Destroying Stars?

SpaceTiger

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, the process would pretty much have to be occurring on the dynamical timescale. Most of the things suggested so far would change the sun on one of the first three timescales. The one exception (which I noted as being the most promising) was the one that involved 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...

EngineeredVision

I was about to suggest resonance. Is it possible to create as SpaceTiger suggested some sort of gravitational resonance in the Sun. I recall Nikola Tesla talking about how the Earth could be destroyed in under 2 years if the correct resonant frequency was achieved through explosive devices (I don't recall if they were in the kiloton or megaton range) set off with each recurring resonant wave. I believe that an explosion on this magnitude would have to be set off every 45 minutes for 2 years to destroy the Earth. However, after only a few weeks Tesla claimed that any surface topology of the Earth would be drastically destroyed as the surface rose and fell several hundred feet. I don't remember the exact setup for this scenario but hopefully everyone gets the idea. Granted this is not gravitational resonance, but if a similar effect could be employed on the Sun would this technique be possible?

JesseM

Ha, that's awesome that Tesla was thinking about ways to destroy the earth. Doesn't sound plausible though--there's no getting around the fact that you need a huge input of energy to get all the mass of the earth up to escape velocity, and the energy generated by some explosions won't come close (the energy needed is estimated at between 2.2 * 10^32 and 3.7 * 10^32 joules on this star wars page written by an engineer). Resonance can't generate more energy than is put in by the external driver, can it? I suppose the stretching and compression of bonds between molecules that make up the object would be converting potential energy to kinetic energy at times, but at other times kinetic energy would be converted back to potential (like an object bouncing up and down on a spring) so there shouldn't be any net increase in energy over time, aside from the energy put in externally by the explosions.

Art

How about artificially creating fusion points in other parts of the sun using the suns own material to feed the process thus causing a cascade effect to destabilize it?

SpaceTiger

To induce fusion, you'd have to artificially increased the temperature and/or density at some arbitrary point in the star. As soon as you did this, however, you'd increase the pressure surrounding that point, causing your enhancement to expand. This would quickly halt fusion. Even if you did this at a large number of points throughout the star, all you would be doing, effectively, is heating and/or increasing the mass of the star. The process would produce pressure waves, so if you tried to coordinate those waves, you might get some interesting results, but the required energy input would still be quite large.

In real astrophysical situations, the best way to produce a nuclear runaway is with degenerate gas. In this case, the star is using degeneracy pressure (instead of gas pressure) to support itself from gravity. If you keep increasing the temperature in a degenerate gas, you can induce fusion reactions. In this case, however, the temperature enhancement won't spread out because the pressure mechanism is dependent only on the density of the gas. The fusion reactions will heat the surrounding gas, causing it to fuse even more. This will continue until the gas becomes so hot that gas pressure overcomes degeneracy pressure and the enhancement is able to expand. Examples of runaways like this are novae (on the surface of accreting white dwarfs) and the "helium flash" (in the cores of moderate-mass stars).

Art

Hi ST,
From your post above it sounds like you are saying this would work for some kinds of stars??

What about a sort of fusion 'missile' rather than fixed points? Using this new super duper technology that will be available in the future if several fusion missiles were fired one after another into the star towards it's core on the same heading using the star's own material en route, much as a jet engine uses oxygen in the air, to power the fusion missiles would it perhaps then fuse a lot more material? Also would penetrating the shell of the core allow pressurised material from within to escape back up the 'tunnel' drilled by the missiles? It sounds like something that would make a big bang at least. :-) BTW as obviously ordinary matter would melt, the shell of the missile would be a containment field rather than solid matter.

SpaceTiger

None of the processes I described above actually destroy the star -- novae occur multiple times and the helium flash just leads to another phase in the star's evolution. If the object were degenerate, however, then the energy requirements for destroying it would be even larger than for the sun because its gravitational binding energy is much larger. The best way to destroy a degenerate object is to give it mass until it passes the Chandrasekhar Limit. When you do this to a white dwarf, you get a Type Ia supernova.


What do you intend to put in this missile? Increasing the rate of fusion in the core was already suggested, so you may want to look at my response to that.

The most efficient missile I can imagine would operate on matter-antimatter annihilation. The amount of antimatter that you would need is given by half the mass equivalent of the binding energy I cited earlier (~10⁴⁸ ergs). Turns out you'd need about 10% of an earth mass. Forget about making/finding that much antimatter, imagine trying to contain it!


Even if it's possible to create a containment field like you suggest, you have to keep in mind that the sun is made of gas. Even if you evacuate a region within the sun, the surrounding gas will quickly diffuse to fill it.

Art

I bow to your greater knowledge as I know next to nothing about the subject but this surprised me. I'd have thought that towards the centre at the pressures involved the hydrogen would have been in liquid form or does the high temperatures prevent that?

Also out of interest presumably the sun and it's planets formed out of the same raw material gas cloud and so was the earth's composition originally the same as the sun's and if so why has it changed so much and how long did the changes take?

SpaceTiger

Yeah, the temperatures are too high for it to be a liquid. The matter at the center of the sun is mildly degenerate, but behaves otherwise as an ordinary gas.


The original disk of gas and dust that led to the solar system would have started with a chemical composition very similar to the sun, but its subsequent evolution led to changes in the chemical abundances at different radii. One reason for the composition gradient in the solar system is almost certainly that there was a temperature gradient in the original disk. Different materials have different condensation temperatures, so while I might be able to form an icy body at the distance of Pluto, such an object would be evaporated near Mercury.

Although many of the key ideas are in place, we haven't actually settled on a detailed theory of the solar system's formation. In particular, some of the mechanisms for the early growth of planetesimals are still uncertain. Perhaps continued observations of extrasolar planets will clear things up a bit.

Bob3141592

This is SF, right, so we can be a little relaxed about engineering effort and expense, OK? What time scale is necessary to disturb the star in, weeks, years, or millenium? How clandestine does the effort have to be?

I suggest, since I don't think a full model of stellar magnetic fields has been perfected, that a constellation of satellites bearing large electromagnetic coils be placed in close stellar orbit. A few billion tons of finely drawn silver wires might suffice for these. These satellites get their huge electric energy from large, very sturdy solar panels feeding really big capacitors or futuristically efficient batteries. At intervals precisely timed to the star's oscillations, they turn on and off in a carfully choreographed sequence until they gradually build a resonance with the star's magnetic field. They can then create huge magnetic storms that cause the star to spew out material in unprecediented flares (okay, so the satellites might have to dodge out of the way, or induce the flares to happen only after the satellite's orbit takes it safely ahead). The choreagraphy could induce a runaway positive feedback loop that becomes self sustaining, or like the magnetic experiments so poorly done in the B movie "The Core" shut down the magnetic field entirely, leading to who knows what consequences (SF imagination to fill in the gaps here). A little input goes a long way, given enough time. Some verbage about chaos theory and pertubation theory always impresses your readership, as shown by that even worse movie "The Butterfly Effect".

If you like these ideas, I want a cameo role in scene three! :-)

Just some guy

ok this is very far fetched but if a baryon entered the centre of a supermassive magnetic monopole then it could decay without it's antiparticle, so if you sent a large volume of monopoles into the Sun shouldn't they eventually annihalate the matter inside it? (I read that the probability of a proton/neutron annihalating in such a manner is only significant if the density of the material is that of a neutron star, so I assume this process would be ridiculously slow).