Were Population III stars powered entirely by p-p fusion?

  • I
  • Thread starter bbbl67
  • Start date
  • #1
165
14
In modern stars, the more massive a star is, the more likely that it is powered by the CNO fusion cycle, where Carbon, Nitrogen, an Oxygen act as catalysts for creating helium from hydrogen. In stars over 1.3 solar masses, this is the primary fusion process. Below that that level it's a mix of the CNO and P-P cycles. P-P is supposed to be much slower than CNO, since it uses no catalysts. Since neither Carbon, Nitrogen, nor Oxygen existed during the Pop III epoch, were all of these massive stars powered simply by the P-P cycle? Well at least entirely P-P until they produced the CNO internally eventually?
 
  • Like
Likes TEFLing

Answers and Replies

  • #2
125
93
Yes. If a star truly has no elements heavier than helium, then it can't use the CNO cycle, at least not at first.

From Section 2.3 of Yoon, Dierks, and Langer (2012), Astronomy and Astrophysics, 542, A113:

"In metal-free massive stars, the CNO cycle cannot be activated initially. Because the energy generation due to the pp [proton-proton] chain is too weak to support a massive star with M ≥ 20 MSun for a significant fraction of the evolutionary time, the stellar core rapidly contracts until enough carbon . . . is produced by helium burning . . . . Hydrogen burning with the CNO cycle only begins thereafter."
 
  • Like
Likes TEFLing, bbbl67, mfb and 2 others
  • #3
Matterwave
Science Advisor
Gold Member
3,965
326
"In metal-free massive stars, the CNO cycle cannot be activated initially. Because the energy generation due to the pp [proton-proton] chain is too weak to support a massive star with M ≥ 20 MSun for a significant fraction of the evolutionary time, the stellar core rapidly contracts until enough carbon . . . is produced by helium burning . . . . Hydrogen burning with the CNO cycle only begins thereafter."
So it sounds like Helium fusion (triple-alpha?) is also going on, and it's not just p-p chain?
 
  • Like
Likes TEFLing and bbbl67
  • #4
mathman
Science Advisor
7,890
460
Something must be going on to get C, N, O.
 
  • #5
phyzguy
Science Advisor
4,717
1,639
I did some simulations of massive Population-3 stars using the stellar simulator MESA (http://mesa.sourceforge.net/). for a course I took in graduate school. For a 50 solar mass star, it only took about 1000 years to produce enough heavier elements for the CNO cycle to become dominant. Since the star lives for 10^5-10^6 years, it spends most of its time with the CNO cycle dominating.
 
  • Like
Likes TEFLing, bbbl67 and Buzz Bloom
  • #6
165
14
I did some simulations of massive Population-3 stars using the stellar simulator MESA (http://mesa.sourceforge.net/). for a course I took in graduate school. For a 50 solar mass star, it only took about 1000 years to produce enough heavier elements for the CNO cycle to become dominant. Since the star lives for 10^5-10^6 years, it spends most of its time with the CNO cycle dominating.
Wow, only 1000 years? I was guestimating a million years at least. So those first stars were already doing simultaneous helium fusion at the same time as hydrogen fusion in their cores?
 
  • Like
Likes TEFLing
  • #7
1,611
226
So it sounds like Helium fusion (triple-alpha?) is also going on, and it's not just p-p chain?
Is it just a matter of these two?
In high metallicity, low mass stars, protium is fused and nearly completely consumed at core temperatures of 15 million K or not much more. Triple alpha only ignites at about 100 million K, by which time protium is nearly completely gone.

But pp chain has branches.
Namely these are:
1)3He+α→7Be+γ
2)7Be+e-7Li+νe
3)7Li+p→2α
and
4)7Be+p→8B+γ
5)8B→8Be+e+e
6)8Be→2α

Note that both these branches cycle to 2α
Triple alpha process is:
7)α+α→8Be
8)8Be+α→12C

Triple alpha has very low probability because the lifetime of 8Be is just 10-16 s to process 6 which is spontaneous. Very many 8Be nuclei must form and spontaneously decay for one to turn to 12C. So triple alpha needs extremely high temperature, where 7) is a bit more frequent.
But 8B has spontaneous decay lifetime of about 1 s, and the lifetimes of 7Be and 7Li are only limited by nonspontaneous processes.
Could there be any noticeable branch processes, at high temperature:
7Be+α→?
7Li+α→?
8B+α→?
 
  • Like
Likes TEFLing
  • #8
stefan r
Science Advisor
882
272
-Pair instability? I suspect you would have carbon after a pair-instability pulse. I have not read anything on that.

-Gravitational collapse should heat population III stars for awhile.

-Tides, should be minor.
 
  • #9
165
14
Triple alpha has very low probability because the lifetime of 8Be is just 10-16 s to process 6 which is spontaneous. Very many 8Be nuclei must form and spontaneously decay for one to turn to 12C. So triple alpha needs extremely high temperature, where 7) is a bit more frequent.
But 8B has spontaneous decay lifetime of about 1 s, and the lifetimes of 7Be and 7Li are only limited by nonspontaneous processes.
Could there be any noticeable branch processes, at high temperature:
7Be+α→?
7Li+α→?
8B+α→?
2He (diproton) also has a pretty low half-life of much less than 10-9, so lots of low-probability reactions happening all at once? These stars were so massive that they brute-forced a whole bunch of low-probability reactions at once, they went straight to helium burning as soon as they got any helium to burn.

Also as a side question, they think they just discovered the first-ever Pop III red dwarf star in our galaxy. A 1st generation red dwarf would have even less CNO present inside it than today's red dwarfs, and almost no way of making their own internally, unlike the Pop III supergiants. So would this type of red dwarf be an even slower burner than modern red dwarfs? Therefore would it last even longer than most modern red dwarfs?
 
  • Like
Likes TEFLing
  • #10
1,611
226
These stars were so massive that they brute-forced a whole bunch of low-probability reactions at once, they went straight to helium burning as soon as they got any helium to burn.
Obviously not. They had primordial helium, yet they were not fusing it as protostars - they were too cool for that.
Also as a side question, they think they just discovered the first-ever Pop III red dwarf star in our galaxy. A 1st generation red dwarf would have even less CNO present inside it than today's red dwarfs, and almost no way of making their own internally, unlike the Pop III supergiants. So would this type of red dwarf be an even slower burner than modern red dwarfs? Therefore would it last even longer than most modern red dwarfs?
No.
A low metallicity red dwarf should be faster burner.
The reason is that the contracting protostar reaches main sequence when heat radiating through the star´s envelope matches fusion heat produced in the core.
A low metallicity, low temperature star should lack the ion opacity possessed by high metallicity, low temperature stars, having only the free electron opacity. Therefore low metallicity, low mass star should, because of its poorer insulation, continue shrinking and reach main sequence at higher interior temperature, higher luminosity and lower lifetime than a high metallicity, low mass star.
 
  • Like
Likes TEFLing and stefan r
  • #11
165
14
Obviously not. They had primordial helium, yet they were not fusing it as protostars - they were too cool for that.
Okay, yes they should have BB helium already inside them, I forgot about that. But does PP fusion alone produce enough power to keep such massive stars inflated, considering it's such a slow fusion process? Or would these stars have to collapse and heat up further to get enough power production?
No.
A low metallicity red dwarf should be faster burner.
The reason is that the contracting protostar reaches main sequence when heat radiating through the star´s envelope matches fusion heat produced in the core.
A low metallicity, low temperature star should lack the ion opacity possessed by high metallicity, low temperature stars, having only the free electron opacity. Therefore low metallicity, low mass star should, because of its poorer insulation, continue shrinking and reach main sequence at higher interior temperature, higher luminosity and lower lifetime than a high metallicity, low mass star.
How much of a difference to lifetimes would this make?
 
  • Like
Likes TEFLing
  • #12
stefan r
Science Advisor
882
272
Okay, yes they should have BB helium already inside them, I forgot about that. But does PP fusion alone produce enough power to keep such massive stars inflated, considering it's such a slow fusion process? Or would these stars have to collapse and heat up further to get enough power production?
...
PP fusion rate is dependent on both temperature and pressure. The rate will be enough to prevent further collapse.
 
  • Like
Likes bbbl67 and TEFLing

Related Threads on Were Population III stars powered entirely by p-p fusion?

  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
5
Views
3K
  • Last Post
Replies
1
Views
623
Replies
8
Views
3K
  • Last Post
Replies
5
Views
1K
  • Last Post
Replies
2
Views
3K
  • Last Post
2
Replies
29
Views
4K
  • Last Post
Replies
2
Views
1K
Replies
1
Views
6K
Top