# A mass of water larger than the sun

My kid wants to know what would happen if a mass of water were to exist that is larger than the sun. (He asked it in terms of “twice as large as the sun” but, just, you know, star-sized.)

I told him I think this might end up just turning into a star due to gravitational forces and whatnot, but I really have no idea.

What would happen?

Is it just water, or is there some kind of solid rocky core with a gravitational field? Is it close enough to a heat-producing star to exist in liquid form?

A body of water the size of the sun has a gravitational field, rocky core or not.

I forget which science fiction story (by Kurt Vonnegut?) involves more esoteric phases of ice, i.e. Ice 9 etc. Under extreme pressure the molecules will pack differently, the weak force that accounts for ice expanding initially after freezing is overwhelmed, etc.

Interesting question. This isn’t my area of expertise, but here’s my guess.

Without any source of heat, I would expect the water to freeze into ice. At the same time, gravity is going to start compressing it. If it’s not already a sphere, it will be soon. As it compresses, the inner core is going to heat up. If the core had a chance to cool down into ice before compression heated it back up, then the core will melt and turn back into water. So for a while, I would expect to have a ball of ice on the outside and a core of water on the inside.

Normally, stars fuse hydrogen into helium, and then fuse helium into heavier elements. At some point the process stops because you don’t have enough mass to keep the reaction going (or the star blows itself to bits during the carbon stage). For larger stars the process stops somewhere around oxygen and silicon. Smaller stars will stop sooner. Since the sun is a relatively smaller star, it’s not going to go very far beyond the helium stage, becoming a white dwarf that slowly radiates its energy away and becomes a dead black dwarf. A star twice as large as the sun could probably ignite carbon but probably not oxygen. So it would die out before it was able to burn oxygen.

This makes me think that your mass of water might get hot enough to separate out the hydrogen and oxygen. At that point the oxygen will sink to the core and the hydrogen will be pushed up to the outside of the sphere. With a mostly oxygen core, while you might get some fusion, you aren’t going to get a sustainable star.

Cecil sort of did an article related to this (he was exploring how many ice cubes to put out the sun), and he seems to think that it would eventually compress and begin to fuse, forming it’s own sun.

Go here http://what-if.xkcd.com/ and hit [submit a question] at the bottom. He’s done a couple of similar items e.g. http://what-if.xkcd.com/14/ and might be glad to tackle your question.

It’s also irrelevant whether it’s “close enough to a heat-producing star to exist in liquid form.”

That much mass is going to produce so much gravitational heating that most of the mass will end up in plasma form, at which point thermonuclear fusion will start. You’re going to end up with a star.

On preview, I see that others have answered…

Even with an oxygen core, if you start off with a mass twice that of the sun, you should be able to support a zone of hydrogen fusion just outside of the oxygen core.

Very shortly, what you’ll have is a star. It’ll collapse under its own weight, and the resulting heat will quickly build up to the point where the hydrogen atoms from the H2O start to fuse. You’ll end up with a star with lots of oxygen kicking around in it.

Am I right in thinking that it doesn’t actually matter what form the mass takes - once it gets up to a certain point, it’s going to collapse and start a sun whatever?

I don’t believe so. Nuclei more massive than iron don’t fuse except in a supernovae. So you couldn’t get a normal star. Whether or not you could get something like a supernova, I’ll leave to experts, but I don’t think so. You’d get some kind of gravitational collapse, but you wouldn’t have the remaining light elements a star has to provide the supernova explosion.

It is not the Weak Force that accounts for ice expanding as it freezes, it is entirely down to EM and the polarity of water molecules that results in the rarified crystal structure of ice-as-we-know-it. As others have noted, extremes of pressure produce alternate phases of ice, some of which are posited to exist in the depths of the water planets that have been observed around other stars.

The Weak Force is the force that governs radioactive decay, nuclear reactions and some particle transformations, such as neutrons becoming protons.

md2000 was refering to weak forces between molecules, not “the Weak Force”.

“The fictional ice-nine should not be confused with the real-world ice polymorph Ice IX, which does not have these properties.”

“While multiple polymorphs of ice do exist (they can be created under pressure), none has the properties described in this book, and none is stable at standard temperature and pressure above the ordinary melting point of ice. The real Ice IX has none of the properties of Vonnegut’s creation, and can exist only at extremely low temperatures and high pressures.”

Here’s an amusing one they did called “A mole of moles” that’s somewhat related, but with an obviously different substance and a lot less mass than specified in the OP at something just bigger than the moon.

Here’s a taste to whet your appetite: “The mole planet is now a giant sphere of meat. It has a lot of latent energy (there are enough calories in the mole planet to support the Earth’s current population for 30 billion years).”

https://what-if.xkcd.com/4/

Which is never described thusly. Crystal structure is governed by the electromagnetic force, the same force responsible for chemistry, it is not some vague “weak force”.

THAT WOULD BE SO PERFECT. The kid asked the question, in part, because he’s reading Randall Munroe’s book collecting entries from that (for lack of a better word?) column!

First, let’s define what we mean by “as large as the sun”. Are we talking mass, or volume? The average density of the sun is actually greater than room temperature water, about 1.4 grams per cm^3, room temperature water is of course 1 gram per cm^3, which was surprising to me, I would have expected it to be a lot less. But the sun is mostly hydrogen by mass, while the water is mostly oxygen by mass, so the water star would probably compress to a lot smaller than the sun. You’d end up with a sphere in equilibrium between the compressive force of gravity and the outward pressure of fusion. The water star would be a lot colder than the sun, because oxygen would release a lot less energy during fusion than hydrogen does. However initially the heat released by gravitational compression could be extremely high until the water star reaches equilibrium.

A good point. The question is underdefined as is typical of these kinds of questions.

But taking the “twice as large” to mean a sphere of ordinary water twice the diameter of the Sun (at least as an initial condition – it won’t stay that large), I find that the mass of water will be over 5 times that of the Sun. (mass of Sun 2 * 10^30 kg; mass of waterstar 1.3 * 10^31 kg)

Since only 1/9 of the mass of water is hydrogen, the waterstar has somewhat less H than the Sun, although still far more than enough to support H fusion. But all that oxygen in it will make for a really strange star. With our current knowledge, we can only guess as to what will happen – astrophysicists don’t study such odd concentrations of matter. My guess is that it would not settle down to a main sequence-type star, but fairly quickly evolve to a supergiant phase. It might even supernova after a relatively* short time.

• Relatively short for the lifetime of a typical star, that is. But that might be thousands or even 10s of thousands of years.

It will form a star. What I don’t know is how quickly it will form a star if it were magically summoned into existence. Another interesting question is what would happen moment by moment (or year by year) if the giant ball of water were placed close to the Sun.

I know I’m gonna feel dumb for asking this, but why is it not 2/10 or 1/5?

ETA: Nevermind. Don’t answer this. I figured out my idiocy on my own. It’s been a long time since chemistry.