Yeah, I’m watching TNG–but it’s not a cafe question–and the plot device is a weapon capable of halting all nuclear fusion within a star.
My question for the nuclear physicists: How quickly would the effects become apparent? ISTR that it takes a photon thousands of years to emerge from our sun.
Since this is the same effect that a star has just before it goes supernova, the timing would be the same. Seconds.
I think it would take a long time to notice the star emitting less light, looking smaller, or appearing cooler. Most stars last billions of years, so in any given year less than a billionth of its atoms undergo fusion. The atoms are generally staying warm and taking a long time to export their heat. After all, the distances are huge inside a star compared to anything we’re experienced with waiting for cooling to proceed.
Since such a thing would involve changing the laws of physics over a substantial volume of space, the question is how long the effect could be maintained. Because the minute the effect ended, fusion would start up again.
These things are sort of like asking what would happen if everything in math stayed the same except that 2 + 2 = 5 now. To achieve your goal would mean that a lot of things have changed, and we’d need to know what they are.
But trying not to fight the hypothetical, here’s my guess.
Stars are sort of a balancing act. Gravity is always trying to pull materials in, and fusion from that compression generates energy that tries to push it out.
So one of the forces goes away. The star is no longer generating as much radiant energy (simple compression will produce some), and will eventually stop “burning”, although the residual heat will keep it visible and emitting for a long time; thousands to millions of years, at a guess, getting dimmer all the time.
It might not have that long, though, because gravity now has unfettered sway. Like much else in astronomy, what happens next will depend on the mass of the star. It may just pack itself in and become a dense planet-like object or something like a brown dwarf. Larger stars will compress through gravity until they start stripping (but not fusing) their atoms, and you’ll get a neutron star…or not, depending on what you mean by “stop fusion.” Larger stars still will compress dramatically, and “wink out” as their gravity becomes able to trap light – becoming a black hole. Pretty much what happens now, without the supernova stellar explosions (which are driven by a sudden reignition of fusion as the star collapses).
I agree, it’s hard to predict the outcome of some nebulous change to physics.
However, I doubt that there’s enough residual heat in the Sun to keep us as warm as we’re used to being. I would expect the difference to be noticeable pretty quickly, assuming that fusion isn’t going on only in the core, and that the nearest photons from fusion aren’t so deep that they’d take 1000 years to reach the surface (if that’s even true of the deepest).
So, experts – what is the speed of light inside the sun, and how deep is the fusion occurring?
BTW, if this “star death ray” works at the speed of light, it would take as long to penetrate into the heart of the star as it takes the photons to get out, wouldn’t it? (But then again, what’s the speed of fiction?)
How long does it take for a star to ignite in the first place. From the moment of ignition to well being a sun.
It takes millions of years for the energy in the core to migrate out to the surface, but that figure is based on the way the core works right now. Turn off all the fusion, and the conditions will change, and thus so will the time.
Each stage of the life of a star takes a shorter time than the previous one, and some of the last stages last only minutes or less. So that’s how quickly changes could be felt, if they were extreme enough.
The 10[SUP]7[/SUP] years estimate for the random walk of photons from the core to the surface of the Sun is a pernicious error that has been much repeated. The actual estimate of the time for a photon created in the core to reach the surface of the Sun is between 10,000 and 190,000 years, based on a weighted 2D random walk Monte Carlo simulation for solar density. The 10My number which is often cited is apparently based on a mean solar density rather than an accurate model of the density at different radii, which gives an unduly short run length.
If fusion in a star magically stopped but the gravitational field remained, I expect you’d see considerable heating from degeneracy pressure and the resulting entropy alone. It would radiate in a different wavelength than that produced by fusion and mediated through the photosphere (which is where the photons we see are actually produced). I expect the total power throughput would drop due to the less energetic and slower photons but temperatures in the core would continue to increase until the degeneracy pressure exceeded the gravitational energy and the Sun would start shedding mass in the form of geysers emanating from the outer core. You would probably also see underdamped oscillatory phenomena as more of the Sun becomes metallic in structure and rings, possibly to destructive resonance. As weird as it may seem at first, the radiating of the photosphere actually serves to cool the Sun and carry away excess gravitational energy that would otherwise create instabilities.
Stranger