1. SRH
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## Question about the Solar Nebula Hypothesis...

I have heard that there is not enough gravity present for hydrogen atoms in nebulas to coalesce to form stars?
Also, wouldn't entropy cause the hydrogen gas to diffuse rather than coalesce?
How does current theory reconcile these issues?

Thank you!

2. Established Member
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There is enough gravity, you heard wrong I am afraid. Read up on Jeans mass and the Virial theorem via a Google search (there are several very good articles out there that will do a better job than me of explaining them). There is also the phenomenon where collisions can trigger star formation, be they from gas expelled by supernovae or just two clouds hitting each other.

Entropy works at a system level and stars seen in the context of the universe (due to their emissions) are a higher entropy state.

3. Order of Kilopi
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Like Shaula said, the Jeans criteria is a good working out on how much gas is needed to collapse. We see nebula that are in the process of collapsing, so theory cannot say it cant happen.

Collapse means radiation means higher entropy. Alot more than diffusion could provide.

It isnt a simple answer to how the theory works, but since theory dosent see either of your issues as problems, it isnt something that needs reconciling

4. Originally Posted by TOEfetish
Also, wouldn't entropy cause the hydrogen gas to diffuse rather than coalesce?
This is a rather insightful question. Entropy is a law of thermodynamics. It's a "gas law" hit upon in the later 1800s. Boltzmann's entropy formula, S = k ln W, where W is the number of microstates corresponding to a given macrostate, does not say anything about gravitation. The idea of entropy was conceived without any consideration of the concept of gravity. Scientists have since attempted to relate the idea of entropy to the universe as a whole. But the universe has a whole lot of gravitational effects going on. As wiki notes, "the role of entropy in cosmology remains a controversial subject. Recent work has cast some doubt on... the applicability of any simple thermodynamic model to the universe in general." In cosmology the general idea of increasing entropy probably holds - in the frame of hundreds of billions of years - but in my mind, something has to be done with gravity to make its relationship to the entropic flow explicit - beyond postulating that black holes are maximally entropic, with the measure of entropy proportional to the BH's surface area....

5. SRH
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(ignoring the application of entropy to the entire universe as a whole, its formation, and BH's for a moment...)

Are you saying that the behavior of gas in a nebula may not conform to the law of thermodynamics?
Do the laws of thermodynamics only apply in certain situations (i.e. in a "system"; on Earth; something else?)
Are the laws of thermodynamics "mainstream" science or are they still being debated?

Thanks!

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1. The nebular gas of course follows the laws of thermodynamics; if you read up on the Jeans mass earlier this month, those articles should have made it clear that thermodynamics is an integral consideration in deriving it. The pressure of the gas resists gravity trying to collapse it. If a cloud gets too massive, this pressure is not enough to stand up to gravity, and the cloud collapses. There are other considerations of course, but that's the basic picture.

It is important to note of course that the gas does not only follow classical thermodynamics/statistical mechanics; it also feels gravitational and other forces.

2. No, they apply generally. What Cougar was talking about is the fact that applying the concept of entropy specifically is difficult on the scale of the universe itself. You can get the Jeans mass from an entropy argument if you want, IIRC.

3. Yes, they are as mainstream as it gets, seeing as they were almost completely established in the 19th century.

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