Expert: Jayendra Upadhye - 6/17/2008

Question
QUESTION: Hi Jayendra, I am interested in nebular hypothesis and I am wondering how a cold gas(hydrogen) cloud can condense into solid objects. I thought hot gas could condense into a colder liquid (and then a solid) but to condense into a hot Star fuelled by fusion seems unscientific to me.

ANSWER: Hi Robert!,
Now i am seriously thinking i should ask you for your "qualification".
No i am not being derogatory / sarcastic etc.
I need to know how much grounding you have had in physics/chemistry etc.
Hope you are not from the other "arts".
If you are, it is going to be rather an uphill task (more than i imagined).

Do learn the "abc" & "multiplication tables" i referred to earlier by brushing up on current theory (90% of which is true verifiable stuff and not some stupid conspiracy theory or speculation).

The true debatable issues are in the metaphorical "advanced maths" not in the "multiplication tables"!

I am again sorry for appearing to talk down.

But the very nature of your questions is making me quail.
(at the prospect of writing another very long answer)! :)
My problem not yours!! :)

Coming to your question....(at last!)..

Robert that would all be about the "phase diagram"s of various elements / compounds.

Hydrogen in the universe exists in these forms
1 - Plasma (pure protions + free electrons).
2 - Gas (natural mormal free space)
3 - Liquid (cryogenic temperature free space)
4 - Metallic Hydrogen (high temperature and pressure, inside stellar cores / gas giant cores). It lies is the first column of the periodic table, the one that contains metals like sodium, potassium, francium etc, the alkali metals, highly electropositive and reactive. Under tremendous pressure it can be a metal!



A phase diagram (not the exact name, i am not one for exactness of that sort), presents all possible phases of a gas / compound, on a single chart. Famous charts of that sort are ones showing "triple points" of water and ammonia and freyon.
These charts are useful in designing refrigeration compressors etc, that use ammonia / freyon if i am not wrong.
[refer:-
1 - http://en.wikipedia.org/wiki/Triple_point
2 - http://en.wikipedia.org/wiki/Hydrogen
3 - http://www.wisegeek.com/what-is-metallic-hydrogen.htm]

Likewise, in stellar cores, hydrogen is compressed to such an extent that is resembes a solid. So much so that when fusion proceeds via the proton proton route, (neutron gamma reaction), the gamma rays released have to come out of the core by multiple absorption and re-emission (compton effect), from atom to atom taking upto a million years on average to emerge into the overlying convective layers!!
(taking only 8 minutes to reach earth therafter).

The other particulate solids found in the solar accretion disk such as water ice, SiO2, NaCl, KCl, NH3, CO2, HCOOH, HCN, O2, CL2, etc are all inherited from more ancient supernova explosions. The water in our ocaens is more ancient than the sun! And so is the carbon, nitrogen, oxygen we breath (in/out).

Some of these elements are gas even in space if the temperature is high, for example the solar constant of radiation at earth orbit is 1.4 kilowatt / square meter! Hydrogen would be blown away at that temperature. (The sun retains hydrogen that it captured gravitationally BEFORE this constant came to be, ie before the ignition of its furnace.

The rest of the particulates that remained soild at that flux level of radiation, accreted into planets, and recieved their gases & liquids in the late bombardment phase via comets!

As usual please do rate the answer.
regards
Jayen

---------- FOLLOW-UP ----------

QUESTION: Hi Jayendra, I get that elements can have different states but what would cause the pressure needed for Hydrogen to go from a gas to a star? This is the part of the model I don't get. i read on wikipedia it was a gravitational collapse. Can clouds of gas collapse in on themselves and become stars. Surely the cloud would have to enveloped by something which could exert enough pressure for this to happen. And is this the model for how all of the billions of stars were formed between big bang and about 5 billion years ago?

ANSWER: Hi,
Robert!
1 - Why do you think an envelop is needed to collapse a gas cloud?
After all you do see daily, the earth's gas envelop, the atmosphere, compelled to stay with the earth, despite the solar flux of 1.4 kilowatt to the square meter, dont you?
What envelop keeps the gases from flying away into free space?
If you can accept that i dont see what woul prevent a gaseous cloud from falling inward on itself under gravity.

In the space shuttle, when people enjoy the floating water globule and watch it assume a spherical shape, they are unwittingly watching the same underlying process of star formation! (but mimicked by viscosity, rather than gravity). There is no external envelop required to pull the drop in on all sides to make it round. Internal uniformly distributed viscous pull does the job. In gases with sufficent mass, gravity does the same thing. The pull is maximum at outer extremities and minimum at cloud center. (zero).

The cloud simply pulls itself in! The outer layers come crushing inwards, and the pressure volume law dictates the temperature of that isentropic compression! (little heat lost by radiation, to surroundings, there is no convection in space between the cloud and surrounding space which is vacuum).

Like the daily mundane tides remind us (we never "see") of the tidal forces lurking around black holes.

Look up my starform html at this site where i have given a rough idea of what happens in an evolving star.

ref:- http://www.geocities.com/jupadhye_99/starform.html

Some gas clouds as seen in the M16 Eagle Nebula are many many solar masses in size! And they show Embedded Gaseous Globules (EGGs) that later evolve into stars. (The process of star formation).

The Orion Nebula actually shows us protostars with their bowshocks where the interstellar ionised gas meets their solar winds!

As i said dont doubt the abc and multiplication tables, the areas of darkness are "in the rarefied field of astrophysics, something much more complex than you might have dreamed of.

Newton / Laplace / Fermat / Lagrange did much more complex tasks than university grads would care to understand, and what they did has stood the test of time, (even if modified by the Einstiens of later time).  
But 'simple' gravitational theory of newtonian vintage is enough to explain a gas cloud collapse even today.

regards

Please do rate the answer if you thought it informative.

Jayen

---------- FOLLOW-UP ----------

QUESTION: Hi Jayen, sorry but I need to ask a follow up.
you said:
"1 - Why do you think an envelop is needed to collapse a gas cloud?
After all you do see daily, the earth's gas envelop, the atmosphere, compelled to stay with the earth, despite the solar flux of 1.4 kilowatt to the square meter, dont you?
What envelop keeps the gases from flying away into free space?
If you can accept that i dont see what woul prevent a gaseous cloud from falling inward on itself under gravity."
I think the gravitational pull of the Earth holds on to the atmosphere. However the Earth loses Hydrogen to outer space every day. So being quite dense with a strong gravitational pull it loses Hydrogen. A Hydrogen cloud needed to make up the mass of the Sun I get to need 17 million km radius. The gravitational pull to the centre would be miniscule with this radius yet it collapses in on itself. This doesn't add up. Also if it contracts and thus due to kinetic energy gets hotter it would then expand again. So is there a scientific explanation that addresses these 2 points?

I just wonder how this could have happened. Also as there are 10^22 to 10^24 stars in the universe this has occurred the same number of times. In a 14 billion year old Universe that means on average this has happened 7.14 x 10^12 to 10^14 times a year (7000 to 700000 billion times a year). This sounds fantastical. Do you know the estimated time period for one nebular cloud collapse?

Answer
Hi Robert!
Old habits die hard..dont they?

We are about to embark on another journey of "bible writing"!! :)

Would have love it if you had resisted the temptation and posted a separate question, as i am asure this will lead to more follow-ups.

I sincerely think you need to brush up a lot on the complex but relatively easy calculus involved in the calculations using Newtonian mechanics of gravity.

I have listed it all out in words on my site http://geocities.com/jupadhye_99/starform.html

You see what happens is that for any multiparticle body (large non-pont bodies of real world, the "surface gravity" is as though the full mass of the real body had been concentrated at a "point" at its center, and then the inverse square law were to be applied with radius of the body (assume spherical) as the separating distance.

Now imagine a huge cloud 1 Astronomical Unit across. Imagine it to be of molecular cold hydrogen, with lot of solid dust (ices and matter like sio2, nacl, kcl etc).

The mass would be HUGE. Even at 1 AU distance, The sun causes severe tides in the earth,s atmosphere (yes it affects the atmosphere in a more complex way as it is an expandable gas), and water. This you have seen for yourself.

Even if the sun were to be a sparse ball of gas 1 AU in radius, it would still cause the same tidal pull on its extremities as it does now on the earth! (the state of the gravitating matter does not matter). If it becomes a black hole by some "divine intervention" tomorrow, it will still pull the earth with same intensity.

In the case of gas, the outermost peripheries represent a spherical surface where that contains all the mass in that 1 AU radius object which is HUGE.

The gas starts gently collapsing in on the center, from ouitsdies first! But as you know, continued gentle acceleration can yield very large velocites over time (integerating processes). In a matter of days the outer envelop would become supersonic!!

Raising the local temperature of the shell as it collapses and accumulates more gas that "comes inthe way". There would be much rebounding, and recontracting, and heating and cooling. As the shell rebounded due to heating, the inner layers would continue thwir own "slower" inward fall, resulting in clear spaces in between! The skeleton of the solar system would be represented by these shells each mighty form a planet if it has sufficent tangential velocity component to equal centriptal acceleration!

the actual equations representing all this are very very complex as it requires great knowledge of thermodynamics, and would involve differential calculus and 2nd, 3rd, 4th order differential equations.

the particulate matter would not rebound like the gas shells and be always found either in the center of the star itself or on the inner periphery of the system. Solid planets on the inside and gas giants outside would seem to be a rule. If not found, it could mean all of the solids were gobbled up by the accreting star.

hope that suffcies.

Please do check out what i have put on that site
http://geocities.com/jupadhye_99/starform.html
There is an unusual "pet theory" conclusion towards the end of that page , regarding starting densities of spiral galaxies.

Please as usual rate the answer if you find it interesting and helpful.

Andf do post new questions separately after my bible writing excercise above!!

regards
Jayen



Post script:-
Robert your questions make me write copiously what to do?
Yes it can happen that a shell of gas continues to collapse inwards on its center even as it glows red hot and is loosing heat via radiation. Chiefly because only route for loss is thru radiation. The inner layers are bing gobbled up by this "spersonic" shell and cannot convect away heat effectively "inwards". Plus if the centripetal acceleration (small g due to graviry) is strong enough, the net outwards push by temperature rise may not be enough to stop its infall. At a ponit, net radiant outflow  will balance heating due to contractual heating, and teh shell will continue its collapse at constant temperature. (this constancy will migrate upwards in temperature over time, till it merges with a "fictitious center point")! Anyone at the center will first observe his "skies" getting reddder and hotter as he is fried by radiation from the shell, and will be convectively/conductively roasted after the shell reaches the center!!
regards
Jayen