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Astronomy question bugging me!

UKBoy1977

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Dec 2, 2002
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Hi, got an assignment to do but stuck on a couple of questions. They only have to be a few sentences, it's only a very basic course.

1) Outline briefly how hydrogen can move from the interstellar medium into stars and back into the interstellar medium.

Now I know this is talking about the stellar life cycle and I understand how stars are formed inside big gas clouds and that is obviously how hydrogen ends up being inside stars, but what I don't get is how does any hydrogen get back into the interstellar medium as I thought it all got used up during the main sequence stage of the stars life cycle?

2) Why would newly-formed main sequence stars have a different composition compared to the initial composition of similar stars formed several billion years ago?

No idea about this one. I would actually appreciate tips rather than full answers as I don't want to feel I've had it done for me entirely!
 
1) it doesn't

2) because of lack of 1.

If you want more than hints, here goes.

1) Hydrogen condense into stars, it gets fused into all the other matter we can see in our world, and when it goes boom(supernova), the material is spread far and wide. This material will end up in other clouds of hydrogen, and condense into a star, and so on and so forth.

Some of it won't end up in the star, but in the planets, which will also catch some of the hydrogen from the cloud.

2) Because 1 hadn't happened before, the stars would be 100% hydrogen. Whereas now, it will be poluted with other matter. Which for one means the stars nowadays will start burning at a much smaller size than the first generation.

The first generation had to get a LOT of hydrogen to get enough pressure to start fussion. Latter generations were poluted with material that was heavier, thus the same pressure was gained with less amount of hydrogen.


Or something like that.. I'm a bit rusty since i haven't really done a school report on this since i was 18, and i haven't touched it outside of school since then...

But, if i'm patently wrong, someone will correct me soon enough :D
 
UKBoy1977 said:
1) Outline briefly how hydrogen can move from the interstellar medium into stars and back into the interstellar medium.
!
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Well I don't think all the hydrogen will be used up. I would guess that the heavier elements (produced by the fusion) will predominantly be in the core, while the lighter elements will tend to be pushed outwards (by the radiant energy) to the outer layers. In other words when the star goes bang (nova or supernova) it is because there is no more hydrogen (as well as other elements) in the core and not because of lack of hydrogen in the star as a whole. Also does the "solar" wind feed the "interstellar medium" with hydrogen. ?
UKBoy1977 said:
2) Why would newly-formed main sequence stars have a different composition compared to the initial composition of similar stars formed several billion years ago?
No idea about this one. I would actually appreciate tips rather than full answers as I don't want to feel I've had it done for me entirely!
Click to expand...
The first generation of stars were formed when the only matter in the universe was hydrogen (bit of helium?). That matter formed into stars and produced lots of other elements. When the star went bang, those elements were spread around the galaxy ("interstellar medium"), which after a while or two , condensed to form new stars, but although being mostly hydrogen .... they now contained other elements.

<..here is where reference to you being made of "star dust", usually goes>

..alternatively have you thought of simply giving the answer "cause god says so?". Apparently its all the rage now a days.

edit: beaten to it by TobiasTheCommie. Interesting I said "goes bang" and he said "goes boom". Surely we both can't be right? :-)
 
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Well I don't think all the hydrogen will be used up.
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I would disagree. As the star burns the hydrogen, the heavier elements(iron, etc) will go to the middle of the core, and the surface of the core would fuse the new layer of hydrogen. Which means that as each layer is turned into heavier elements the core's surface area will increase.

In the end the core is almost the entire size of the star, and, due to the size of the core, the remaining hydrogen will be fused a lot faster.

I believe this is called a flash.

But then again, i may be wrong. :)

edit: beaten to it by TobiasTheCommie.
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nah nah nah :D
 
Thanks guys!

So there seems to be doubt about the hydrogen getting back into the interstellar medium? Hmm, maybe I'll answer the first part of the question about hydrogen forming part of new stars and then say 'and then God makes new hydrogen and the cycle repeats' (credit to 'but does it work')

I mean they can't prove me wrong can they?
 
UKBoy1977 said:
Thanks guys!

So there seems to be doubt about the hydrogen getting back into the interstellar medium? Hmm, maybe I'll answer the first part of the question about hydrogen forming part of new stars and then say 'and then God makes new hydrogen and the cycle repeats' (credit to 'but does it work')

I mean they can't prove me wrong can they?
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Except, perhaps, for the solar wind(i can't say one way or the other if that is hydrogen, or part hydrogen), any hydrogen from the star would be minimal, imo.

Kinda like a bit of gasoline may get out of the car... but most of it is burned.


But i would like a real scientist to say if the solar wind is hydrogen, or what exactly it is. Since i have no idea. :)
 
Just been on Wikipedia and it seems that when stars start to become Red Giants it is only because they have exhausted the hydrogen in the core. There is still hydrogen contained in the outer parts which are generally lost to the interstellar medium as the red giant expands.

Quote from Wikipedia on Red Giants:

"They are believed to be stars of near solar mass or higher which have exhausted the supply of hydrogen in their cores and started fusing hydrogen in a shell outside the core. Since the source of energy is closer to the surface, the star begins to expand"

Also I checked Solar Wind, and that does not contain hydrogen. It is actually a stream of protons and electrons.
 
interesting....

In that case, the flash i talked about would be confined to very heavy stars(the ones that become supernovae). They would have a hotter surface, so all the hydrogen should, imo, be burned.
 
UKBoy1977 said:
Also I checked Solar Wind, and that does not contain hydrogen. It is actually a stream of protons and electrons.
Click to expand...

Ummm... that just means it's ionized hydrogen. Most hydrogen only consists of a single proton and a single electron bound to each other (I say most because deuterium and tritium, isotopes which make up a small fraction of the hydrogen in the universe, have one or two neutrons in the nucleus as well, respectively).
 
TobiasTheCommie said:
1) Hydrogen condense into stars, it gets fused into all the other matter we can see in our world, and when it goes boom(supernova), the material is spread far and wide. This material will end up in other clouds of hydrogen, and condense into a star, and so on and so forth.
Click to expand...
I think that there other ways than supernova for matter to disperse from a dying star. As mentioned above when a star goes into the red giant phase it expands a lot (and still has quite a bit of hydrogen), I imagine quite a bit of matter is lost in this process.
Not an expert, just some thoughts...

Some of it won't end up in the star, but in the planets, which will also catch some of the hydrogen from the cloud.
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A question for the experts: Does this heavier matter tend to end up disproportionately in planets, or in the star?
Or is this only apparently so because much of the hydrogen that formed with the planets was blown out of the solar system as the sun started to shine?

2) Because 1 hadn't happened before, the stars would be 100% hydrogen.
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And some helium, which also formed shortly after the big bang.
 
Actually the star does not go super nova until it IMplodes. Some of the material rushing inwards then bounces off the star's core and is ejected into space. As part of this process elements heavier than iron are created.
 
UKBoy1977 said:
Hi, got an assignment to do but stuck on a couple of questions. They only have to be a few sentences, it's only a very basic course.

1) Outline briefly how hydrogen can move from the interstellar medium into stars and back into the interstellar medium.

Now I know this is talking about the stellar life cycle and I understand how stars are formed inside big gas clouds and that is obviously how hydrogen ends up being inside stars, but what I don't get is how does any hydrogen get back into the interstellar medium as I thought it all got used up during the main sequence stage of the stars life cycle?

2) Why would newly-formed main sequence stars have a different composition compared to the initial composition of similar stars formed several billion years ago?

No idea about this one. I would actually appreciate tips rather than full answers as I don't want to feel I've had it done for me entirely!
Click to expand...

You probably just need to read a text book, or general pop science like John Gribben's Stardust.

Hydrogen is expelled from the surface of the star normaly, there are also shell producing events for red gients as they collapse and start to fuse other elements.

The second question mean you weren't palying attention at all!

Fusion of elements and nonae and supernovae are part of the answer.
 
rjh01 said:
Actually the star does not go super nova until it IMplodes. Some of the material rushing inwards then bounces off the star's core and is ejected into space. As part of this process elements heavier than iron are created.
Click to expand...

Gribben states that it is the shock wave from the explosion in the collapse of the nuetron star and similar events that causes the fusion to heavier than iron.
 
Ziggurat said:
Ummm... that just means it's ionized hydrogen. Most hydrogen only consists of a single proton and a single electron bound to each other (I say most because deuterium and tritium, isotopes which make up a small fraction of the hydrogen in the universe, have one or two neutrons in the nucleus as well, respectively).
Click to expand...

and there's the answer to (1), it seems to me. Once they cool off, I don't believe single electrons and protons are not going to get together...
 
To a certain extent it depends on the size of the star, but when it says in the textbook, and on wikipedia, that "A star exhausts all of it's core Hydrogen" it doesn't literally mean that there is absolutely no Hydrogen left. Hydrogen burning stops (and usually Helium burning takes over) when the proportion of protons gets too low to allow large numbers of proton-proton interactions. This means that there is some Hydrogen left, just not that much.

Shell burning takes place in most stars which uses up the majority of the Hydrogen in their outer envelopes. The exceptions are very low mass stars which are fully convective, so material is dredged up from the core to the outer envelope and vice versa, making them very efficient Hydrogen burners.

Proto-stars have huge stellar winds which also blow huge amounts of Hydrogen back into the interstellar medium.
 
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