I'm not a physicist so I don't even know what he's talking about.

is that a comment to "history of the entire world, i guess"?

is that a comment to "history of the entire world, i guess"?

I'm not sure, a friend asked me on what he should say since he doesn't know either.

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!

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!

Thanks! Just waiting for a reply now!

Be prepared for a reply along the lines of "your a dumb who is brainwashed by NASA and I'm blocking you" because that's the sort of thing I usually get.

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.

???
I sent him this btw.
https://physics.stackexchange.com/questions/216629/how-is-hydrostatic-pressure-overcome-when-a-star-is-formed

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.

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.

Thanks for the help, mate!

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.

In theory, except said arse-talkers tend to delete the comments too.

You could (crudely) say that once gravity wins, it 'keeps' winning over pressure because of the '4/8 ratio'. If you consider very heavy partices in a small area, the gravity should win over pressure, at that limit. On the oher hand, if the particles are very light, pressure will win. There has to be an 'mid-point' in between where they are roughly equal. Stars form on the gravity side of that limit.
(sorry for necroing)

I agree you can argue that there is a midpoint between gravity winning and pressure winning, but I don't follow your argument that collapse can continue (that gravity keeps winning).  What is the 4/8 ratio and how does it help?  Does it still hold if the collapse is adiabatic?

Rigorously, the midpoint where gravity balances pressure is given by the Jeans criterion.  It depends on the average mass of the particles, their temperature, and the cloud's total mass, size, or density.  In astrophysical contexts, the average particle mass is more or less fixed (it's mostly hydrogen and helium), and it's the total mass, temperature, and density that matter more.  

If a cloud exceeds the Jeans criterion, then it may collapse.  But does a collapsing cloud continue to collapse all the way to becoming a star?  Conservation of energy tells us that as the cloud collapses, the gravitational potential energy is converted to thermal energy, which increases the pressure.  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).  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!

So again I think the route to showing that collapse is able to continue is to show that there are ways for this thermal energy to be lost.

Yeah, I missed that temperature increases, so pressure does NOT vary as [tex]\dfrac{k}{x^3}[/tex], because all they did was [tex]PV = Constant[/tex], which holds only for isothermal collapses.

The 4/8 ratio is what was mentioned in the original post (https://imgur.com/a/85D81) in the second image.

Watsisname wrote:
 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).  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!

So, once it starts collapsing, it heats up, but simple radiation goes like [tex]P = k*T^4[/tex], so it loses a lot of energy (and cools) and then shrinks to heat up?

Ah, I gotcha.  And yeah, I would say the mathematical form of the argument in the original post is not adequate for the application.  It doesn't reveal anything about conditions for stability. The correct way is to examine the changes in energy as the cloud shrinks, and the key principles come from thermodynamics ([tex]dE = TdS - PdV[/tex] and the Ideal Gas Law), gravitational physics ([tex]U=-\frac{GM^2}{r}[/tex]), and some quantum mechanics (binding energy of the electron and H2 bonds).

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?

Basically, but think of it being more continuous than jerky, and the radiation isn't the most important cooling mechanism early on.

Initially the collapse may be virtually in free-fall, and proceed from the inside out (the inner regions become denser and collapse faster, which removes pressure support from the outer regions).  The collapse is also isothermal during this phase, since hydrogen dissociation is important to preventing the increase in thermal energy (costs 4.5eV per H2 bond broken).  

Then there is another phase of the collapse which is mediated by the gas being ionized (13.6eV per electron ripped from an H atom).  When the gas is mostly ionized the temperature will rise more dramatically, and radiation becomes the dominant cooling mechanism.  But by this time the dissociation and ionization have already allowed the collapse to result in a number of nearly protostar-sized, virialized regions.  These continue to shrink as they radiate, and accretionary processes become important to shaping the protoplanetary system (then we have to account for transference of angular momentum and all sorts of fun things).

I think it's very cool to see what this whole process looks like in simulations.

[youtube]YbdwTwB8jtc[/youtube]