From Quora, an answer to the question “If we pour water on the sun with a bucket as big as the sun, will the sun be extinguished?”
The probable answer is “no.” The Sun involves a special type of fire that is able to “burn” water, and so it will just get hotter, and six times brighter.
Water is 89% oxygen BY MASS. And the Sun’s overall density is 1.4 times that of water. So if you have a volume of water the VOLUME of the Sun, it will have 1/1.4 = 0.71 times the mass of the Sun, and this mass will be .71*.89 = 63% of a solar mass of oxygen and 8% of a solar mass of hydrogen. The Sun itself is 0.74 solar masses of hydrogen and 0.24 solar masses of helium.
So you end up with a 1.7 solar mass star with composition 48% hydrogen, 37% oxygen, and 14% helium (with 1% heavier elements).
Now, will such a star burn? Yes, but not with the type of proton-proton fusion the Sun uses. A star 1.7 times the mass of the Sun will heat up and burn almost entirely by the CNO fusion cycle, after making some carbon and nitrogen to go along with all the oxygen you’ve started with. So with CNO fusion and that mass you get a type F0 star with about 1.3 times the radius and 6 times the luminosity of the present Sun, and a temperature somewhat hotter than the Sun (7200 K vs. the Sun’s 5800 K). It will be bluish-white, with more UV. That, along with that 6 times heat input, will cause the Earth’s biosphere to be fried, and oceans to probably boil.
Well, we probably shouldn’t do that then. (via gizmodo)