A mass of water larger than the sun

H20 is 18 daltons. (1+1+16)

The hydrogen in H2O is 2 daltons.

2/18=1/9.

We still have names for hydrogen bonding, van Der walls forces, friction, pushing, etc. even though it’s all EM force.

You would get a weird star.

Like others said, the gravitational heating would turn the thing to plasma. The hydrogen should pretty much fuse like it would in a normal star. So what happens to the oxygen?

Well, fusion in the sun mostly proceeds by the CNO cycle. Instead of hydrogen directly fusing with itself as you might expect, it instead fuses with several isotopes of carbon, nitrogen, and oxygen in a repeating chain C[sub]12[/sub] -> N[sub]13[/sub] -> C[sub]13[/sub] -> N[sub]14[/sub] -> O[sub]15[/sub] -> N[sub]15[/sub] -> C[sub]12[/sub]

Note that normal O[sub]16[/sub] is not included, and that’s the stuff we’d find in our normal water. So we need another cycle–CNO-II (which should probably be called FNO…) comes to the rescue: N[sub]15[/sub] -> O[sub]16[/sub] -> F[sub]17[/sub] -> O[sub]17[/sub] -> N[sub]14[/sub] -> O[sub]15[/sub] -> N[sub]15[/sub].

So all that O[sub]16[/sub] should eventually get fused away via this secondary cycle (as well as some even less common ones). Might take a while, though.

You could argue that “large” might mean size or mass, but when discussing 3-dimensional objects, size means volume, not width.

Well, you could, but that doesn’t seem to be the way that most people (i.e. non-technical people) mean it. As far as I can tell, diameter seems to be the parameter they care about most. This shows up very commonly in reports/discussions about exo-planets as well as the big todo about Pluto a few years back.

Your same source says otherwise, only a vanishing fraction of the Sun’s fusion is CNO, it is primarily the p->p process. Hydrogen fusion dominance seems to be the definition of main sequence.

A few years ago I calculated the maximum diameter of a ball of water that did not have any high-pressure ice in the centre (or rather someone else calculated it and I checked the figures); this turns out to be 4800km, more or less. at room temperature. Bigger than I expected, but much smaller than the Sun. It would have such low gravity that it would evaporate, and cool, until it was a much smaller iceball. In a ball big enough to have fusion, I presume, the ice would melt.

Ahh, my bad. I had thought the threshold was just below the sun’s mass instead of just above. Nevertheless, some CNO-II activity will still take place which should eventually convert the O[sub]16[/sub] into helium. It was going to be slow anyway since that cycle is even more rare than CNO. Though I do wonder what the final density will be–although the mass of the water star will be less than the sun, it may end up more dense due to the heavier nuclei. Could be that the increased pressure starts to favor CNO again.

I’m not familiar with that. Can you point to an example?

Our water star will be unlike any star existing in nature; it will by mass consist mostly of oxygen (comparable to some end-stage white dwarfs), but will also have a non-negligible hydrogen content. Moreover it won’t have a differentiated core, at least not at first. IANAastrophysicist but my wild-assed guess is that along with H-H burning it’s core it will be hot and dense enough to see the various oxygen-fluorine-nitrogen cycles along with some triple-alpha carbon production. Given the relative scarcity of hydrogen however I don’t know if there will be time for enough carbon to be produced to significantly alter the water star’s evolution. I also don’t know if it will simply burn in equilibrium until it’s hydrogen fuel runs out, or if it will go through various expansion and contraction phases.

It really depends on context.

If we’re buying milk and I say “Get twice as much” then you assume I mean 2 gallons instead of 1 gallon.

But if we’re building a model train set and I say “Make it twice as big” then I’m probably referring to its linear dimensions and not its volume.

And if we’re apartment hunting and I want something twice as big, I almost certainly mean total square feet.

All of these usages are correct, they’re just leaving certain parameters unspoken.

Stars are usually measured in mass rather than volume - after all, the volume can change a lot over a star’s lifespan. So now we’ve got a fourth context and a fourth assumption for “twice as big” and this one doesn’t care about size at all.

Agreed that in the context of stars mass is probably the best metric for size, since the volume/diameter can vary a lot during the star’s evolution.

Look up at the sky at that big glowing ball of light. THAT is what would happen.
The sun and a big ball of water are mostly the same stuff. The giant water balls gravity would cause it to collapse in on itself. I don’t know the exactly chemical and nuclear process, but it would basically be heat and pressure causing a combination of the water molecules breaking apart and the hydrogen combining in a fusion reaction into heavier elements. Basically doing what any star does now.

Not even close. The sun is something like 98% hydrogen. Water is 11% hydrogen and 89% oxygen by mass. This would make an enormous difference in how hot and dense the water star would be, and what nuclear reactions could take place in it.

Yeah, a high oxygen low hydrogen star would be interesting.