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35 votes
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Why does nuclear fusion generate heat?

The products of nuclear fusion have either kinetic energy (the products with mass) or just plain energy in the case of massless photons. The kinetic energy of particles in the gas is heat. The energy ...
ProfRob's user avatar
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27 votes

Do stars become more metal-rich as they evolve?

If the star is a solar mass or below it will not produce any metals (anything heavier than helium) within 10 billion years of birth. It will be on the main sequence, fusing hydrogen to helium via the ...
ProfRob's user avatar
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15 votes

Do stars become more metal-rich as they evolve?

Yes, it will Metals in astronomy, are simply elements that are heavier than hydrogen and helium. From that perspective, even oxygen and carbon are "metals" in a astronomical sense, although ...
Alastor's user avatar
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12 votes
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Within the Milky Way, how much helium is made each second?

I think this is really hard for the Milky Way because we are in it and have no accurate evaluation of how many stars it contains or its overall luminosity. Some sort of answer would be to assume that ...
ProfRob's user avatar
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12 votes
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What is it like to see a brown dwarf turn into a star?

Exactly what this would look like would depend a great deal on how (fast) the mass is accreted and whether there is a significant amount of energy accreted along with the mass. There are two ...
ProfRob's user avatar
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9 votes
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Why do stars usually stop fusion at iron, even though nickel-62 has the highest binding energy per nucleon?

Contrary to common myth, the addition of alpha particles to iron-peak nuclei is exothermic. However, in the nickel/iron core of a massive star, there are no free alpha particles, they must be broken ...
ProfRob's user avatar
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8 votes
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Is stellar ignition all-or-nothing?

No fusion isn't all or nothing. Given the same chemical composition of constituents then there will be a smooth ramping down of the nuclear fusion rate as the mass decreases. The lower mass objects ...
ProfRob's user avatar
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6 votes
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If two fast asteroids collided, would the temperature and pressure be high enough to cause nuclear fusion?

A different approach, ignoring any details of the collision. Note extreme simplifications will occur, do not try this at home... The easiest hydrogen isotope fusion process is D-T, with a cross ...
Jon Custer's user avatar
5 votes

In layman's terms, why does the cold C-N-O process end in Carbon?

This question has several misunderstandings. The CNO cycles (there is more than one CNO cycle) in a main sequence star have nothing to do with the triple alpha process. A star that is using the triple ...
David Hammen's user avatar
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5 votes
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In layman's terms, why does the cold C-N-O process end in Carbon?

Both 15N($p,\alpha$)12C and 15N($p,\gamma$)16O can and do take place. However, the former is a strong force interaction, whilst the latter is mediated by the electromagnetic force and is known as a &...
ProfRob's user avatar
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4 votes

Do brown dwarf 'stars' fuse simple hydrogen (p-p reaction) at all?

Does coal burn at room temperature? Most people would say "no". For an oxygen molecule to react with an atom of carbon in the coal, a certain amount of energy is required to break the ...
James K's user avatar
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4 votes

Bremsstrahlung's role in the sun

The interior of the Sun is almost at perfect thermodynamic equilibrium. The mean free path of a photon before scattering or absorption is on a scale of mm. This is true even in the convective region. ...
ProfRob's user avatar
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3 votes

If two fast asteroids collided, would the temperature and pressure be high enough to cause nuclear fusion?

Not even close. This would have to be a rough "Fermi" estimate, Lets suppose the asteroids are 50km in diameter and moving at a relative velocity of 50km/s. And so lets say the collision ...
James K's user avatar
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3 votes

Why does nuclear fusion generate heat?

In nuclear fusion, generally a nucleus plus impactor splits into two tunnelling-separated particles. Those two particles, having overcome the tunnelling barrier are now repelling each other via the ...
AtmosphericPrisonEscape's user avatar
3 votes

Pure Lithium Star

Depends on the mass of the star. For a low-mass object, it will form a cooling lithium brown dwarf supported by electron degeneracy pressure before it gets hot enough to ignite lithium. A higher mass ...
ProfRob's user avatar
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2 votes

Do brown dwarf 'stars' fuse simple hydrogen (p-p reaction) at all?

Wikipedia discusses this matter. Essentially, the mass limit of brown dwarfs is defined in a way that excludes proton-proton fusion, which requires higher temperature and pressure (thus stronger ...
Oscar Lanzi's user avatar
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2 votes

Does most of the Sun's energy and light come from its black-body temperature, due to its massive size alone? Constant crashing of particles?

If you calculate the total thermal energy of the Sun now - i.e. the sum of the thermal (kinetic energy) of its constituent particles - then it could be used to supply it's current luminosity for 40 ...
ProfRob's user avatar
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2 votes

Does most of the Sun's energy and light come from its black-body temperature, due to its massive size alone? Constant crashing of particles?

A collapsing cloud of gas will generate heat from the release of gravitational potential energy, and it's core temperature will continue to rise. Indeed this is the mechanism that Kelvin and Helmholz ...
James K's user avatar
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2 votes
Accepted

Is there a way to compare input and output energy of solar fusion to the input and output energy of man made fusion?

Roughly $25,000$% for main sequence nuclear fusion. The internal energy of the Sun is roughly $$U \sim \frac{3M_\odot k_B T}{m_p}\ , $$ where $T \sim 10^7$ K is an average interior temperature and $...
ProfRob's user avatar
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2 votes

How does carbon end up in the remnants

First Carbon + Carbon makes Magnesium, Neon, or Sodium. It's Carbon + helium that makes Oxygen. Secondly, it's not true that all carbon is converted into oxygen. At the end of core helium burning (...
Rob's user avatar
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2 votes

In layman's terms, why does the cold C-N-O process end in Carbon?

The CNO process does not end in carbon. It's part of the hydron burning processes. Let's look at the different processes inside main sequence stars: Hydrogen burning describes the fusion process from ...
planetmaker's user avatar
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2 votes
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What are the temperature and pressure requirements to fuse hydrogen and deuterium in Jupiter?

The answer you link to contains this self-contradictory statement. The minimum temperature required to fuse hydrogen is about 100 million Kelvin, which is about six times the temperature in the core ...
ProfRob's user avatar
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