This is a fairly straightforward question, yet for some reason Google only sends me to a poly-mega-truckload of creationist sites when I prompt it. Since I’d rather have a real answer…
Anyhoo.
The Sun is a big ball of gases, on fire. Since combustion turns fuel into inert components, it would make sense that the fireball shrink over time, if ever so slightly, unless somehow the products of the combustion are also its fuel, or are turned back into fuel… somewhere. What’s the Dope ?
(the bonus question would be : why the slew of creationist crap ? Is Sun shrinkage or lack thereof a bullet point in their nonsense ?)
Stars are somewhat variable in size, as gravity compresses them and heat expands them. The sun is relatively stable as stars go, and I don’t know exactly what it’s doing at the moment. Eventually (in billions of years) we can expect it to expand to many times its current size.
As for “combustion” - that’s not what is happening in the sun. It is powered by nuclear fusion
Yes it’s losing mass. But it’s not on fire in a regular combustion by combination of a fuel with oxygen kind of way. It’s performing fusion - hydrogen into helium mostly.
There’s a small loss of mass in the fusion conversion. Also some material is ejected by the solar wind and by flares.
No idea what creationists are doing with the sun. Don’t want to know, really.
Also, as dracoi said, its size and its mass are two different things. It’s losing mass, but may blow up to a larger, though somewhat lighter, size when it starts running out of hydrogen to convert.
As far as I understand it, creationists claim that the sun is indeed shrinking by a small amount each century, 0.1% or something like that. They say this is not a problem if (as they think) the Earth is only 6000-10,000 years old, but hundreds of thousands of years ago this would mean the sun was so enormous that it would have been impossible for the Earth itself to even exist in its current orbit, let alone to sustain any life.
Of course, scientists dispute many aspects of the argument, including: whether the sun is in fact shrinking right now; whether the sun is shrinking or not in the long-term; and if it is shrinking in either the short or long term, whether it’s been shrinking at that same consistent rate for hundreds of thousands of years.
For interest’s sake, it’s worth noting that the sun fuses about 700,000,000 tons of hydrogen per second, producing about 695,000,000 tons of helium. The remainder is released as gamma ray energy, which is absorbed and re-emitted at lower and lower temperatures for about 80% of the way to the surface of the sun (after which it’s mostly convection that carries it). The sun has apparently used about half of it’s starting supply of hydrogen.
About half of its usable starting supply of hydrogen. About 90% of the hydrogen in a star will never be fused at all, but just form part of the remnant after the star dies.
By number of particles, they’re about the same. But a helium nucleus is four times as massive as a hydrogen, so for a given pressure and temperature, helium will be four times as dense (and hence, a given mass will take up 1/4 the volume). Of course, temperature won’t be constant, either.
Thanks everyone for answering and setting me straight. I admit, I slapped myself about the mass/volume confusion. I should know better, that’s high school physics… Anyway.
Ok. So, what happens to the resulting helium then ? Does it decay overtime, splits back into hydrogen through nuclear fission, just stays around ? And if it just sticks around, doesn’t it interfere with the fusion reaction ?
Ah yes, now that you mention it, I recall something like that as well. They really like freewheeling extrapolations, don’t they ? Then again, I guess it’s sort of a prerequisite
If the Sun is losing mass, then is its gravity reduced? And if its gravity is reduced, isn’t it expanding because there is less gravity to pull the gas in?
If you make things hot enough, you can fuse anything; really heavy stars can fuse helium and more, creating all of the heavier elements that we see (though anything over iron is actually a net energy loss, as I understand it). Helium and other elements are stable, so they don’t go anywhere and they don’t really interfere with the reactions until later in a star’s life. You might start by reading about how stars work during their “normal” lives on Wikipedia here: Main sequence - Wikipedia and then follow the links to learn more about how they age and finish up.
I would guess that there is some kind of dynamic equilibrium governing the volume of the sun: If the sun would expand, and its density is reduced, less energy is produced per volume, which would lead to a reduction of the temperature, which would reduce the pressure, which would lead to shrinkage. On the other hand, if the sun would shrink, its internal temperature would rise, causing it to expand.
here’s a pdf of an old paper in the British Journal of Applied Physics analyzing this in much more complexity
Because that would entail an energy loss. Or, to voice a more general objection, Thou Shalt Not Create A Perpetual-Motion Machine. But normally fission only goes on with heavy nuclei, where the resulting nuclei have less net energy than the original one had (thus leaving an excess to be radiated away). Similarly, normally fusion doesn’t make anything heavier than iron, which is the bottom of the trough whether you’re fissing or fusing. The only time anything heavier results from fusion is when there’s a lot of surplus energy about, specifically, during a supernova explosion. Then, all kinds of stuff gets smooshed together, which is handy for those of us who like to be able to use the resulting heavy elements, some stable, some less so.
The creationist claim that the sun is shrinking is a holdover from the 19th century, when even real physicists didn’t have a clue about nuclear fusion. One theory was that the sun’s mass must be falling in towards its center, releasing energy as the sun shrunk. Physicists calculated the rate at which this must be happening in order for the sun to be radiating the energy we measure. They found that at that rate, the sun’s surface must have reached as far as Earth’s orbit 70-80,000 years ago, which was much too recent for any kind of geology or Darwinian evolution to occur since.
Of course, at the time, both evolution and geology had loads more evidence to back up their estimates of the age of the Earth. Any real scientist at the time knew that this meant the “Sun is shrinking” theory must be wrong. But the idiots somehow turned it around and figured physical speculation must outweigh the mountains of geological and biological evidence for the age of the Earth. You were an idiot in the 19th century if you thought this was evidence for creationism. Any 21st Century individual seriously advancing this theory must be devoid of any brain matter at all.
Others addressed this, but since you quoted my post, let me see if I can expose more of my ignorance.
As Chronos says, not all of the hydrogen that starts out in a star is available for fusion. A lot of that has to do with the very problem you mention; at some point, the proportion of helium to hydrogen means that the pressure of gravity isn’t enough to keep enough hydrogen atoms bumping into each other to sustain the fusion pressure (a star is essentially a stable system of outward pressure from the fusion reaction balanced by the inward pressure of gravity). As the fusion pressure drops, the star collapses a little bit – which drives the pressure up until the point that helium starts fusing (although a star has to be more than half as massive as the Sun for this to be possible). At a certain point, depending on the size of the star, the drop in fusion pressure does not result in enough gravitational pressure to fuse the next element up. At that point, the fate of the star depends on its mass.
Very large stars (>9 Solar masses) manage to sustain fusion through the steps all the way to iron, which as others have mentioned is a special stage. Iron fusion is endothermic, which is to say that it results in a net loss of energy. As soon as iron fusion begins, the fusion pressure that balances gravity goes away in a hurry. The mass of the star collapses inward very quickly, driving up the pressure immensely, and bingo! You have a [Type Ib, Ic or II] supernova. Usually.
Really immense supernovae will overcome the gravitational binding energy of the stars, essentially blowing it away, leaving no remnants, such as black holes or neutron stars.
Now, if an intrepid person who actually knows what they’re talking about could just come along and clean up my mess…
The ideal gas law suggests another reason the sun may be shrinking:
At any given temperature and pressure 2 moles of hydrogen will take up twice as much volume as 1 mole of helium.
Now the core of a star is a somewhat extreme environment in which to apply the ideal gas law, but even more complex versions of PV = nRT follow the same general trend linking volume, moles, pressure and temperature.
Good heavens. I can’t believe so many dopers don’t know how the sun shines!
The short version:
The sun started out as a giant cloud of mostly hydrogen that collapsed in on itself due to gravitational attraction. As it collapsed, the pressure all that material was under caused the temperature to rise and rise. At a critical point, the temperature became so hot that the hydrogen atoms began to fuse together into helium, releasing enormous amounts of energy in the process.
Now, the sun is in a long-term tug-of war between the gravitational attraction of all of that gas and the outward expansion pressure caused by the fusion heating. Hot gases want to expand. Gravity makes that mass want to come closer together. So you have the sun at a basic equilibrium between these two forces for about 10-12 billion years (we’re halfway through, now).
Once the easily-fused hydrogen is used up, the expansion pressure will let up, allowing gravity to cause the sun to contract, creating MORE pressure, which then will create enough heat to fuse helium into carbon, and result in a new equilibrium (that only lasts about 130 million years). This is also when the outer layers of the sun balloon out and envelop some of the inner planets. Official end of life on Earth, if not earlier.
Finally, things become unstable enough during these sensitive helium fusion reactions that the sun blows off huge quantities of gas to form a planetary nebula, and the stuff that’s left becomes a white dwarf, which cools over billions of years.
There’s a lot more going on (esp. in regards to the switchover from Hydrogen fusion to Helium fusion, when there are different layers of different gases in the sun, which expand and fuse at different rates, etc.), but that’s the gist of it.
