How did water get on the sun?

This answer courtesy of Rob Rutten (Sterrekundig Instituut, Utrecht University, Netherlands).

How can there be water on the Sun? Good question! As you will see below, a one-sentence question with a many-sentence answer. Often, questions of astronomy are, at the same time, questions of physics and questions about the universe. The same with your question.

Water consists of tiny particles called molecules. You can't see them, not even under a microscope, they are much smaller than that. But even each water molecule consists again of smaller particles called atoms. Each water molecule consists of three: one oxygen atom (called O) and two hydrogen atoms (called H). Therefore, the formula for water is H2O.

Atoms are the stuff the universe is made of. The atoms themselves consist again of smaller particles (protons, neutrons and electrons) and the first two of those are again made up of smaller things called quarks. By then you get to very small things! But basically, it is fair to say that the universe consists of atoms. Those are what is called "the elements"; each atom type making up an element depending on how many protons and neutrons and electrons (usually the same number of protons and electrons) it has. For example:

H  = hydrogen  = 1 neutron + 1 proton + 1 elecron
He = helium    = 2 neutrons + 2 protons + 2 electrons
Fe = iron      = 74 neutrons + 56 protons + 56 electrons
Au = gold      = 118 neutrons + 79 protons + 79 electrons
and so for all elements (silver, aluminum, carbon, copper, mercury, nitrogen, sulphur, sodium, calcium, uranium) -lots! Over a hundred in total. Other substances (like water, oil, plastics, sugar, air) are mixtures of different atoms within a molecule as in water or mixtures of different molecules (like air).

You and me and the earth and the other planets and the sun and the other stars are all made up from these hundred or so elements. Hydrogen is a very lightweight gas, having just those three particles (and electrons don't count, they are much lighter than neutrons and protons), and therefore much of the earth's hydrogen has escaped from the earth. But there is still enough left to make oceans. On the earth, most hydrogen is bound with oxygen into H2O molecules = water. But that only happens where it is cool enough. If you would cook the whole earth, first the oceans would boil off into water vapor. That is still made of H2O water molecules, but they are now flying around freely, as a gas, instead of sitting in drops making up a cloud or a liquid. But if you then go on with cooking the earth (don't do that please), the H2O molecules will split into loose atoms H and O.

The sun is heavy enough that hydrogen has not escaped from it. The sun also consists of all the elements, but hydrogen is the one it has most: nine/tenth of the solar particles are hydrogen ones. And the sun is very hot (over 10 million degrees in its inside) so that there no molecules, only loose atoms. (In the inside, these have also lost their electrons).

So how can there be water molecules on the sun? Well, there are a few cool places, just cool enough that some hydrogen (H) atoms and oxygen atoms (O) can team up to become a water molecule (H2O). Those are the centers of sunspots on the outside of the sun. Sunspots are dark places on the sun with a lot of magnetic field, much much stronger than the earth's magnetic field which pushes your compass needle to the direction of the North pole. In sunspots, the magnetic field is so strong that it pushes the gas largely aside and makes it cool as well. The remaining gas is so cool that there can be a bit of water by H atoms joining O atoms. That is still water vapor, not fluid! You can't go surfing on the sun - and I can't go kayaking there (that's my sport).

There is more to this. When the universe started in what we think was a big explosion called the Big Bang, it made only hydrogen and helium, not all the other elements. Those were made later, inside stars and when heavy stars exploded at the end of their life. All the atoms other than hydrogen and helium around us and inside us (yes, you too) were made in stars long before our sun and our planet were formed. You are made of star stuff! In five billion years from now, the sun will die but not explode, it is too small for that. But it will swell up first and push the earth out of the way. It may happen that the H and O atoms making up the water in your body right now end up in some other living body say ten or more billion years from now. I like that idea.

Jump to the press report to read more about this exciting discovery.

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Special Thanks to R. Rutten.