Chemistry of Star Formation

[Pianosorplanets]

In the last year, I've done a lot of reading and study on most subjects surrounding astronomy and astrophysics. (Not that I'm laying claims to a deep understanding of astrophysics which would not be true.) I've been trying to wrap my brain around the concept of new star formation but no source I have found really answers my personal questions. I was hoping someone else frequenting this area would be able to guide me to somewhere that I could learn about that which I seek.

First, I know that nebulae like those organized within the Sharpless Catalogue are the "breeding" grounds for new stars. I understand the Herbig-Haro formations and pillars within HII regions that signal new stars being born and fertile ground in which they could form.

I've read the Wiki article on Gravitational collapse but it doesn't help me understand how hydrogen of all things can manage to become so heavily compressed without anything to compress it. I can begin to see it at the molecular level when we're talking about galactic collisions where gas fields are rammed into each other at incredible speeds but am still lost concerning those fields of gas which were never under any discernible outside pressures managed to develop the necessary pressures to begin the fusion process.

I'd also like to know how we go about measuring (or is there a static figure) the amount of pressure needed to cause hydrogen to begin fusing into helium?

I understand that hydrogen gas (which is very rarefied in its general form) condenses and compacts until it is brought under such extreme conditions that the forces within the hydrogen (be they weak or strong forces) find themselves behaving in an excited way that initiates the fusion of this hydrogen into helium and so forth. I even get the other end of the story of how large and small stars meet their ends although I'm not laying claim to expertise in that matter either (but I, at least, feel that I can understand what is going on during nova/supernova events and the causes and functions of SNRs and Planetary Nebulae. It's why my handle is Pianosorplanets since I am a piano rebuilder by trade and the stuff we are made of and work with each day had to be formed by converting simpler elements into more complex via fusion within a star and/or a (super)nova event. I can even wrap my brain around the basic principles of the chemistry going on there. But the chemistry associated with the beginning of star formation eludes me in its specifics.

What I can't figure out and have not found a source to learn from is how this initial situation (wherein the hydrogen becomes adequately excited to begin fusing) works. On Earth, if we want to condense hydrogen, we freeze it into a liquid or even a solid. I suppose a solid block of hydrogen could eventually accumulate additional hydrogen atoms until it gets larger and larger. Eventually, the shear size of this massive planetoid of hydrogen would find its inner core under such pressure from gravity that it sublimates, finds no avenue of escape and begins reacting with itself in the manner of a star.

But, I'm making this up out of whole cloth. I don't know that this is the case at all. The molecular and subatomic conditions, reactions and formations are glossed over everywhere I have looked in favor of sticking to the "big picture." I would really like to understand this in a much more specific way.

How can hydrogen gas end up under adequate pressure to begin the atomic chain reaction? How can a field of straight gaseous hydrogen end up so heavily compressed in the freezing vacuum of space with nothing to pressurize it but, well, "it."

Can anyone help?

Pianosorplanets

[geckzilla]

I think of it like eddies and whirlpools in a pool of water with bodies forming in places where knots of density occur. I know it's a no-no to compare fluid dynamics to large-scale galactic structures, but it's the best I can do. At some point when the star and surrounding materials are forming into planets, apparently it becomes something like a fluid environment. I'm not really sure when that transition happens... The gravitational forces are so weak it's hard to imagine them coming together. I think it's something beyond human intuition, or at least without quite a bit of work understanding the matter it is. The shockwaves of previously exploding stars help to create strands of density and that ignites into a chain reaction of star formation until so many stars exist that they exert forces which drive away the brick and mortar of planetary and stellar formation. One does wonder how the Big Bang managed to form the earliest stars at all. Different times.

[Chris Peterson]

I've read the Wiki article on Gravitational collapse but it doesn't help me understand how hydrogen of all things can manage to become so heavily compressed without anything to compress it.

Why do you think there's nothing to compress it? As you note, the article is called "gravitational collapse". Gravity is not a strong force, but it acts over very large distances. If you have a large enough clump of gas (as when shock fronts from supernovas create vast regions with relatively high density) the gravitational self-attraction causes a further increase in density- a process that continues until you have most of the hydrogen in a small volume. It's all held together by its own weight, not by some external force.

I'd also like to know how we go about measuring (or is there a static figure) the amount of pressure needed to cause hydrogen to begin fusing into helium?

It has nothing to do with pressure. We could (in principal) put some hydrogen in a tank and pressurize it to the same levels found in the Sun and there would be no fusion. Fusion occurs when the hydrogen gets sufficiently hot. The proton-proton chain reaction requires a temperature of about 14 million K. As gravity causes a region of hydrogen to fall in on itself, that gas heats up (an adiabatic process). Once it becomes hot enough, hydrogen fusion begins and the resultant energy release prevents further contraction, and we get a stable system.

[Pianosorplanets]

Thanks for your quick answers Chris and Geck. I dislike asking others to "spoon feed" me answers to questions which are readily answered on my own with the proper research but I had found myself at a logical bind and didn't know which way to "tug" to loosen the mental knot.

I had recently become acquainted with the adiabatic condition of star behavior. I'm glad I already had a little understanding of it or I'd have been confused by your explanation. Thanks so much for investing time in helping me learn.

I can't say that your answers settle all my questions but they open up venues I can seek out in order to gain further understanding. I will certainly need to reread them and spend some time thinking on them to let each idea coalesce in my mind. But, then again, I'm on my fourth or fifth pass through the very basic beginnings of Einstein's work along with the contributions and disagreements that occurred with his peers. i.e. The Lorentz transformations, The Poincaré Conjecture and so forth. I figure I'll start understanding them in a manner that makes that knowledge useful to me after I've reread the material the sixth time or so...

Logically, I thought that it had to be heat that started stellar fusion but I couldn't understand how the gasses could produce this heat unless they achieved a state of the most extreme compression be it from outside forces or internal ones. And I still can't readily wrap my brain around this fully although I'm convinced you've given me tools that will speed me on the way.

I believe my next research project should involve the force of gravity, what is understood about it, and learn just how powerful it is as it acts within interstellar gas clouds. Logically, I never did believe that liquid or frozen hydrogen were involved but they are the only earth bound examples of hydrogen atoms achieving "density" I could think of. I'm still wrapping my brain around the concept that hydrogen can generate heat without extreme density. I need to study more of the behavior of hydrogen and learn more about the forces that exist in it that can lead to its become insanely hot.

You have opened my eyes to new ideas on the behavior of hydrogen at the very least. I'll follow that lesson up with further research. I gather that the heat is generated at the subatomic level as the forces that act upon all matter (according to Chris - especially gravity) but I will examine the weak and strong atomic forces as well as I study further.

I already knew that the act of freezing hydrogen would remove the energy needed for fusion. It was simply the best example I could lay my hands on to ask how these atoms can achieve adequate temperatures in the freezing conditions of space and with nothing I could discern capable of producing adequate density so that the new star would have sufficient material on hand to live out a typical life span once its stellar wind started.

Should I, perhaps, look at it as a gathering process that, perhaps could be described as starting its initial life as a form similar to a gas giant like Jupiter, grows in size to that of a huge exoplanet, grows again to the size of a brown dwarf until it finally accrues enough mass to behave as a star?

It's already a very interesting exercise in logic but I'll need to temper my logic with more adjudicated facts as I go along.