Star formation

"Mike Williams" wrote in message
...
Wasn't it John Brockbank who wrote:

"Sjouke Burry" wrote in message
l...
John Brockbank wrote:
Sorry if this is an FAQ, but it5 has long made me wonder and I have
never
seen an explanation.
The Earth massive as it is, is too slight to have hydrogen in the
atmosphere. I think that is because the speed of the molecules
exceeds,
just about, the escape velocity. Given that, how did stars form from
hydrogen?
Surface gravity at the sun 27 G?(260 /s/s)
And that is for a yellow dwarf.
Even Jupiter/Saturn/neptune hold onto their
hydrogen pretty well.

Of course once a star has formed it has a strong gravity field. However,
presumably a vary large volume of hydrogen, having a huge mass, in fact
will
have a low gravitational force because it is spread out (inverse square
law).
Sorry, but I do not think this is a no-brainer.

Remember that it will also be very cold. There's no heat source until
the star starts to form. So the hydrogen will be moving very much more
slowly.

Ah, yes, temperature is a big factor with gases. I wonder whether there is
some sort of snowflake effect, like water droplets in a cloud. I will
certainly look at the simulations, and do some more looking.


Escape velocity is proportional to sqrt(mass/distance). We only have to
consider molecules escaping from the outer fringes, and don't need to
worry about the gravity in the interior of the condensing cloud.

The typical speed of molecules in the gas is proportional to the square
root of the absolute temperature.

Presumably there's certain critical mass/distance/temperature ratio. If
external forces cause a cloud to cross this boundary then it starts to
contract under its own gravity. If a cloud stays outside the boundary,
then it continues to be a cloud or may even disperse.

--
Mike Williams
Gentleman of Leisure