Why are stars mostly hydrogen?

[AverageGuy]

I am sorry if the preamble is long but I want to establish the logic of my question. Using the Sun as an example of a third or fourth generation star made from the leftovers of older novae, which created all of the heavy elements in the solar system. As I, an admitted neophyte (no formal advanced science education but I watch a lot of science TV shows), understand the process, the dust cloud condensed under gravity to form the Sun, planets, and everything else. The heavy rocky inner planets, the lighter gas giants farther out and miscellaneous asteroids and comets in their places. This is logical placement order to me.

What does not make sense to me is the Sun. Hydrogen, the lightest (least mass) element and Helium, the next lightest make up the bulk of the Sun. If gravity from the condensing cloud created the Sun, shouldn't it be made of mostly heavier rock and metal? How did the hydrogen differentiate from the other material? Is there some type of attraction between hydrogen molecules or repulsion to other molecules that I have never heard of?

[Davey-T]

All there was to start with was hydrogen so not a lot of choice

Dave

[Dave Lloyd]

Presumably one proton and one electron is the easiest  configuration and everything else builds on that.

[michaelmorris]

Even with 3rd and 4th generation stars have formed, hydrogen is still by far and away the most abundant element.

[Alien 13]

I too struggle with this, my own view would say all the heavy gas coalesced first due to gravity over large distances the heavy stuff clumping together due to small attractive forces the clumps would then move towards the gravitational center but perhaps not reaching it before the Sun ignited. 

Alan 

[DRT]

Wiki has a good explanation of this: https://en.wikipedia.org/wiki/Star_formation

 

[AverageGuy]

1 hour ago, Davey-T said:

All there was to start with was hydrogen so not a lot of choice

Dave

Fist generation stars have only hydrogen to work with, I am talking about later generations like the Sun which came from the remnants of previously exploded stars with all the elements created in their novas.

[AverageGuy]

40 minutes ago, michaelmorris said:

Even with 3rd and 4th generation stars have formed, hydrogen is still by far and away the most abundant element.

Thanks, I understand that. Maybe I am dense but it does not fully explain to me why the lightest element is at the center and the heaviest are further out.

[AverageGuy]

46 minutes ago, Alien 13 said:

I too struggle with this, my own view would say all the heavy gas coalesced first due to gravity over large distances the heavy stuff clumping together due to small attractive forces the clumps would then move towards the gravitational center but perhaps not reaching it before the Sun ignited. 

Alan 

Thanks Alan, this makes a bit more sense to me.

[saac]

Michael effectively had it, notwithstanding the heavy elements created by dying stars,Hydrogen and Helium remain the most abundant elements in the universe by some considerable orders of magnitude.  In fact the very abundance of  these light elements support the big bang theory in that the predicted values correlate well to observed values.

Jim 

 

Elemental Composition of the Sun

Element	% of total atoms	% of total mass
Hydrogen	91.2	71.0
Helium	8.7	27.1
Oxygen	0.078	0.97
Carbon	0.043	0.40
Nitrogen	0.0088	0.096
Silicon	0.0045	0.099
Magnesium	0.0038	0.076
Neon	0.0035	0.058
Iron	0.030	0.014
Sulfur	0.015	0.040

source 

 NASA - Goddard Space Flight Center

[SilverAstro]

57 minutes ago, AverageGuy said:

Fist generation stars have only hydrogen to work with, I am talking about later generations like the Sun which came from the remnants of previously exploded stars with all the elements created in their novas.

Not from only the remains of previous stars, they still came from a cloud of mostly primordial hydrogen and helium just like the first gen. but with a weeny little bit of stuff leftover from previous stars mixed in as well. The cloud collapsed under gravity, the center got so dense that fusion was triggered, just like in the first gen. The new star/sun solar wind blows away the remaining less dense bit of the hydrogen cloud into the outer regions - leaving the dusty stuff in the inner bit cos it was by then clumping together to form the rocky planets which did not have enough gravity to hang on to their bits of the hydrogen cloud. (Well that was the theory for most of the past little time, but recently the ghost of Velikovsky (sp?) is beginning to ride again )

Meanwhile, further out there was still hydrogen hanging about. :--

45 minutes ago, AverageGuy said:

does not fully explain to me why the lightest element is at the center and the heaviest are further out.

But it isnt only at the center ! You are forgetting the gas giants, or am I misunderstanding you ?

 

[DRT]

1 hour ago, SilverAstro said:

Not from only the remains of previous stars, they still came from a cloud of mostly primordial hydrogen and helium just like the first gen. but with a weeny little bit of stuff leftover from previous stars mixed in as well. The cloud collapsed under gravity, the center got so dense that fusion was triggered, just like in the first gen. The new star/sun solar wind blows away the remaining less dense bit of the hydrogen cloud into the outer regions - leaving the dusty stuff in the inner bit cos it was by then clumping together to form the rocky planets which did not have enough gravity to hang on to their bits of the hydrogen cloud. (Well that was the theory for most of the past little time, but recently the ghost of Velikovsky (sp?) is beginning to ride again )

Meanwhile, further out there was still hydrogen hanging about. :--

But it isnt only at the center ! You are forgetting the gas giants, or am I misunderstanding you ?

 

...and it is not one star that is the centre of the collapsing cloud, it is a cluster of stars, each (probably) with its own planetary system. These clouds of gas and dust are enormous - just look at our closest one in Orion and then look up the enormous distance between the stars that form the tiny little Trapezium in the core. Then picture it all in three dimensions rather than the two you see in an eyepiece. Where is the gravitational centre? Where do the heavy and light elements want to coalesce? The reality is that it is happening in many, many places, and as the table above shows the vast majority of it is Hydrogen and Helium

[AverageGuy]

My thanks to all of you, now I think I have it. I knew this forum looked like the place to get an answer I could understand. As I said, most of my information comes from the science shows on TV where they gloss over things like this to fit in their hour long shows and from other internet sites where they get so technical my head swims. The vast majority of these clouds is gas with a relative smattering of other elements as dust. The shows just say "cloud of gas and dust" which makes it sound more like equal parts.

I appreciate the quick and informative responses.

[Helen]

On average clouds are about 99% gas and just 1% dust  

[Macavity]

It's an interesting topic that has lots of implications and INPUT parameters?

Because of the nuclear binding energy, only elements upto and including Iron
can be formed by "fusion" and exothermic release of energy. Beyond that, the
process is endothermic - consumes energy in massive stars. Ergo supernovae! 



You can be confident that elements beyond Iron were create "in some way" in
the brief moment of supernova explosion where HUGE energies are available?

The MIXING of space is clearly important. Quite a lot of Pre-Iron elements are
locked in by the slow "freezing" of smaller stars to a white drawf, black drawf
black hole etc. Mixing must be favoured by more "explosive" behaviours?

As a "random factoid" re. nuclear stability and exothermic / endothermic 
reactions: Every nucleus beyond GOLD is theoretically unstable to fission!
(Quantum mechanical tunnelling and all that) But thankfully the universe
is too young to have witnessed ONE gold nucleus decay in your "bling"?

At least, I think that's right? This was from (comparatively) a while back!  

 

[SilverAstro]

And that graph graphically ( !) illustrates why a 'Hydrogen' bomb is so much more powerful than a Plutonium or Uranium weapon ('atom'- bombs of the WW2 type). The binding energy difference between the original stuff and the products is so much greater (for deuterium/tritium). Sorry for the technical terms like "stuff" but it isnt quite as simple as that, fortunately !! Or maybe unfortunately, as why ~1/2 century after Zeta we are still not much closer to domesticating fusion energy

 

 

[billyharris72]

I get the point about it seeming odd that the lightest stuff is at the centre, but that's not really the case. When you look at the material in the solar system that is not at the centre (planets, asteroids etc) that material is also mostly hydrogen, with some helium and a smattering of the heavier elements. We tend to think of the solar system as containing lots of rocky bodies, but they account for a tiny faction of its mass.

For example, if we take all the mass of the rocky planets, asteroid belt and large moons (don't know exactly, but pack of a fag packet calculation suggests less than 2e25 kg excluding the Kuiper Belt) and compare it with the amount of hydrogen in Jupiter (around 1.8e27) we see that the hydrogen and helium in Jupiter alone (well over 95% hydrogen / helium) has about x100 the mass of all the rocky bodies combined.

As for why all the rocky material is not concentrated at the centre, if you think of the cloud collapsing it will contain a wide mix of material, no more rocky than the mass of the cloud. Heavy, dense objects will tend to fall towards the (vastly more massive) core at the same speed as everything else (gravitational force is proportional to the mass of the object, but its acceleration is inversely proportional, and the two cancel out). When it becomes a star the radiation pressure and solar wind "blows" the remainimng disk out from the new star, the material then entering orbit around the star as an accretion disk. At this stage, the material in the disk will still be quite evenly spread. The reason the rocky inner planets don't contain so much gaseous hydrogen is not that hydrogen is light and hence at the outsides of the disk, but that, being light, its molecules are excited to higher velocities by the Sun's radiation, exceding the planets' escape velocity and being blown out by the solar wind. Further away from the star, the exposure to radiation is less, enabling these planets to cling on to their hydrogen and helium and grow to the huge sizes that we see.

That's the basic story as I understand it, but happy to be corrected on points of detail, as I'm still learning, much like everyone else.

Billy.

 

 

[lansonlad]

May I suggest the "open learn" modules provided by the Open University? They are free to study and provide a good overview of the sort of subject matter you're interested in.

I've been studying the Natural Sciences for the last 4 years, and over the last couple of years have focuses on Physics and Astronomy/cosmology and the materiel is fantastic.

[Knight of Clear Skies]

It takes a while to get your head around this kind of stuff. Another thing to bear in mind about stellar formation in nebulae is that only a fraction of the material ends up in new stars. Most of the gas is ejected back out into space once large hot stars form, by radiation pressure and strong stellar winds; only something like 10% of the material in a star forming region will coalesce into stars. This is why the Orion Nebula has the appearance of an open rose, it's at a fairly late stage of evolution where the gases are being dispersed. While each generation of star formation increases the amount of 'metals' (elements other than hydrogen and helium) they still only make up a small fraction of the interstellar medium.

[SilverAstro]

3 hours ago, Knight of Clear Skies said:

It takes a while to get your head around this kind of stuff.

And just when you thought you had ( I remember the nice stable clockwork '50s) some pesky theoretician, or even an astronomer with a new observation, comes along and messes it all up * Now it planetary/solar system formation is beginning to sound like a game of snooker ( which is why I mentioned poor ol' Mr Velikovsky earlier, google him and "Worlds in Collision" and have a chuckle )

* and restarts the search for planet X, or is it 10, or even 9 ? on the basis that Jupiter and Saturn only got to where they are now after beating up on and ejecting a similar one.