In the birth of stars, how does gravity-induced condensation (increasing density) of hydrogen atoms result in increasing temperature?


I was just re-watching a video in the Cosmology and Astronomy section of Khan Academy titled Birth of Stars.

Sal begins the video by describing a huge mass of hydrogen atoms which gradually becomes more dense as the volume understandably decreases due to gravity. At the 1:00 minute mark, he states that as this is occurring, the temperature increases — but he does not explain why.

What is the connection or what is the mechanism by which increasing density results in increasing temperature?

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Adiabatic Ideal Gas Law
Or gravitation potential energy converting to kinetic energy
poterntial energy goes to kinetic energy which implies increase in speed of the molecules which is the same thing as an increase in temperature.
Force (of gravity) * Distance (span of gas cloud) = Work ( aka Energy)

The force and mass density are tiny (at first).
But the distances are galactic and the energy confinement before ignition is close to perfect.
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WeThotUWasAToadAuthor Commented:
Thanks for the responses.

I know that later on when the force of gravity becomes extreme, it causes fusion of hydrogen to form helium which gives off energy. However, I'm having a brain cramp or something regarding the increased temperature before fusion begins.

I've always understood temperature to be a measure of the movement of atoms/molecules but I guess I haven't really understood a more detailed or specific definition. For example, does "movement" mean the velocity at which they are traveling, or does it mean their movement back and forth or "jiggliness" (for lack of a better term), or is it somehow related to the frequency of collisions with other atoms/molecules?

When an object falls and accelerates towards earth, it is converting potential energy to kinetic energy, right? And then when it hits the ground, some of that kinetic energy is released (I suppose part into the ground and part into the atmosphere) but then, following the impact and depending on the object's elasticity, it moves more slowly indicating it now has less kinetic energy, right?

Is that what happens to the hydrogen atoms? Do collisions result in them moving more slowly? And if so, then shouldn't that equate to a lower temperature?

Thanks for the patience. :P
Imagine a cold dense cloud at the center of a galaxy of widely spaced stationary hydrogen atoms.  The cloud is the center of gravity, and all the atoms accelerate toward the center and pick up kinetic energy.  When the atoms hit the cloud, they collide with the cold atoms and lose energy.  The cloud gains energy.  The process continues, with the cloud gaining mass and energy until it fuses or runs out of fuel.
Well, one tends to associate temperature more with disordered energy than with ordered energy, but technically it is related to how the number of states change as the energy changes, and that can be tricky to talk about with ordered motion, since the the energy can look different from different reference frames, so it's easier to interpret the temperature of atoms in random motion with respect to their average momentum, but in terms of ability to cause fusion, it only matters how hard the atoms collide, not how much they jiggled before colliding.

Total energy is conserved, so if an object moves more slowly after an impact, some other object has to move faster.
If it collides with a colder object, its temperature can decrease, but only by increasing the temperature of the colder object.
Such impacts tend to turn ordered motion into disordered motion, which is easier to understand in terms of heat.
It can make the energy spread out, which can tend to cool a hot spot down to the average, but if you are talking about a big cloud of hydrogen all collapsing together, then impacts would not change the average kinetic energy, it would just make it more random, and so easier to think of in terms of temperature.

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WeThotUWasAToadAuthor Commented:
Many thanks. All responses were helpful.
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