelvin-Helmholtz timescale - The time it takes for the sun to radiate away its gravitational binding energy. If nuclear burning were to stop or become insufficient for compensating the energy losses, this would be the approximate lifetime of the sun. It comes out to around 10 million years.

Thermal timescale - This is the time on which the temperature profile of the sun changes. The virial theorem makes it so that the gravitational and thermal energies of the sun are about the same, so this timescale turns out to be approximately equivalent to the Kelvin-Helmholtz timescale.

Diffusion timescale - The average time it takes for a photon to undergo a random walk from the core to the surface. I'm guessing this is the one you were referring to. I've seen estimates that range from 50,000 to 10 million years, but to my knowledge, this timescale doesn't play a big role in calculating changes in the sun, so the precise time isn't very important.

Dynamical timescale - The time on which gravitational perturbations are communicated across the sun. It turns out to be comparable to the sound-crossing time, the free-fall time, and the hydrostatic time. For the sun, all of these come out to about 30 minutes.

If we wanted the sun to be "destroyed" on any timescale that was relevant for a science fiction novel, for example, the process would pretty much have to be occurring on the dynamical timescale. One of the most promising scenarios would involve a collision with another object. When the sun and the object collided, the sun would be disrupted on the dynamical timescale.

Another process that's known to occur on the dynamical timescale is stellar pulsation. When a star is unstable to hydrostatic perturbations, it can pulsate. If the amplitude of these pulsations were large enough, presumably the star could be blown apart. Perhaps if there was some contraption that created gravitational perturbations at one of the sun's resonant frequencies, a futuristic society could succeed in causing the sun to pulsate itself to pieces. It's a bit far-fetched, but then science fiction usually is...