Let us say, that in light of recent events, Congress passes a law stating that the sun’s nuclear reaction must be stopped. Seawater shall be pumped in immediately in order to cool it down. Ignoring the small fact that this is blatantly impossible, how much seawater would be required to stop the sun from being, well, the sun.
By molarity, the majority of seawater is hydrogen. The Sun is a mass of [del]incandescent gas[/del] hydrogen being fused into helium, meaning that once the seawater is hot enough to stop being seawater and be plasma instead, most of it (by molarity) is fuel for the reaction.
My best guess is that the Sun is so huge and the Earth is so tiny we could dump the entire ocean into the Sun and not have any impact beyond giving it that much more hydrogen fuel and, of course, mass.
But that would kill off all life on Earth and render this planet an uninhabitable rock pit, like Mars or Wyoming. And as an environmentalist, I’m against that.
Let us assume for the sake of argument that there is an infinite supply of seawater.
Then you keep pouring until the star becomes a supernova. Since the Sun’s fusion is the result of gravity, the more mass you add, the larger the star becomes. However, given an infinite amount of seawater, you would shorten its life, as it would grow into a supergiant, which burns much faster than stars like the Sun.
Of course, the end result would be like saying we solve the nuclear power plant crisis by blowing it all up.
My main point is that cooling it down with seawater isn’t going to work unless you add enough elements the sun can’t fuse to poison the reaction. It will get heavier, and more fuel-rich, long before that happens, even assuming it ever will.
It might be more interesting to feed the sun massive amounts of iron. Iron is heavy enough of an element it can’t be used in fusion: You lose more energy trying to fuse it than you can get back from the reaction. It’s the final ash of the normal part of a stellar lifecycle, and it seems the Sun isn’t even massive enough to produce it. This should produce an iron core in the middle of the Sun that gets increasingly massive as we pour more iron in, dragging the rest of the stuff the Sun’s made of into an increasingly thin, dense fusion shell around the core. I can only imagine this would eventually cause a helium flash as the shell components became degenerate, and then who knows what.
This is wrong. Ignore it.
Hmm.
You are saying that if we were to feed the sun WATER, it won’t grow and become a larger, brighter sun?
What if we fed it a sun-mass of water? What if we fed it a hundred thousand sun-masses of water?
Well, as said it’s basically just more fuel. And going by this Wiki page, the largest known star in somewhere in the range of 265-320 solar masses. So at a guess we should be able to add at least 264-319 solar masses of seawater.
There’s a problem with what I suspect is your underlying assumption here.
Pumping cooling water into a shut-down reactor works because the fission reaction has stopped and the cooling water just has to cope with decay heat (which is a small fraction of the heat the reactor produces when fissioning).
The sun is not like a shut down reactor: it is a reactor with an open throttle, and a fusion (not fission) reactor to boot.
The seawater doesn’t stand a chance.
Just read Dahnlor’s post. I completely messed up on what happens at the limit.
What could you add to the sun to quench it? Would adding large amounts of a heavy metal like iron work?
I don’t think you could add anything to the sun to quench it. The rate of fusion in the core of the sun is determined mainly by the gravitational pressure. Adding mass, no matter what that mass is, will increase the gravitational pressure and the rate of fusion. I suppose you could drop a black hole on it, and that would eventually shut off the fusion after absorbing the entire sun.
You aren’t thinking in terms of enough water, Presumably if you immersed the entire sun in water, the suns gravity would be counteracted by the mass of water water surrounding it and fusion would stop.
Stopping the initial mass of water from collapsing into something else first is left as a minor technical exercise for whoever is doing the experiment 
Actually, no, it wouldn’t. A spherical shell has zero gravitational force on its interior. You can’t pull something outwards by surrounding it with a shell of matter like that.
Is there any known substance in the real universe that could stop solar fusion if pumped into the sun in sufficient quantity? Iron perhaps, since that is the highest element formed by exothermic stellar fusion processes?
I never said anything about a spherical shell, or that the sun would be at the centre of it. Obviously the minor technical exercise bit gets slightly more complicated if it’s not a sphere.
No idea about the plausibility, but I’ve definitely come across the idea of destroying a sun by poisoning it with Iron. I’m pretty sure that’s how Charles Stross’s Iron Sunrise starts.
Well, presumably if iron would work, any element heavier than iron would work too.
But the water would displace the hydrogen and helium at the core. If you used iron, wouldn’t the iron sink to the center of the star and stop fusion at some point before the star would be so massive that it could fuse iron?
I figure if you pump enough in there, then it’ll become less Sol-like and more white dwarfy.
Quoth AndrewL:
You certainly could. When the Sun eventually dies a natural death, it’ll still have about 90% of its original hydrogen remaining. The dying is triggered by having too much waste helium, not by having not enough hydrogen. And anything more massive than helium will work even better. My guess is that by adding water to the Sun, you’re doing more to quench it with the oxygen than you are to fuel it with the hydrogen.