# Questions tagged [nucleosynthesis]

Nucleosynthesis is the process by which stars create heavier elements from hydrogen and helium

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### Why is the baryon to photon ratio assumed to be constant?

For standard BBN calculations, we use the baryon to photon ratio $$\eta=\frac{n_b}{n_\gamma}$$ that we get from CMB. Now, this clearly assumes that $$\eta_{BBN}=\eta_{CMB}.$$ There are around ...
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### Can particles (atoms, protons, neutrons and quarks) combine together in empty space? [closed]

Can particles (atoms, protons, neutrons and quarks) combine together in empty space without the use of energy to form macroscopic objects? Can elements form with an atomic number that is greater than ...
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### Can someone clarify this quote about weak interactions causing neutron-to-proton ratio to go to zero if equilibrium were possible?

I'm self-studying Modern Cosmology by Dodelson and Schmidt 2nd edition, and I've found a certain passage in Section 4.2.1 particularly interesting. It says, Therefore, at high temperatures, there ...
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### Ratio of specific entropy after a particle decay

I am working through the following problem (not homework, but revision), and would appreciate some feedback on my answers to parts (b) and (c). For part (b), I used Friedmann's first equation ...
1 vote
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### Conceptual problem understanding chemical equilibrium in the context of cosmology

In the context of describing the departures from equilibrium in the early universe (i.e. Big Bang Nucleosynthesis, recombination), one uses Boltzmann equations. Let's suppose we have a binary ...
1 vote
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### Would a head-on collision between two stars create heavier elements?

I was thinking about Przybylski's Star, and I was wondering how it was possible that so many heavy elements ended up in the star, such as einsteinium, californium, berkelium, etc. But there is ...
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### What is the reason for density fluctuations in the primary matter of the early Universe? [duplicate]

After a period of inflation and nucleosynthesis, small density fluctuations appeared in the primary gas, which became the "seeds" of the first stars and galaxies. What led to the appearance ...
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### How is formation of elements from particle accelerators different from the fusion process in a star?

I have a query assigned with the formation of elements in particle accelerators. We know that two hydrogen atoms for example fuse under extreme conditions to give a helium atom to release a lot of ...
1 vote
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### Does pressure alter nuclear reaction rates?

I'm studying the nuclear reactions involved in nucleosynthesis. As I understand it, the primary variables are the density of the particles, the cross section of the particle, and the average ...
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### What are the number densities of neutrinos and positrons during BBN?

I'm attempting to work through the third chapter of Dodelson's Modern Cosmology. He gives an approximation for the neutron-proton conversion rate, $\lambda_{\rm np}$ that he lifted from Bernstein, but ...
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### Why are lithium-$7$ and boron-$11$ respectively more abundant than lithium-$6$ and boron-$10$?

Lithium, beryllium and boron are unusually low in abundance for low-$Z$ elements, because their stable isotopes aren't the ones stellar nucleosynthesis makes without consuming them. What little ...
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### If for heavier atoms to form, you need a lot of pressure, then why do we say that the early universe was too hot for atoms to form?

I have read this question: Over the next 380,000 years, the universe gradually cooled down enough for the sub-atomic particles to condense and form the first Hydrogen atoms After the big bang As far ...
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### Photon temperature at nucleosynthesis

I've been asked about photon temperature at nucleosynthesis (3 minutes from Big Bang). So I guessed this is the moment when Matter and Radiation where in equilibrium: $$\rho_M(T) = \rho_R(T)$$ Taking ...
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### In Big Bang Nucleosynthesis (BBN), why aren't anti-protons created and destroyed?

I'm working through BBN theory with Dodelson and other papers. I have a question about the reactions between particles in the $1\space MeV$ range. We have reactions that produce and annihilate ...
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### Why is Big Bang Nucleosynthesis favored over Stellar Nucleosynthesis?

We know that a hot soup of plasma as it cools (as space expands) can produce light elements if the baryon-photon ratio is just right and the space expands quickly enough so that the neutrons and ...
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### Can someone please explain how Dodelson derives his formula for the neutron to proton ratio during BBN?

On page 66 of Modern Cosmology by Dodelson, we have the formula for the change in neutron to proton ratio as a function of time:$$\frac{dXn}{dt}=\lambda_{np}\left\{(1-X_n)e^{-\frac{Q}{T}}-X_n\right\}$$...
1 vote
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### How does Dodelson calculate the number density of a particle species?

In chapter three of Modern Cosmology, Dodelson models the evolution of a particle plasma as the universe expands. On page 61, the author gives the formula for the species-dependent equilibrium ...
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### Why is aluminum more common in the solar system, etc. than sodium and fluorine?

I know that odd-numbered elements are less common than even-numbered ones, but why is aluminum more commonly created in the galaxy, apparently, than lighter odd-numbered elements? Sure, sodium and ...
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### Why is nitrogen (with 7 protons and 7 neutrons) so much more abundant than any other odd-odd isotope?

Having an even number of neutrons and/or an even number of protons tends to make a nucleus more stable against radioactive decay. There are only 5 stable nuclei with both an odd number of neutrons and ...
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### Is the beta-plus decay that converts two colliding protons into deuterium more likely (energetically favorable) than beta-plus in a larger nucleus?

In the Sun, two nearby protons have to get extremely lucky and have one (or both?) of the protons convert to a neutron in a positron-emission, or beta-plus decay... I would think that there is less ...
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### Why is the isotope Nitrogen-14 formed preferentially to Nitrogen-15 in the CNO-cycle in stars?

Most of the universe's nitrogen is formed in larger, main sequence stars using the CNO Cycle, right? But I cannot find a good, specific explanation as to why $^{14}$N, with both an odd number of ...
1 vote
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### Nuclear synthesis and neutron star collisions

When I look up the wikipedia article on nuclear synthesis, this https://en.wikipedia.org/wiki/File:Nucleosynthesis_periodic_table.svg picture tells me that quite a lot of chemical elements would have ...
1 vote
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### Can Uranium-235 absorb neutrons and become U-238 instead of splitting?

Does that happen, on occasion? Does anyone know? I suppose it is quite rare, ... To absorb three neutrons without once fissioning... Could U-235 absorb three neutrons at once? Or just one, becoming ...
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### What is the reheating temperature of neutrinos during the BBN?

I want to integrate a differential distribution function of sterile neutrinos and the boundaries are the decoupling and the reheating temperature of neutrinos, but I don't know what is an acceptable ...
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### Why is the density of atoms in space so much higher than the baryon density from BBN and CMB?

If you ask the question: "how many atoms per cubic meter in space", you can get a consensus of around $5$ atoms per cubic cm or about $5\times 10^6\space \text{atoms}\space m^{-3}$. If we ...
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I'm reading through Dodelson chapter on BBN. I'm trying to follow the examples, but having trouble with the basics. First, the proton to neutron ratio is quoted as: \frac{n_p}{n_n}=e^{\frac{Q}{T}}$... 0 votes 1 answer 55 views ### Could one, with enough electricity to spare, use particle accelerators to mass produce heavy elements If electrical, and economic limitations are ignored, would it be possible for a civilization to mass produce many millions of tonnes of useful heavy elements using particle accelerators? Could they ... 0 votes 1 answer 54 views ### Weak interactions in nucleosynthesis During the Bing Bang nucleosynthesis the particles present in the universe are, photons ($\gamma$), neutrinos ($\nu$), antineutrinos ($\overline{\nu}$), electrons($e^{+}$), positrons ($e^{+}$), ... 19 votes 3 answers 5k views ### Why do main sequence stars get bigger and more luminous as they age? As stars age, the concentration of hydrogen in the core decreases, which lowers the power output, causing an imbalance between outward radiation pressure and inward gravitational pressure. This causes ... 8 votes 1 answer 1k views ### Why did the Big Bang produce hydrogen? I know that first generation stars' main fuel was Hydrogen. I know the Big Bang happened at some point in time. Now if the strong force exists, then why aren't different, higher mass, number elements ... 0 votes 1 answer 27 views ### What means$h$which appears following cosmological measured parameters? For example, Big Bang Nucleossynthesis says that baryonic content in the Universe is around$0.018 \leq \Omega_{b}h^{2} \leq 0.024$. I know it is something related to measurement error but I do not ... 1 vote 1 answer 54 views ### How the dark matter density parameter appears in Big Bang Nucleosynthesis (BBN)? I know that it is possible to obtain the light element abundances ($^{4}\mathrm{He}, \mathrm D, ^{7}\mathrm{Li}$etc.) and to determine the baryon-to-photon ratio$\eta$from the total baryon ... 0 votes 1 answer 89 views ### Thermal fission of gold? [closed] Here's a slightly silly idea that arose from this question "What elements would be created in a star composed entirely of gold?": If we accumulate a lot of gold (really a lot) fast enough, it will ... 0 votes 2 answers 87 views ### Why does the formation of iron in stars not produce any energy? Why does the formation of iron in stars not produce any energy. I have a hunch that it might have something to do with the fact that iron has the highest binding energy per nucleon and hence (... 3 votes 0 answers 52 views ### Intermediate product of CNO cycle CNO cycle of stellar nucleosynthesis involves several steps in which different isotopes of Carbon, Nitrogen, and Oxygen are transmuted into each other when hit by a ... 1 vote 0 answers 50 views ### CNO-cycle in Population III stars [duplicate] It has been stated several times here and probably elsewhere that the first population III stars were unable to undergo H-to-He fusion via the CNO cycle as there was virtually no carbon-12 at the ... 0 votes 1 answer 374 views ### Energy budget of of proton-proton fusion (p-p chain) How much energy is released in the first nuclear reaction of the p-p chain which produces energy in lower-mass stars like the Sun? The value of 1.44 MeV is given in many sources, but I don't ... 0 votes 1 answer 152 views ### What are physical obstacles for rare earth metal synthesis in nuclear reactors in industrial scale? [closed] Diminishing availability of rare earth metals prohibits expansion of traditional and quantum computing and low temperature technologies. So - my question is about artificial synthesis of rare earth ... 0 votes 0 answers 67 views ### BBN: How is the expansion rate related to the freeze-out of Neutrons? I've been making my way through several papers and textbooks on the subject, but they all seem to gloss over this subject: how is the equilibrium of protons, neutrons and photons related to the ... 1 vote 2 answers 69 views ### Where do large deposits of heavy elements come from? I understand that the heavier elements were produced by a series of fusion reactions in a series of stars. What I've never understood is why the atoms of these elements would ever coalesce together ... 5 votes 1 answer 202 views ### What is the current measured (i.e. model independent) density of hydrogen$H\$ in the universe?

The question came up in this thread: What is the current value for the temperature at which Recombination took place? I was under the impression that we had an independent measurement of Hydrogen in ... 173 views

### How large was the universe at the start of BBN? How large was it after?

I'm trying to get a handle on the basics of BBN. I've read several authors quote the figure of 17 minutes. So BBN started at 3 minutes and ended at 20 minutes after the start of time. So how large ... 1 vote
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### How to calculate angular momentum given angular distribution

I have recently attended a colloquium on transfer reactions. At one point, the speaker was talking about calculating the angular momentum of a nuclear transfer reaction from an angular distribution ...
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### Why is hydrogen the most abundant element in the Universe?

Hydrogen is the most abundant element in nature. Does cosmological nucleosynthesis provide an explanation for why is this the case? Is the explanation quantitatively precise?
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### Why did CMB decouple when neutral atoms formed?

Atoms can absorb and emit photons. But it is said that when the neutral atoms were first formed the photons got decoupled. If the atom-photon interaction is allowed how can we say that photons were ...
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### Why do the sun and planets differ in their chemical elements composition?

The chemical elements composition of the sun changes as its nuclear furnace changes elements into heavier ones. But it is supposed to be mostly Hydrogen at the start. The planets are too cold for ...
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### How likely is it that the mass of all humans was generated by the same supernova? [duplicate]

All humans are brothers. They came from the same supernova. ─ Allan Sandage (1926 - 2010) My gut (not a physicist) says not very likely at all. That being said, he was probably referring to the ...