Column:
http://www.straightdope.com/columns/read/3071/how-many-ice-cubes-would-it-take-to-put-out-the-sun
This might be the best column of all time. A proper level of non-strained humor, with a real and surprising answer to a question well beyond the reach of Google. Huzzah.
Thanks for posting that. Looks like we will have to give up on that strategery for fighting globular warming. Next?
Yeah, that’s a fun question. My followup is to Una, asking about one critical assumption: How cold is the ice cube? I would think it would matter somewhat if we’re taking about ‘so warm it’s melting’ ice or ‘fraction of a degree above absolute zero’ ice - in terms of being able to temporarily halt fusion by reducing the sun’s core temperature, that is. 
Suppose you could cool the sun by some kind of magic freeze ray, and keep cooling it as it contracted under it’s own gravity. Would you eventually get a ball of superdense but too-cold-to-fuse hydrogen? Or would you reach a point where the hydrogen would be dense enough to start fusing even if it started out at just 1 degree Kelvin?
I assumed 0 F. However, it makes little difference to the calculations what you assume for the ice, as you’re quenching something so many orders of magnitude hotter that a few dozen degrees one way or the other fall out in the error band. Even accounting for the heat of fusion and heat of vaporization doesn’t compare much with the sensible heat of raising the water, steam, then plasma to 10,000,000 C.
How much dry ice would it take?
Why is nickel and iron incapable of undergoing fusion?
That was truly a classic SD article.
Una, the next time you speak with Cecil please remind him, for me, that he rocks!
Damn, I came in as well to compliment Cecil on an excellent article. He’s been on a roll lately, I think, but this one is really classic. Maybe the competition from Randall Munroe has inspired him!
Things only fuse if you make energy out of the deal. Similarly, things only fission if you make energy out of the deal. Nickel and iron don’t have any fission or fusion products that would be lower-energy than them, so they can’t do either. Of course, things can happen if you have a large energy excess, such as in a supernova - that’s the only reason why fissionable elements exist, they would never result from normal fusion. Just as oil and coal are fossilised sunshine, uranium is fossilised supernova.
Some people believe that you can get energy out of nickel fusion. Of course, as far as I know, their claims haven’t been verified (some suggest that, assuming fusion is indeed occurring, the hydrogen fuses, although the reaction is supposed to produce copper).
I thought the problem was that fusion in the heavier elements required more energy than it produced. Someone please enlighten me.
The Wikipedia article about Nuclear Binding Energy has a graphthat illustrates the mechanical reason for this. Nuclear binding energy, as the name implies, is the energy that binds subatomic particles (nucleons) in the nucleus of the atom. It’s a very short-ranged attractive force; in a large atom (like uranium), the binding force of a given proton or neutron may not reach all the way to the most distant other particles in the same nucleus; at the same time, the electrostatic repulsion among all the protons tends to push the nucleus apart a tiny bit. This puts a limit as to how strongly-bound a large nucleus can be, and explains why some very heavy atoms can be split (they’re weakly bound and have internal repulsion pressures). On the other hand, a nucleus with too few particles doesn’t have as much total binding energy as a more massive one, because there are just fewer particles. It also means that the lighter nuclei can accept more particles by fusion.
The peak in the curve between “low per-particle binding energy because of too few particles” and “low per-particle binding energy because of too many particles” is in the vicinty of Nickle-62 and Iron-56. This is the “sweet spot” in the binding energy curve, in which the atom is not too small and not too big, but just right. It actually takes more energy to disrupt the balanced nuclear binding than can be liberated by breaking the binding force.
If a star begins to pour energy into nickle-iron fusion, it’s a doomed exercise. Nickle-iron fusion sucks up all the energy you care to put into it and comes back for more. It takes, and takes, and takes.
(This has to be a big star, around a dozen to a few dozen times more massive than the Sun. There’s not enough mass for a smaller star to burn into such massive core elements as silicon.)
Since energy is the ultimate thing resisting the huge gravity of the entire mass of a star, when the energy is gone, gravity takes over. The entire mass of the star’s inner layers accelerates towards the center and compresses into a humougous instantanous explosion: a core-collapse supernova.
Good summary article specifically about the Iron peakof nuclear binding. Relevant quote:
Left unstated, elements around iron can’t be fissioned or fused profitably. (Or at all? I’ve never heard of fissioning anything as stable as iron.)
Awesome explanations, thanks all!
I wonder if Cecil (and Una) might have left something out. Tossing ice cubes into the Sun will add more hydrogen, true, but it’ll also add more oxygen. While it’s true that oxygen will fuse under the right conditions, the core of the Sun isn’t those conditions, and too much non-fusing material will “poison” the reaction. A sunlike star can only fuse about 10% of its total hydrogen supply before the helium is enough to start having drastic effects; surely adding material that’s 1/3 nonfusible by atom count will hurt more than it’ll help.
Oxygen is heavier per-atom than hydrogen or helium. The massive gravitation gradiant of a star’s innards means that elements are sorted pretty efficiently by atomic mass, so all the oxygen would sink deeper into the core (past the helium “waste” of hydrogen-burning fusion) and sit there inert until temperatures increased high enough to start it fusing… much later in the life of a main-sequence star.
If all the oxygen settled into the sun’s core, wouldn’t it form a denser core that would stimulate hydrogen fusing at it’s boundary, the way mainstream stars become red giants once they accumulate enough “ash”?
That is sort of the thought experiment we went through. However, I admit there was some significant uncertainty in this speculation.
Possibly. I think the greater effect will be the addition of matter. A lot of matter. As the article itself indicates, more than 1/3 of the mass of the sun. That’s a lot of mass. In the case of the sun, enough topush it into the next luminosity class(F5 or F4 versus current G2), becoming hotter and a little whiter. Global warming-wise, CO2 has nothing on that.
Nuclei can be pushed the “wrong way” on the binding energy curve with sufficient energy input. For instance, the artificial elements at the ass end of the periodic table are created by slamming lighter nuclei together, causing them to fuse with a net energy loss. On a larger scale, elements heavier than iron exist because the energy levels found in supernova explosions push fusion beyond the bottom of the binding energy curve.