Re: Is evolution an example of decreasing entropy?

David Ewan Kahana <dek@xxxxxxx> wrote:

[...]
> Gravity is an attractive force that acts between the atoms of a
> gas cloud. Gravity is also a conservative force. If such a cloud
> begins to collapse, and it is sufficiently large in size, then
> gravity will make it continue to collapse. Opposing the tendency
> to collapse is the pressure in the gas that builds up, because
> the collapse occurs at first mainly conserving energy, due to
> the low density of the initial cloud, which limits the rate of
> collisions among its particles. As the gravitational binding
> energy of the cloud increases, this binding energy is thus
> transformed generally into more rapid motion of the
> particles of the gas, that is to say into kinetic energy.
> This kinetic energy is what results in pressure and
> temperature of the cloud.

> Without dissipative forces (friction), and energy loss,
> a large gas cloud COULD NOT COLLAPSE under its
> own gravity into a star. The whole process of collapse is
> critically affected by the ability/inability of the gas to radiate
> away excess kinetic energy created by its own collapse.

> Radiation occurs because atoms/molecules of a collapsing
> cloud smash into eachother more and more frequently
> as the cloud gets denser and denser, and these collisions
> are not completely elastic, but instead sometimes excite
> the atoms/molecules into excited states. The electrons
> drop back down to lower states, emitting photons. The
> photons are able to carry energy out of the cloud.
> This is what makes the whole process of collapse
> possible.

> If energy couldn't be radiated away, the cloud wouldn't
> collapse. It would stabilize at some point, when the kinetic
> energy became large enough.

One minor quibble. You're correct, of course, in your description
of how gravitational collapse takes place. But even if there were
no radiation of photons -- even if the gas only interacted through
gravity, with no dissipative forces -- collapse would still occur,
albeit at a much slower rate. What happens in that situation is
that the cloud sheds energy by flinging off a small number of very
high-velocity atoms/molecules.

While this is largely irrelevant to star formation, it is probably
important in, for example, the formation of globular clusters.
(The key phrase, if you want more, is "gravothermal catastrophe.")

Steve Carlip

.

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