Star Formation (Star Condensation)

I've got what should be a really simple question about star formation...

We are told that after the big bang, the universe expanded and eventually stars formed from the immense clouds of hydrogen and helium gas that stretched across the galaxy.

However, one of the fundamental properties of a gas, is that it attempts to expand as much as possible and will repel other molecules around it. This principal is what keeps the tyres inflated on our cars.

This being the case, I would have expected no star formation to ever have taken place. What force could act upon a body of dispersed gas and initiate the process of causing it condense into a liquid? I can understand perfectly that our sun now holds itself together by gravity. But I would have thought that this force only comes into being when a vast amount of matter is densely packed into an object such as our sun.

Is it possible that gravity can overcome a gas's attempt to expand even when there is initially no concentrated mass of matter?

Ian

Edited by Hoof Hearted, : No reason given.

Edited by Hoof Hearted, : No reason given.

Edited by Adminnemooseus, : Added the "(Star Condensation)" part to the topic title.

Thread moved here from the Proposed New Topics forum.

What force could act upon a body of dispersed gas and initiate the process of causing it condense

Gravity

I can understand perfectly that our sun now holds itself together by gravity. But I would have thought that this force only comes into being when a vast amount of matter is densely packed into an object such as our sun.

You would be wrong, though. Gravity also holds together objects much bigger then stars, like Galaxies

Is it possible that gravity can overcome a gas's attempt to expand even when there is initially no concentrated mass of matter?

ideed it is. But you are right, this is not a trivial question at all. There is the need for some sort of seed (a region that is slightly denser then its neighbourhood). because of that, I gigant cloud of gas can stay stable for very long periods and then go through a period of intense star formation. The point is that once it becomes unstabl, there is no stopping it. the little seeds atract some matter becoming a little bigger, and so forth, until the radiation produced by the newly formed stars dissipates whatever is left of the gas, and the process halts itself

Well, let us look at the mathematics behind all the theoretical mumble jumbles, shall we?

Ideal gas law PV = nRT = NkT
P = pressure
V = volume
n = number of moles
R = universal gas constant (8.3145 J/mol K )
T = temperature
N = number of molecules
k = boltzmann constant (1.38066 * 10-23 J/K)

Our sun is an average star and has about 10^57 atoms of hydrogen and helium. A typical stellar cloud is about 20K, which is about -250 degrees celcius. A typical interstellar cloud is about .1 LY across, which is about 9.46 * 10^14 meters. Volume of a sphere is 4/3*pi*r^3. Therefore...

k=1.38066 * 10-23 J/K
T=20K
N=10^57
V=4.41*10^44 m^3

P = (NkT)/V = 6.26*10^-10 J/m^3 which by the way is 6.26*10^-10 N/m^2

Now, the gravitational force on each hydrogen atom on the outer rim can be calculated roughly by the following equation.

F = (G*M*m)/r^2

F = force
G = universal gravitation constant (6.67 * 10^−11 N m^2 /kg^2)
M = mass of an object (1.7*10^30 kg)
m = mass of the other object (1.7*10^-27 kg)
a = surface area of hydrogen atom(3.52*10^-20 m^2)

Therefore, F/a = 1.16*10^-2 N/m^2

So, the pressure on each atom on the outer rim of the gas cloud is 6.26*10^-10 N/m^2 while the gravitational force on each atom's surface is 1.16*10^-2 N/m^2. As you can see, gravity clearly wins out on this one.

I move that the topic be closed.

Moose

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Professor, geology, Whatsamatta U
Evolution - Changes in the environment, caused by the interactions of the components of the environment.

"Do not meddle in the affairs of cats, for they are subtle and will piss on your computer." - Bruce Graham

"The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness." - John Kenneth Galbraith

"I know a little about a lot of things, and a lot about a few things, but I'm highly ignorant about everything." - Moose

You know, I have 3 dogs. My dogs would own your smiling cat there.

{Are you threatening my cat??? - ADMINNEMOOSEUS}

Edited by Adminnemooseus, : I love my yawning cat.

Thank you for the involved reply. I accept now that a dispersed cloud of gas can indeed collapse and form a star. But would I be correct in assuming the following?:

1. A tiny amount of gas would always expand because of the lack of gravity.

2. Once seeded, a large cloud will contract because of the effect of gravity.

3. So presumably there is an intermediate size of cloud which causes an equilibrium. The two forces cancel each other out and a stalemate occurs.

Ian

Here are some links on the topic, if you're just looking for more information.

http://www.astro.virginia.edu/class/hawley/astr124/starbirth.html
http://creationwiki.org/Star_formation
http://creationwiki.org/Nebula_hypothesis
http://en.wikipedia.org/wiki/Boyle%27s_Law

I haven't done the math myself, but it used to be commonly taught that Boyle's Law would prevent a gas cloud in the vacuum of space from gravitationally collapsing and forming a star. Stars were said to form when a shock wave from a nearby supernova explosion disturbed the cloud(s) enough that a high-density area could form and start the process of stellar formation.

Obviously the first generation stars would have no supernova explosions to get them started, so new ideas have been advanced to get around the problem. Some require very high temperatures, and others go the opposite way and use very low temperatures. Some seem to use both simultaneously, but I must be mistaken about that - who wouldn't spot such an inconsistency?

I haven't seen dark matter invoked yet in star formation scenarios, but it's only a matter of time. Such a handy tool cannot be neglected forever.

These calculations assume the "typical stellar cloud" has the same mass as our sun, occupying a spherical space "about .1 LY across". Perhaps this is why your results are the first I've seen that indicate gravity is sufficient to form stars from these clouds.

I think the alternative approach would be to take a given amount of gas and calculate it's volume at various temperatures, or take the volume and calculate the mass.

I could be wrong. I don't have a lot of experience with this kind of thing. But there must be some explanation for your unique results.

I was just reading those articles and realised something quite fundamental in all of this...

When I mentioned the scenario of compressed air keeping my car tyres inflated. I was forgetting that the air in my car tyres is somewhere close to 300°K. If the same quantity of air was at the temperatures encountered in deep space, then I suspect that I would be running on my rims.

So of course a cold gas is relatively easy to compress, and it's a lot easier to for me to understand now how a star can be formed from gravity.

CTD writes: These calculations assume the "typical stellar cloud" has the same mass as our sun, occupying a spherical space "about .1 LY across". Perhaps this is why your results are the first I've seen that indicate gravity is sufficient to form stars from these clouds.

You misunderstood me. I made that calculation off the top of my head to disprove the assumption that the tendency of a gas to expand is stronger than gravity. Clearly, a quick look at the some actual math says otherwise.

But on the other hand, our sun make up the bulk of the mass of our solar system. Why can't I use the sun's mass to calculate the initial gravitational force?

Edited by Tazmanian Devil, : No reason given.

The Angry Astronomer has a short piece on this. People here might find it interesting. On comment in particular:

Evolutionists claim that stars form from swirling clouds of dust and gas. For this to happen, vast amounts of energy, angular momentum, and residual magnetism must be removed from each cloud. This is not observed today, and astronomers and physicists have been unable to explain, in an experimentally verifiable way, how it could happen. First off, evolution and cosmology are two distinct fields and are wholly unrelated. Their conflation of the two does not impress me. Regardless, the claim that energy must be lost is absolutely true. However, the claim that we do not observe such things is a blatant lie. Newly forming stars are able to shed this excess energy in numerous ways, the most prominent among them being mass outflow in both the form of stellar wind as well as jets. Many stars in regions which are forming new stars have been observed with precisely such properties.

(Note: the discussion about energy loss is related to the gravitational forces overcoming the pressure.)

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Actually, if their god makes better pancakes, I'm totally switching sides. -- Charley the Australopithecine

Part of Chiroptera's quote from the Angry Astronomer (the fine print being the quoted creationist, the regular print being the AA):

Evolutionists claim that stars form from swirling clouds of dust and gas. For this to happen, vast amounts of energy, angular momentum, and residual magnetism must be removed from each cloud. This is not observed today, and astronomers and physicists have been unable to explain, in an experimentally verifiable way, how it could happen. First off, evolution and cosmology are two distinct fields and are wholly unrelated. Their conflation of the two does not impress me.

My "bolding".

From the second paragraph of the cited article:

The Angry Astronomer writes: However, during the course of it, I pointed out the similarity of our understanding of common descent to the understanding of stellar evolution within my own field.

Again, my "bolding".

The creationist perspective may indeed be totally bogus, but I can't fault the use of the term "evolutionists" (see my "signature"). I guess the AA is miffed that they didn't use the term "stellar evolutionists". Not a graceful was for the AA to start his article.

Moose

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Professor, geology, Whatsamatta U
Evolution - Changes in the environment, caused by the interactions of the components of the environment.

"Do not meddle in the affairs of cats, for they are subtle and will piss on your computer." - Bruce Graham

"The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness." - John Kenneth Galbraith

"I know a little about a lot of things, and a lot about a few things, but I'm highly ignorant about everything." - Moose

I'm sorry if I misread your post. I was somewhat surprised at the conclusion, since to the best of my knowledge the supernova shockwave scenario has been the front-runner for quite some time.

Thus I took a closer look to see just exactly what your math was modeling. If you were to take the mass of the sun, translate it to gas, and calculate the volume it prefers to occupy at a given temperature, I think your results would probably match most models. If the gas is sufficiently "pre-condensed" at the start of the calculation, gravity alone should suffice; but then how does one account for the "pre-condensed" state?

Gases in space aren't easy to deal with. I'd prefer to leave it to persons more capable than myself. For example, they can turn to liquid if they decompress rapidly. Also, the ideal gas theory is said to only approximate many real-world gases. So I try to leave the nitty-gritty arguments to others.

There is some controversy between Big Bang advocates and Young-Earthers, but I don't think this thread was started for the purpose of arguing it. I see it as informational in nature. I think it would be more appropriate to explain the popular model. The difficulties it faces must be fairly minor, or they would already have applied dark matter to the model long ago.

CTD writes: If the gas is sufficiently "pre-condensed" at the start of the calculation, gravity alone should suffice; but then how does one account for the "pre-condensed" state?

Pre condensed? If you haven't noticed, 20K is a very low temperature.

The volume of a typical interstellar cloud is 4.43*10^44 m^3. Divide that by 10^57 and you get 4.43*10^-13 m^3. Each hydrogen gas takes up a volume of 6.2*10^-31 m^3. I'd hardly call that condensed.

If you were to take the mass of the sun, translate it to gas, and calculate the volume it prefers to occupy at a given temperature, I think your results would probably match most models.

I think you've read too much creationist misinformation. You seem to think that there is a predetermined volume for gas at certain temperature.

But let me ask you this. How the hell do you calculate it's prefered volume at a given temperature? At least give me that much to go on.