I am 5, and have been wondering about this for almost half my life now: How many ice cubes would it take to put out the sun? My mum found your website and thought you must know the answer, since you know everything.
— Rei Mordue
You're one articulate 5-year-old, Rei, and plainly your mum is also no dummy. Your question has obliged us to rethink the basics, always a useful exercise. Plus you've given us yet another opportunity to brainstorm cosmic extinction, and what former 5-year-old could fail to get a kick out of that?
Let's clear up some misconceptions. First, and you'll excuse me if this seems picky, ice cubes have never been the firefighting methodology of choice.
Second and more important, technically the sun isn't on fire. What you've got up there is a nuclear furnace. Having started out as a huge cloud of gas and dust, the sun eventually was condensed by gravity into a ball, its temperature rising steadily in the process.
After the summer we just had, you may think you know hot. You don't. When a gas-and-dust ball gets up to about 10 million degrees Celsius, nuclear fusion spontaneously begins. That's hot. During fusion, hydrogen atoms combine under enormous pressure and temperature to make helium and release energy in the form of heat, light, high-energy radiation, neutrinos — always with the neutrinos, the crumbs of the universe — and other miscellaneous particles.
Long story short, there's no fire in the usual sense. In fact, if you were somehow to throw an ice cube into the sun, the effect would be the opposite of what you're hoping. First the ice would quickly melt and turn to steam. After heating to more than 5,000 degrees Celsius the steam would turn into plasma, meaning the hydrogen and oxygen atoms would fly apart and shed their electrons. The hydrogen would serve as more fuel for the nuclear reaction, and given the right conditions so would the oxygen. In other words, you'd just make things worse.
Another nontrivial problem is how you'd get the ice to the sun without having it melt. Comets, which are largely composed of ice, plunge sunward once in a while, but from what I can see they rarely arrive intact. Last summer, for example, comet C/2011 N3, fairly hefty at 132 million pounds, got within about 60,000 miles of the solar surface and vaporized completely.
That's not to say you couldn't theoretically douse the sun with ice cubes. Suppose you could teleport a monumental quantity of ice into the heart of the sun. The sun's core provides almost all the nuclear fusion that powers it, and currently bubbles along at about 15.7 million degrees Celsius. Bringing that temperature down below 10 million degrees might halt fusion, if only briefly. Gravity pulling everything together is what led to all that heat in the first place, and that's not going away. So after the initial temperature drop and a period of reorganization, fusion would start up again — this time with even more hydrogen to burn.
But we'll ignore that for now. How much ice would you need? It's tricky, since the laws of Newtonian physics don't apply in the heart of a star, and some stellar properties are only conjectural. Never fear. Making certain bold assumptions, my assistant Una determined that chilling the sun's core to below 10 million degrees would require an ice cube 562,000 miles on a side. If you were planning on using standard-sized cubes from your kitchen freezer, you'd need about 45 nonillion of them.
That's 45 followed by 30 zeroes. Never mind how long it would take to make that many ice cubes, Rei — it'd take you a while just to write it out.
Anyway, it's a lot of ice, more than twice the volume of the sun's core. Its size alone might be enough to tear the sun apart — but again, only temporarily. Gravity would eventually consolidate the solar fragments, the temperature would rise, and the thing would reignite like one of those trick birthday candles.
OK, so how might one permanently extinguish the sun? A nearby black hole could tear the sun apart and swallow it, as was witnessed recently by astronomers in a cataclysm so powerful it was detected 2.7 billion light-years away. It's also possible that collision with enough nonfusible material, such as a mass of nickel-iron asteroids, could dissipate the nuclear fire (and almost certainly lead to formation of a black hole, a cool concept all by itself).
One last thing. The cosmic ice cube posited above would have one-third the mass of the sun. Left floating in space and given enough time to compact itself, it would eventually heat to the point of fusion and become its own little sun. A mere 0.08 solar masses is required for this purpose.
So, Rei, when you're packing snowballs this winter? Make sure you don't make them too big.
Send questions to Cecil via straightdope.com or write him c/o Chicago Reader, 350 N. Orleans, Chicago 60654. Subscribe to the Straight Dope podcast at the iTunes Store.