Science a GoGo's Home Page Forums General Discussion Physics Forum Suns with heavy elements

In the March 2012 Scientific American there is an article about the future of the universe "The Far Far Future of Stars" by Donald Goldsmith. I was reading it when I was struck by something that I wondered about.

My first thought when I read that was that if the radiation is trapped in the star then it will be hotter. That would imply that it would burn its fuel faster, not slower. I may be all screwed up, the way it is stated just doesn't look right to me.

Bill Gill

My first thought agreed with yours, Bill; then I thought about how small amounts of heavy elements might reduce luminosity.

Luminosity might be reduced by something less transparent covering the outer surface, and this, as you point out, would tend to increase the temperature and therefore the rate of fuel consumption. It would also require large amounts of whatever it was, which is not what we are dealing with.

We are dealing with small amounts of heavy elements, which one would expect to find in the interior of the star. This might reduce the rate of reaction, thus resulting in lower fuel consumption, and reduced luminosity. So it is the lower fuel consumption that causes the reduced luminosity, rather than the other way round.

Perhaps “traps radiation” was not the best term to use. “Inhibits radiation” might have been better.

Just a thought while we wait for the experts..

The quote from SciAm clearly says that the heavy elements increase the opacity of a stars outer layers. This is the factor the author was discussing. He mentions another factor which is that heavy elements are "nuclear deadweight". They don't burn so they reduce the lifetime of the star.

My impression is that when he is talking about increased abundance of heavy elements he is talking about a large increase. The time frame he is talking about in which the abundance of heavy elements increases is in the billions and possibly trillions of years. He is concerned with an increase in heavy elements due to the death of a huge number of massive stars which spit out, over a long enough time, a huge amount of heavier elements.

Bill Gill

The problem is you are trying to understand quantum behaviour with conventional physics

http://www.physorg.com/news/2012-02-transparent-iron-atomic-nuclei.html

There are limits to understanding these sort of problems classically in a sun you are entering the quantum domain :=)

DG is saying that the addition of heavy elements “augments the opacity of a star's outer layers”, but if EIT is influencing the scenario, would that not negate effect?

Orac, the article says that because the radiation is held in (my rephrase) by the increased opacity of the surface then the luminosity of the star would be lower. He then equates a lower luminosity to a slower consumption of the fuel. With a star of the current generation this would be true, luminosity would be an indicator of the fuel consumption. But if the luminosity is lower because the energy can't get out that would produce a different effect, at least I think it would. If the energy is contained within the star, which is what the author implies, then the sun would actually be hotter than its luminosity would suggest. And it seems to me that that would imply a higher burn rate. I'm not sure that the EIT thing would have much impact, since we are talking about a higher opacity, not one that is lowered for certain conditions.

Bill Gill

Sorry I only quickly read your last comments I didn't read back to the original post and so my response is out of confusion :-)

Having read it lets see if we can clear up the confusion.

You have a nuclear fusion reactor ball in space.

The nuclear reaction will be producing force trying to throw the ball apart, gravity will opposing that so you have an equilibrium.

The equilibrium will be between the radiation held into the ball increasing the chain reaction versus the radiation that leaves the ball so it doesn't continue to go into runaway chain reaction.


Any other material other than hydrogen or helium introduced into the system will act like a moderator. That is it will absorb and therefor inhibit the reaction process trying to achieve runaway so you lower the chain reaction and therefore lower the equilibrium.


If you were trying to run a classic nuclear plant you would call it poisoning the core (http://en.wikipedia.org/wiki/Neutron_poison)

Hope that makes sense.

The problem with the original article is the term "keeping the radiation in" the scientist would have said "absorbed" and they are not identical expressions in this case, a problem someone to whom english is a second language like myself is all too familar with.

Thanks Orac. I think that explains the matter. Another thing that the writer said about the effect would be that a larger percentage of heavy elements would reduce the amount of hydrogen and helium, so that the burn would be slower due to that. So the star would be cooler and have a longer lifetime.

Further into the article the author suggested that there might be many more stars with planets in the life zone, and therefore much more life would develop, as the galaxies got more spread out. I think that was his major point. Even though the galaxies will be much more isolated in the far future there will be more planets capable of sustaining life for a very long time.

Bill Gill

What I learnt was one should not come home late at night full of beer and hit the forums :-)

I should add a rather neat thing about suns being so big gravitationally it is actually hard for things to hit them they tend to get slingshotted as they spiral inwards and given the above effect you can see it is rather important that they do.

Hits would be rare but not impossible.

Well explained, as usual, Orac.

Would you expect these heavy elements to migrate towards the centre under gravity, or would the process of combustion keep them circulating throughout the body?

Sorry Bill S that one is way outside my ability to speculate.

If I had to guess and it would be only a guess I would say go towards the middle because it is denser. However that would give the sun a radiation cold spot in the core and I imagine forces opposing that would be immense so I think that one is a question for the experts in the field.

When a star gets old and has burned up all of its hydrogen and helium it starts working its way up the table of elements toward iron. Iron of course is as high as a star can go during its normal life. So at the end of life a star will have an iron core. Actually I think that is only for heavy stars. The iron in that case of course is formed near the center of the star, which is where the fusion reaction takes place. So that isn't quite the same as having a lot of heavy elements in the mix when the star is formed. I don't have a good answer as to what would happen, but I suspect that the heavy elements would settle towards the center, but they would do it very slowly. I suspect it would be like a suspension of dust in a glass of water. If the dust is fine enough it can stay mixed in the water for a long time before it all settles out.

Bill Gill