The sun is burning gas.
The Sun is burning gas now, but that in no way answers the OP’s question.
The sun is not nor has it ever been burning gas. Burning (combustion) is a chemical process. The sun is hot today because of its constant fusion reactions, a nuclear process.
You can also look at some of the cites at the bottom of those pages on Google Books, which use those terms.
ETA: Chapter 2 on one of those cites is on gravitational contraction.
Just being under pressure doesn’t cause any heat at all. But the process of going from low pressure to high pressure generates heat.
Gas heats up as you compress it for the same reason that a tennis ball speeds up as you whack it with a racket. The surface of your piston is constantly bombarded with bouncing molecules (which is the origin of pressure in general). If the piston is motionless, they bounce away with the same speed as they started with. But if the piston is moving towards the gas, the molecules gain a tiny bit of speed when they bounce. Zillions of bounces later, they’ve gained quite a bit of speed/energy and thus temperature. The reverse happens when the piston moves in the other direction.
Taking the statement
**Having started out as a huge cloud of gas and dust, the sun eventually was condensed by gravity into a ball, its temperature rising steadily in the process… When a gas-and-dust ball gets up to about 10 million degrees Celsius, nuclear fusion spontaneously begins.
**
I am not a scientist, just an old snipe who has worked with steam and refrigeration systems.
But what stood out to me was that the cloud of gases “condensed”, that is changed state from gas to liquid. With a change of state from gas to liquid heat is given off. That hear will have to be removed from the mass if it is going to stay the same temperature. If it is not removed it will raise the temperature of the mass.
And there is the added heat of compression.
Is that true of all substances? I thought it was one of the quirky properties of water, but that it wouldn’t necessarily be true of hydrogen.
Any gas to liquid.
Don’t take that too literally. “Condensing” here doesn’t mean gas->liquid, just that you’re starting with diffuse material (gas) and ending up with something more structured (a star).
In fact, a phase change does happen, but it’s gas to plasma (which happens when you get to a few thousand degrees).
The point is that the molecules collide with each other more often and more quickly once pressure is applied. Apparently they can’t just stop, so squeezing them together just makes them move faster- and Speed x Mass is ultimately all ‘heat’ really is. It is the collisions that transfer heat/kinetic energy. In a huge cloud of hydrogen gas in space, as the molecules get squeezed closer and closer together, they bounce off each other faster and faster the more ‘crowded’ they get. Eventually they move so fast that when they collide they have greater energy than the… uh, either weak or strong nuclear force, protons repel each other being positively charged, unless they get close enough to ‘stick’… anyway, they have enough energy to overcome their mutual revulsion and instead fuse, which releases an enormous amount of energy (the explanation why would make a great thread by itself). Voila, a star.
That brings up an interesting question; obviouly, the sun is a big fusion reactor, but what it happening to the elements that are not Hydrogen or Helium? - there must be small amounts of most other elements, just because the sun is massive enough to have swallowed some metals, etc.
What is happening to any oxygen that is part of the sun? Is it just part of the plasma? - i.e. too hot to combine with anything?
Are there any chemical compounds (i.e. products of the heavier contaminants) that are known to, or could concievably exist as part of the processes ongoing in the sun.
Although it’s not the dominant cycle for our Sun, the CNO cycle does occur, and in larger stars is the dominant form of fusion. In short, hydrogen gets fused into helium in a somewhat roundabout way, with carbon, nitrogen, and oxygen all playing a part. These are all nuclear reactions.
But yes, all elements in the core will be ionized and not chemically combine with anything. Some molecules (like water) can exist in the very coolest parts of the surface (sunspots, mostly), but deeper down it’s all plasma.
I don’t think you can get any molecules at all on the Sun, even in the spots. You can with cooler red stars, though.
This is a bit old, but as far as I can tell hasn’t been invalidated since then.
Been a long time since college chemistry, but I believe that the amount of heat doesn’t change–a fixed amount of heat energy just gets confined to a smaller space therefore the temperature goes up.
This explanation might be equivalent to the others above, but to me, it’s the simplest and clearest. The implication is that temperature is proportional to heat density, all other things being equal.
It’s also false. The hot compressed gas has more thermal energy than the uncompressed gas. We know this must be the case because we performed work on the gas when moving the piston. Temperature is based on the average kinetic energy of the particles, not their density. Two cylinders of the same gas can have different thermal energy densities, even when they are the same temperature, if the pressures are different.
I won’t try to argue the point, but then what is going on when a hot gas is allowed to expand? When I spray my compressed air can that sucker gets cold. Is the heat used to do work on the escaping gas?
Sorta. Mostly, the escaping gas does work on its surroundings. In the case of compressed air, I’d guess that most of the work goes to waste in pushing the atmosphere out of the way. Some will be lost to aerodynamic losses in the valve and such. Anyway, exactly the same thing goes on in an internal combustion engine: the fuel burns to a hot gas, and it does work as it pushes the piston down, cooling in the process.