The sun is a giant ball of gas that burns at the centre of the solar system. And the adjective giant is not unearned: the sun constitutes 99.86% of the mass of the solar system. Without it life as we know it would not exist. Water would not be liquid. Plants would not be able to photosynthesise. Without the sun the earth would just be a big icy rock floating through interstellar space.
But let’s assume you have a death wish and want to try to put it out. How much water would you need?
Even though we think of the sun as being on fire, it’s actually not. What we normally consider as fire is combustion, a series of exothermic (heat-releasing) reactions between a fuel and an oxidant (usually oxygen but other substances can occasionally take part, like fluorine). For example, methane (CH4) reacts with oxygen (O2) to make water (H2O) and carbon dioxide (CO2).
To get a chemical fire you need a fuel, an oxidant, a high temperature (above the flash point of the fuel) and a chain reaction has to occur (these 4 elements are commonly called the fire tetrahedron). To put out this fire one of the four elements needs to be taken out. Water is used as it takes away more heat from the fire than the fire can produce, lowering the temperature. In some the reaction is energetic enough that water is not sufficient to put out the fire. In that case other substances (such as CO2) are used to cover the fuel, blocking atmospheric oxygen from coming in contact with it.
So that’s why water puts out fires. Except, as mentioned before, the sun is not on fire. It’s definitely very very hot but the energy does not come from an exothermic oxidation reaction but from nuclear fusion.
The sun is mostly composed of 2 elements: hydrogen and helium. Hydrogen makes up close to three quarters of the sun’s mass and helium makes up most of the rest. Just 2% of the sun’s mass is composed of heavier elements. The surface of the sun is “only” 5000-9000 K, which may seem like a lot, but compared to the temperature of the sun’s core which hovers between 9 and 17 million K, it’s actually not that hot. A bit chilly, even.
The immense pressure caused by the rather very large mass of the sun causes such high temperatures in the sun’s core. After hydrogen gas temperature reaches more than 10 million K nuclear fusion happens spontaneously. Two hydrogen nuclei combine and form a single helium atom. When they do so they shed a little mass which is converted to energy according to the most famous equation in physics: E=mc2 (energy is equal to mass times speed of light squared). Even though the mass lost in a single fusion reaction is very small, the speed of light is a very big number and anyway every second the sun ‘burns’ through 620 million metric tons of hydrogen. Which can only be described by a specific scientific term: that’s A LOT of hydrogen (and therefore a lot of energy).
So what would happen if you were to douse the sun with water. Well, the water would quickly turn into steam. This would absorb some of the energy, cooling the sun slightly. However, as the steam gets hotter and hotter the water turns into plasma. The hydrogen and oxygen atoms separate and lose their electrons. Congratulations, you have added more of the proverbial fuel to the fire. Oxygen would be used as a catalyst in a slightly different type of fusion reaction called the CNO cycle.
And even if you were to add so much water that the sun core‘s temperature would drop below 10 million K (and therefore under the threshold for spontaneous nuclear fusion), the drop would only be temporary. The high temperatures are caused by the mass of the sun which causes incredibly high pressures in the core. And all you’ve done is increase it. It wouldn’t be too long before the sun would start back up.
And of course, carrying such a large amount of water to the sun would be problematic for a couple of reasons. It could very well boil off before you’ve even reached the surface. The sun absorbs lots of passing by comets, but they usually evaporate before they get anywhere near it. Comets are mostly made of ice, and the only one we’ve observed plunging into the sun (comet C/2011 N3) completely evaporated 100 thousand km above the surface.
Of course, a huge amount of water would have its own very large gravitational force. Big enough, and it could become its own star. The smallest star we know of (AB Doradus C) only has 93 times the mass of Jupiter, or 0.08 of that of the sun.
So no, no amount of water would be enough to put out the sun. Too much and you’re probably just going to create a second star.
So how could you put out the sun? Well, it could be absorbed by a black hole, just like it was observed in 2011 when a dormant black hole in a distant galaxy (3.9 million light years away!) devoured a nearby star.
Of course, stars also die of old age. Though the sun is predicted to be around for another 5.4 billion years so you might have to wait a little while.
-Francesco (follow me on G+)
References and further reading