Watsisname wrote:I have seen people make this or similar arguments before. They think they are showing that it is impossible for a cloud of gas in space to collapse to form galaxies and stars, because the pressure will always overwhelm the gravity.
So, you think this argument is wrong. I agree!
When I encounter something like this, I like to ask questions that test that person's knowledge of the subject and whether they can really support their claim with rigor.
For example, in this case I might ask them to consider a spherically symmetric cloud of gas with the Jeans mass just starting to collapse, converting gravitational potential energy to thermal energy. Then show that if the energy is radiated so that the collapse is isothermal, then the cloud becomes more unstable as it collapses.
I find that they have a very high failure rate. Of course, asking such questions and judging their answers requires understanding the subject, too. In your case, you could try describing the physics of it and point out what they did not account for:
As the cloud collapses, gravitational energy is converted to internal energy, in the form of thermal motion of the particles (i.e. the cloud heats up and pressure increases). But what they are forgetting (or are simply unaware of) is that there are mechanisms to remove the heat, such as by radiating it away (a dusty molecular cloud is especially good at this), or by dissociating and ionizing the gas. The collapse continues, until a new equilibrium is established by some other means. E.g. the formation of the star, supported by radiation pressure.
Or if you rather not try to explain that, you simply could show them what a textbook on introductory astrophysics has to say.
Hope that helps!
PS: Another fun fact is that if the gas cloud is so massive and dense that it lies within its own Schwarzschild radius, then it will collapse into a black hole regardless of how strong the pressure is. In fact, the stronger the pressure, the more rapid the collapse will be!
Watsisname wrote:Doesn't matter that you can't do the math. He can't either. But you know people who can, and they presented it for you.
Realize of course that this is an ignorant and unreasonable person "debating" anonymously on the internet. You are not going to convince him. You can instead be satisfied that you now understand the subject better than he does. The best you can do is show other viewers that the physics of cloud collapse can be demonstrated with rigor, and that he is unable to engage with that rigor.
Watsisname wrote:Pretty much, but it's productive for any other readers to see the actual astrophysics behind it, and that the person making the argument is talking out of their arse.
Another powerful tool is the Virial theorem, which says that for a gravitationally bound system, the thermal energy is -1/2 of the gravitational energy (and the total energy is negative because it is bound).[font=Roboto, sans-serif] This is interesting because it says that these systems (molecular clouds, stars, etc) have a negative heat capacity. If they lose energy, they shrink, and heat up![/font]
realmezzzz wrote:Source of the post So, once it starts collapsing, it heats up, but simple radiation goes like P=k∗T4P=k∗T4, so it loses a lot of energy (and cools) and then shrinks to heat up?