Star formation

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?

Wasn't it John Brockbank who 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?

Stars formed from gas clouds that are considerably heavier than the
Earth, and therefore have higher escape velocities.

--
Mike Williams
Gentleman of Leisure

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.

"John Brockbank" wrote in news:43fcaa89$1_3@mk-nntp-
2.news.uk.tiscali.com:

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?



Note that the planets that formed further out in the solar system (Jupiter,
Saturn, Uranus and Neptune) still have plenty of hydrogen. This is mainly
because of their greater mass but also the solar radiation is much lower so
their atmospheres are cooler than the Earth's and thus a lower percentage
of the hydrogen can reach escape velocity (which is considerably higher for
those planets than for the earth).

The details of star formation are still being worked out but the basic idea
is that the increase in escape velocity caused by the accretion of matter
exceeds the increase in the speed of the accreting molecules or atoms
caused by increasing temperature, i.e no significant portion of the
accreting matter heats up enough to gain a speed suffient to overcome the
ever increasing escape velocity caused by accretion. Once the stars' core
initiates nuclear fusion the radiation from the new star heats the
atmosphere of any planets formed. In the case of the Earth, the heat from
newly formed sun would have caused the Earth to gradually lose hydrogen
from its' atmosphere. It is thought that hydrogen was probably being
outgassed by volcanism on the Earth for some considerable time after its'
initial formation. This is deduced from the probability that the Earth
primarily formed from chondritic material.

See:

http://www.sciencedaily.com/releases...0911103921.htm

and

http://en.wikipedia.org/wiki/Star_formation

Klazmon

"Sjouke Burry" wrote in message
. ..
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.

Wasn't it John Brockbank who wrote:

"Sjouke Burry" wrote in message
...
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.

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

John Brockbank wrote:

"Sjouke Burry" wrote in message
. ..

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).

For any given density of material there is some ultimate size beyond
which a clump of matter will necessarily and inevitably contract into
ever tighter clumps until stars are formed. There are some really nice
simulations online at a cosmological scale and star cluster scales too.

Sorry, but I do not think this is a no-brainer.

It only requires that there is enough attractive force from the clump of
material to ensure that parts of it slowly become ever denser.

There was a very nice illustrated talk about this at Astrofest by
Matthew Bates of Exeter, simulations in video for braodband only at:

http://www.astro.ex.ac.uk/people/mbate/

Regards,
Martin Brown

JRS: In article , dated
Thu, 23 Feb 2006 18:04:28 remote, seen in news:uk.sci.astronomy, John
Brockbank posted :


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.

It's not the force that matters, but the integral of the force over the
distance from the putatively-escaping particle to infinity. The field
of a large volume drops off slowly.

Most people find climbing a ladder easier than climbing Snowdon,
although in the latter case the slope is generally much less.

The larger Gas Giants of the Solar System hold hydrogen more because
they are big than because they are cold.

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

Martin Brown wrote in
:

John Brockbank wrote:

"Sjouke Burry" wrote in message
. ..

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).

For any given density of material there is some ultimate size beyond
which a clump of matter will necessarily and inevitably contract into
ever tighter clumps until stars are formed. There are some really nice
simulations online at a cosmological scale and star cluster scales too.

Sorry, but I do not think this is a no-brainer.

It only requires that there is enough attractive force from the clump of
material to ensure that parts of it slowly become ever denser.

Yes but I agree with the previous poster that it is not a no brainer. As
the collapse procedes, the temperature rises and then the question is if
the increase in temperature gives a significant portion of the molecules
enough speed to exceed the escape velocity. The increase in temperature
acts against further collapse but of course heat is also being radiated
away. The radiation details depend on the composition of the collapsing
nebula. It is a genuine complex problem, worthy of computer simulation
which you point out below.

Klazmon.








There was a very nice illustrated talk about this at Astrofest by
Matthew Bates of Exeter, simulations in video for braodband only at:

http://www.astro.ex.ac.uk/people/mbate/

Regards,
Martin Brown

"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