Can Protons and Electrons Combine to Form Neutrons in Stars?

[Stephanus]

There's a reaction where a proton and electron can combine to form a neutron and a couple of neutrinos...

This the answer that I have from Chalnoth in my other thread about nuclear fusion inside the sun (or star in main sequence).

And after iron burning in the core of massive star, the star explodes and leaves a neutron star (or a black hole) behind.

Is producing neutron in P+P reaction chains and producing neutron star a similar process?

 

[ChrisVer]

I think we had talked about it in the neutron star formation question you had posted...
The interaction is an inverse beta decay (or electron capture)
pe^- \rightarrow n \nu_e
and it's what happens at the collapsing core of the sun before forming a neutron star. (sidenote: Not all protons or electrons have to disappear)

Is producing neutron in P+P reaction chains

for which branch are you asking about?

 

[ChrisVer]

https://www.physicsforums.com/threads/proton-electron-neutron.814101/

 

[ChrisVer]

Also check here:
http://en.wikipedia.org/wiki/Proton–proton_chain_reaction
where it gives the reactions...
For the pp I branch (temperatures 10-14 MK) you don't have any electron capture processes vs pp II branch (temperatures 14-23MK) you have electron capture processes (Berylium gets an electron to give the Lithium)

 

[Stephanus]

I think we had talked about it in the neutron star formation question you had posted...
The interaction is an inverse beta decay (or electron capture)
pe^- \rightarrow n \nu_e
and it's what happens at the collapsing core of the sun before forming a neutron star. (sidenote: Not all protons or electrons have to disappear)
for which branch are you asking about?

Yes, I do remember it ChrisVer.
It's just that I realized in P + P reaction P + P -> Deuterium
It is different I think with He3 + He 3 -> Two Hydrogens + He 4.
There's no change of particle here, but in P + P there is a change particle. Proton becomes a Neutron. So I remember back then when I ask that previous question about neutron star. Is it the same process.
In neutron star it is pressure, right?
And in P + P reaction it is collision (and pressure), right?
Because there's also fusion reaction in tokamaks, but in atmospheric pressure.

 

[ChrisVer]

that change is just a usual beta+ decay and not an electron capture.

Both are due to pressure...

i am not sure if Tokamaks have achieved fusion.

 

[Stephanus]

http://www.citycollegiate.com/atomic_structureXIg.htm[/URL]
CHARACTERISTICS OF PROTON
Mass of proton: Mass of proton is 1.0072766 a.m.u. or 1.6726 x 10-27 kg.
Comparative mass: Proton is 1837 times heavier than an electron.

CHARACTERISTICS OF NEUTRON
Mass of neutron: . Mass of neutron is 1.0086654 a.m.u. or 1.6749 x 10-27 kg.
Compartive mass: Neutron is 1842 times heavier than an electron.

And at some other links
[URL]http://education.jlab.org/qa/particlemass_02.html

It seems that proton is heavier than neutron.

http://en.wikipedia.org/wiki/Proton–proton_chain_reaction#Energy_release

Comparing the mass of the final helium-4 atom with the masses of the four protons reveals that 0.007 or 0.7% of the mass of the original protons has been lost. This mass has been converted into energy, in the form of gamma rays and neutrinos released during each of the individual reactions.

Is He4 heavier than 4 Hydrogens?

 

[Stephanus]

i am not sure if Tokamaks have achieved fusion.

Haven't they? Perhaps for 10 minutes then dies I think. ITER is designed to sustained the reaction much longer.

 

[ChrisVer]

It seems that proton is heavier than neutron.

the proton is lighter than the neutron, and that's why a free proton cannot decay to a neutron.

Is He4 heavier than 4 Hydrogens?

no... Nuclei are not "balls" made up of constituents you can sum up and get their mass. There is a difference between summing over the neutron/proton masses and the nucleus mass, and that's because the nucleus is a bound system.

 

[Stephanus]

the proton is lighter than the neutron, and that's why a free proton cannot decay to a neutron.no... Nuclei are not "balls" made up of constituents you can sum up and get their mass. There is a difference between summing over the neutron/proton masses and the nucleus mass, and that's because the nucleus is a bound system.

Thanks for your invaluable help ChrisVer.
Wikipedia says "Comparing the mass of the final helium-4 atom with the masses of the four protons reveals that 0.007 or 0.7% of the mass of the original protons has been lost"
The proton is lighter than the neutron. As you say and many links in internet and my textbook in high school. Okay...
Or a SINGLE proton is lighter than a SINGLE neutron?
Or somehow 2P+2N (as in He4) if combined together becomes even more lighter than 4 single protons?

 

[mfb]

i am not sure if Tokamaks have achieved fusion.

They have.

Fusion needs high pressure and/or high temperature - both are important. We cannot reproduce the pressure in the interior of the sun in tokamaks, but we can reach higher temperatures.

Or a SINGLE proton is lighter than a SINGLE neutron?

Right. It does not make sense to ask about the mass of a single proton or neutron inside a nucleus. That is a meaningless concept.

 

[ChrisVer]

Or somehow 2P+2N (as in He4) if combined together becomes even more lighter than 4 single protons?

That's what it happens... Again I'm repeating that the mass of the Helium is not 2m_p + 2m_n, but less than that... The difference is due to the binding energy of the four nucleons within the Helium nucleus.

The proton (or hydrogen nucleus) has a mass of around 1.007825u
So 4 protons have a total mass of 4.031300u...

The Helium has a mass of 4.00260u...

Take the difference 4p-He: 4.031300u-4.00260u =0.02870u (or 26.72 MeV )
That's positive so M_{4p} > M_{He}
And so that's why it says that the protons lost some of their mass when they got combined into the Helium nucleus.

Making a check of the percentage: \frac{\Delta M}{M_{4p}} = \frac{0.02870}{4.031300}=0.00711929154 \approx 0.007 or 0.7%

 

[ChrisVer]

They have.

OK, thanks for that :)

 

[ChrisVer]

Or a SINGLE proton is lighter than a SINGLE neutron?

I don't understand this question.
A proton is lighter than a neutron... it can decay to a neutron only by finding from somewhere the needed energy [and it does so when it is within a bound state like a nucleus, and so you have nuclei that can undergo beta+ decay]

 

[Stephanus]

I don't understand this question.
A proton is lighter than a neutron... it can decay to a neutron only by finding from somewhere the needed energy [and it does so when it is within a bound state like a nucleus, and so you have nuclei that can undergo beta+ decay]

I mean the mass of a positive ion of Hydrogen is lighter compared to the mass of a single free neutron.

 

[ChrisVer]

positive ion of Hydrogen

You are rephrasing the word "proton" in a complicated way here

and yes... a free "positive ion of Hydrogen" has a mass m_p while a neutron has m_n...and we already agreed that m_n> m_p.
But still, two free "positive ions of Hydrogen" are lighter (mass=$2m_p$) than two free neutrons (mass=$2m_n$)... (that's why I had problem with the word "single" but I'd prefer "free").

But for example (from the wiki article on the pp chains) the diproton which is not "free", is heavier than a deuterium...

 

[Stephanus]

"Or a SINGLE proton is lighter than a SINGLE neutron?"
Right. It does not make sense to ask about the mass of a single proton or neutron inside a nucleus. That is a meaningless concept.

Okay, okay I understand now. Really do.
Thank you everybody.
Can I add a question here?
Beta decay.

A proton is lighter than a neutron... it can decay to a neutron... so you have nuclei that can undergo beta+ decay

I think we had talked about it in the neutron star formation question you had posted...
The interaction is an inverse beta decay (or electron capture)

I can understand (if not imagine) beta decay in P+P reaction chain in the Sun.
What about in neutron star.
Are you saying that the whole iron core, 1.44 solar mass I think, decays in 1 seconds to become a big ball of neutrons?
I mean tritium decays in 12 year, is it?
And Carbon 14 decays more than 5000 years.
But if somehow P + P decays becomes a deuterium in more than 1 second, I can imagine. At least, they say the proces for P + P becomes He4 in the sun takes more than 100 000 years. Forgot how long.
But in neutron star, does beta decay take place in 1 second?

 

[Stephanus]

...But still, two free "positive ions of Hydrogen" are lighter (mass=$2m_p$) than two free neutrons (mass=$2m_n$)... (that's why I had problem with the word "single" but I'd prefer "free").

FREE is the word. I understand now. Thanks.
It means a big different than the proton in nucleus.

 

[ChrisVer]

Are you saying that the whole iron core, 1.44 solar mass I think, decays in 1 seconds to become a big ball of neutrons?

I don't know about the time scales to be honest...
However when the core collapses beyond the white dwarf stage, it gets heated up... this heat up provides enough energy for:
e^-+ p+1.36MeV \rightarrow n \nu_e
As I have already explained, the existing electron/proton degeneracy blocks the neutron from decaying back and makes it stable within the core.
As collapse keeps on going and temperatures keep on growing (more free energy), any surviving atomic nuclei will undergo the inverse beta decay, with a peak appearing for the iron at ~3.7MeV.
So you get suddenly too many neutrons in the core. After that point the collapse is getting slowed down from free-fall by the neutrino pressure.
When neutrons are enough to become degenerate, the collapse is halt and immediately you go back into equillibrium ...this immediate transition produces an outward going shockwave (being also boosted by the neutrino pressure) because of the bounce of the infalling matterial over the neutron degenerate core. And that shock is the supernova.

 

[Stephanus]

I don't know about the time scales to be honest...
However when the core collapses beyond the white dwarf stage, it gets heated up... this heat up provides enough energy for:
e^-+ p+1.36MeV \rightarrow n \nu_e
As I have already explained, the existing electron/proton degeneracy blocks the neutron from decaying back and makes it stable within the core.
As collapse keeps on going and temperatures keep on growing (more free energy), any surviving atomic nuclei will undergo the inverse beta decay, with a peak appearing for the iron at ~3.7MeV.
So you get suddenly too many neutrons in the core. After that point the collapse is getting slowed down from free-fall by the neutrino pressure.
When neutrons are enough to become degenerate, the collapse is halt and immediately you go back into equillibrium ...this immediate transition produces an outward going shockwave (being also boosted by the neutrino pressure) because of the bounce of the infalling matterial over the neutron degenerate core. And that shock is the supernova.

Thanks ChrisVer, it's good enough for me.

 

[ChrisVer]

Now if all that happens in ~1sec, I don't know... But I wouldn't be surprised if it would...an astrophysicist might help you in that...

 

[Vanadium 50]

t seems that proton is heavier than neutron.

1842 > 1837.

 

[Stephanus]

1842 > 1837.

Ahhh, typo.
Proton is lighter than Neutron.
Thanks.

 

[mfb]

I can understand (if not imagine) beta decay in P+P reaction chain in the Sun.
What about in neutron star.
Are you saying that the whole iron core, 1.44 solar mass I think, decays in 1 seconds to become a big ball of neutrons?
I mean tritium decays in 12 year, is it?
And Carbon 14 decays more than 5000 years.
But if somehow P + P decays becomes a deuterium in more than 1 second, I can imagine. At least, they say the proces for P + P becomes He4 in the sun takes more than 100 000 years. Forgot how long.
But in neutron star, does beta decay take place in 1 second?

You are mixing several different concepts here.
p+p happens in main-sequence stars over millions to trillions of years. A stellar remnant that might form a neutron star fused most of its hydrogen to heavier elements already.
The neutron formation for a neutron star is not a decay in the usual sense - it is more a collision process of high-energetic protons and electrons.
There are very short-living nuclei, by the way. 10^-22 seconds is nothing special.

 

[Stephanus]

You are mixing several different concepts here.
p+p happens in main-sequence stars over millions to trillions of years. A stellar remnant that might form a neutron star fused most of its hydrogen to heavier elements already.
The neutron formation for a neutron star is not a decay in the usual sense - it is more a collision process of high-energetic protons and electrons.
There are very short-living nuclei, by the way. 10^-22 seconds is nothing special.

10^-22seconds, oh okay...
Thanks MFB for the answer.
I mean the beta decay process to convert the all of the protons to neutrons in supernova is 1 sec?
The process from hydrogen burning, helium, CNO, up to iron burning as you say, could be millions of years. 1 day for silicon burning.
But can't be more than 20 millions years I think from what I read, more than that the mass is not suficient to provide iron burning.
And how long this "iron burning" process? I read that it only takes 1 second and the star explodes into a supernova and leaves a neutron star behind.
So this beta decay in supernova is 1 second?

Sorry, I think I'm going off of topic here. I just want to know the nuclear reaction in main sequence, but,...

 

[mfb]

The nuclear reactions happen much faster (of the order of those 10^-22 seconds), all those stellar timescales are the timescales of the stellar evolution. Imagine building a house - the whole process can take a year, but you do not need one year to put one stone on top of other stones (as one tiny step in building the house).

But can't be more than 20 millions years I think from what I read, more than that the mass is not suficient to provide iron burning.

There are stars that do not end in a supernova. Very light stars can burn for trillions of years.