Andy Meadowcroft - Introduction (and a question about nuclear fusion)

[Andy Meadowcroft]

How did you find PF?: I searched google for a Physics Forum

I searches Google for a Physics Forum, to enable me to pose a question to people who know more about physics than I do. I have no detailed physics knowledge, but do read a lot of science articles, so understand most of the basics.

A few months ago I was watching a documentary (How the Universe Works on Discovery Channel) about the end of the universe. The last generation of stars were described as being so cold that you could ski on their surfaces and the reason given for this was that the materials of which they were formed included large amounts of heavy metals. These heavy metals act to reduce the temperature at which fusion can start, thus reducing the overall temerature of the star itself.
Since this was a documentary for relative laymen there were no further details given, but it got me thinking about the long-running problems which are being encountered with creating working nuclear fusion reactors on Earth, the main one being the incredably high temeratures required.

My question is this: could the fuel used in a fusion reactor be seeded with heavy metals, to reduce the temperature at which fusion can commence to levels more easily created in the laboratory (thousands of degrees instead of millions of degrees)?

I don't know the protocols for this forum in particular, and forums in general, since I usually avoid most social media, so I apologice in advance for any faux pas I may have made/may make in future.

Andy

 

[TeethWhitener]

I think you're @Lord Jestocost in disguise Welcome to PF!

 

[berkeman]

(thread moved from the New Member Introductions forum to the Physics forums)

 

[Andy Meadowcroft]

Sorry - didn't read the guidance properly.

I'll repost my question in General Discussion.

Andy

 

[Andy Meadowcroft]

I have never posted to this or any other science forum before.

Andy

 

[berkeman]

Sorry - didn't read the guidance properly.

I'll repost my question in General Discussion.

Andy

No need to repost. You will get good replies here in this technical forum.

Have you read anything about Binding Energy and nuclear fusion?

https://en.wikipedia.org/wiki/Nuclear_binding_energy

 

[Drakkith]

The last generation of stars were described as being so cold that you could ski on their surfaces and the reason given for this was that the materials of which they were formed included large amounts of heavy metals. These heavy metals act to reduce the temperature at which fusion can start, thus reducing the overall temerature of the star itself.

I've not heard of this before, so I'm a bit skeptical. Fusion in stars usually starts with the proton-proton chain (ignoring the much easier deuterium-deuterium fusion since there's only ever a miniscule amount in any star), with heavier elements only undergoing fusion at much higher temperatures and densities. I'm not quite sure what would happen if a star formed with a significant portion of its core composed of heavier elements. My first thought is that it would reach a higher temperature initially and have less fuel to burn while on the main sequence (the portion of a star's life when it burns hydrogen).

My question is this: could the fuel used in a fusion reactor be seeded with heavy metals, to reduce the temperature at which fusion can commence to levels more easily created in the laboratory (thousands of degrees instead of millions of degrees)?

Not a chance. Current fusion research focuses primarily on dueterium-tritium fusion, or D-T fusion. D-T fusion is by far the easiest reaction to sustain, as both deuterium and tritium have very high masses compared to their electric charge and this reaction also releases the most energy per fusion event. We can fuse D-T at a lower temperature and a lower density than we can any other materials.

Heavy elements are MUCH more difficult to fuse, requiring both higher temperatures and higher densities for the same reaction rate. Throwing heavy elements into the reaction chamber will do nothing but introduce 'ash' to the reaction chamber, ash being non-burnable material, since heavy elements simply don't fuse at the temperature that D-T does.

 

[Andy Meadowcroft]

Thank you for your reply.

The documentary was talking about the last generation stars which will form in trillions of years after the last of the red dwarfs have burned out. It did not state that the heavy metals themselves fused, merely that the presence of the heavy metals in the forming stars lowered the temperature at which fusion could happen from millions to thousands of degrees, but with a lower energy output, hence the cold surfaces of the stars.

The episode of How the Universe Works that I saw was either Season 3 Episode 2 End of the Universe or Season 8 episode 4 Death Of The Last Stars (I am not sure which as I did not note down the title but these are the only two episodes listed on their website which could cover this subject).

For this statement to have been made in the programme by the apparently reputable scientists they consult there must be some scientific basis behind it.
I was just wondering if there could be a practical application of this phenomena

Andy

 

[Vanadium 50]

"Watch two hour-long shows and explain to me what I don't understand" is a big ask indeed.

Fusion at thousands of degrees (at substantial rates) sounds dubious. Of course stellar surfaces are that cool.

 

[Astronuc]

These heavy metals act to reduce the temperature at which fusion can start, thus reducing the overall temerature of the star itself.

It did not state that the heavy metals themselves fused, merely that the presence of the heavy metals in the forming stars lowered the temperature at which fusion could happen from millions to thousands of degrees, but with a lower energy output, hence the cold surfaces of the stars.

No, heavy metals do not lower/reduce the temperature at which fusion can start/occur. The higher the Z, the higher the temperature to fuse, due to coulomb repulsion. Heavy metal nuclei also recombine with electrons which produces relatively low energy photons that radiate from the plasma. At some point, like around Ni (Z = 28), fusion doesn't occur.

Heavy metals are the 'ash' of the fusion process, not the fuel. It's complicated, but heavy metals do not lower the temperature at which fusion occurs.

 

[berkeman]

The documentary was talking about the last generation stars which will form in trillions of years after the last of the red dwarfs have burned out. It did not state that the heavy metals themselves fused, merely that the presence of the heavy metals in the forming stars lowered the temperature at which fusion could happen from millions to thousands of degrees, but with a lower energy output, hence the cold surfaces of the stars.

The episode of How the Universe Works that I saw was either Season 3 Episode 2 End of the Universe or Season 8 episode 4 Death Of The Last Stars (I am not sure which as I did not note down the title but these are the only two episodes listed on their website which could cover this subject).

"Watch two hour-long shows and explain to me what I don't understand" is a big ask indeed.

So @Andy Meadowcroft obviously it would help your case if your could at least provide a time stamp directly into the long videos that you posted.

 

[Drakkith]

I found the part of the video in question. It's around 38:30 in the episode Death of the Last Stars. The show claims that there is a hypothesized star that might exist in the far future that has a surface temperature of just 32 degrees F, and says that this is because heavy metals let stars form cooler and smaller. It says nothing about the internal temperature, only that the star will have nuclear fusion inside it, yet have a surface that's as cold as ice. I'd provide a link to the video but I'm fairly sure the website I watched it through pirated it.

I'm not an astronomer or astrophysicist or anything, so I can't say with any certainty where they got this idea from. I know that heavy elements make it easier for a forming star to fragment, leading to multiple smaller stars instead one big star, but I don't know how heavy elements affect the minimum temperature required for fusion. My understanding was that it didn't.

 

[Andy Meadowcroft]

Thank you very much for your hard work, Drakkith, and for everyone else's comments..
I guess I must've misunderstood the implications of what they were saying.
It looks like the addition of heavy metals in the formation of the hypothisised last stars would reduce size at which that star can achieve the necessary pressures and temperatures to ignite and also reduce the surface temperature of the star once it is burning rather than the temperature at which fusion can commence in the core.

Andy