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3

$^{56}Ni$ is produced in silicon-fusion stars. The fusion process doesn't "stop" at $Fe$. Several A=56 nuclides show up. See the Wiki-pedia article on :Silicon burning. Also, Introductory Nuclear Physics by Krane, Chapter 19, Section 4.


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From Wikipedia: Because the nuclear force is stronger than the Coulomb force for atomic nuclei smaller than iron and nickel, building up these nuclei from lighter nuclei by fusion releases the extra energy from the net attraction of these particles. For larger nuclei, however, no energy is released, since the nuclear force is short-range and ...


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In physics, the universe may be defined as the smallest system containing the earth and not interacting with something outside it. This is in agreement with the common usage of the term in astronomy and cosmology. It has a precise meaning in any concrete mathematically formulated theory of physics. In each such theory, there is only a single such system. ...


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Black body radiation is given by Planck's formula (see link for variables) Here is the measured irradiance of the sun and the attempt to fit it with the black body formula: The effective temperature, or black body temperature, of the Sun (5,777 K) is the temperature a black body of the same size must have to yield the same total emissive power. ...


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Maybe the simplest way to think about this is that the Sun is in approximate thermal equilibrium and would absorb any photon, of any frequency, that is incident upon it. This is essentially the definition of a BB. There are many radiative processes that can absorb (and hence emit) radiation at all frequencies, not just those corresponding to atomic ...


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Draw a picture of the geometry, including the cloud, the earth, and the galactic center. The earth and the cloud are rotating around the center at the same speed (that is your flat curve). You have two sides and one angle of the triangle. The radial velocity gives you the projection of the difference of the velocity vectors onto the line of sight.


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If the question is asking whether there is a definition that encapsulates our universe, then I believe the answer is No. This is because encapsulating a "space" into a formal system requires defining bounds. However, we don't know the bounds of our own universe--let alone what bounds might be possible for any universe. We can only describe what we can ...


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Sometimes the word universe is just used colloquially and can just refer to everything on some side of a horizon (an event horizon, a causality horizon, etc.) But when used precisely, I'm sure different definitions are used in different fields. For instance, in mathematical general relativity, you assume that your universe is a connected four dimensional ...


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You would be very interested in one of the recent Kepler discoveries - Kepler 444. The star is estimated to be 11.2 billion years old (using asteroseismology) and is surrounded by a number of rocky exoplanets. These planets are all too close to their parent K-dwarf star to be in the habitable zone, but there is no reason there couldn't be planets further ...


4

I think you already know the answer... Pop III stars, by definition, are born from primordial gas that is basically Hydrogen, Helium with trace amounts of deuterium, tritium, lithium and beryllium; they initially contain almost no C, N, or O. Therefore the primary fusion in massive Pop III stars has to be (well, initially the deuterium is burned but this is ...


1

After (re)combination (I never understand why the "re" is used) and the formation of the CMB, the universe was transparent and the only light in it was from the rapidly cooling CMB. The baryonic universe was composed almost entirely of neutral hydrogen and helium. After perhaps 100 million years, the first galaxies and stars (assisted by dark matter) were ...


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the answer is actually very simple. Electrons, protons and neutrons and other subatomic particles don't exist in the degree of proximity that a collapsing start forced them into. The massive gravitational force overcomes the equilibrium forces that exist in matter in its "normal" state ("normal" here refers to the state of the gas/dust cloud before ...


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I'm not going to attempt to usurp Chris White's perfectly good answer - but just fill in some detail and answer the edit. For a star like the Sun, the collapse proceeds in 4 basic stages, each takes about 10 times as long as the previous one. Pseudo-spherical collapse of the cloud - not far from a free fall timescale, often quoted as a few $10^4$ years. ...


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Short answer: gravitational potential energy is converted into heat. Let's look at the Sun as an example. Its mass is $M_\odot = 2.0\times10^{30}\ \mathrm{kg}$ and its radius is $R_\odot = 7.0\times10^8\ \mathrm{m}$. If its density were uniform, its gravitational binding energy would be $$ U_{\odot,\,\text{uniform}} = -\frac{3GM_\odot^2}{5R_\odot} = ...


1

A star is neither "flaming" nor "fire" in the sense that we use those words about things on Earth. It's just a big, hot ball of ionized gas. The only thing that happens "to" it is that it gets hotter and denser. At some point the temperature rises high enough to ionize the gas. Later still fusion becomes possible at non-vanishing rates. The energy for the ...


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Well there are problems in your question and analysis. First off, there have been a few SE questions recently about this "Keplerian" treatment of dark matter. The shell theorem, that the gravitational field is the equivalent of that due to the mass inside radius $r$, and that exterior masses can be ignored is only true for spherically symmetric mass ...


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The accretion of matter onto a compact object cannot take place at an unlimited rate. There is a negative feedback caused by radiation pressure. If a source has a luminosity $L$, then there is a maximum luminosity - the Eddington luminosity - which is where the radiation pressure balances the inward gravitational forces. The size of the Eddington ...


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A 12 billion Solar mass black hole sounds massive, but actually it's not all that big. The radius of the event horizon is given by: $$ r_s = \frac{GM}{c^2} $$ and for a 12 billion Solar mass black hole this works out to be about $1.8 \times 10^{13}$m. This seems big, but it's only about 0.002 light years. For comparison, the radius of the Milky way is ...


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I don't see an "equivalence between the partial and total derivative" of H in anything you've written; it's always written $dH/dt$ as a total derivative. The reason that the partials emerge within the volume integral is because A and B are also functions of space, and so the time derivative must be taken while holding position constant; it's basically that: ...


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Theoretical physics, in general, does not have to be confirmed by observations. Theories are proposed as an effort to explain observations, so some consistency with observations is expected. However, it's not necessary to wait for observations. A theoretical astrophysicist can propose work that is consistent with current observations and build off of it, ...


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It's not really related to your question but I've read that heavier white dwarfs are smaller than lighter white dwarfs and heavier neutron stars are thought to be smaller than lighter ones. When you get that much mass together, gravity tends to win. Even on the scale of the earth or Mercury, the planet's cores are crushed into greater density. I'm not ...


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The reason is electron degeneracy pressure. The cores of giant planets are dense enough that the electrons in the gas occupy about $h^3$ of phase space each. The Pauli exclusion principle means that they cannot all occupy low energy/momentum states. This means that even at relatively cool temperatures the gas can still exert considerable pressure due to the ...


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Throw a handful of marbles up. Assuming they all were given the same upward velocity, they will be as far apart as possible when they leave your hand. As they climb, they will come together (in the direction toward the center of mass of the earth) as they decelerate. At the top of the parabola they will be as close together as possible. And as they fall back ...


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There seems to be no direct evidence that the comet has any ice at all. The statement that there is ice below 8 to 10 inches of dust was based on an assumed hardness of the substrate based on how high the lander bounced after initial impact. It could be ice or it could be another hard material like rock under that dust. The other significant "evidence" given ...


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In stellar pulsations, multi-periodicity literally just means that there is more than one period present. In the cases of the classical pulsators (usually Cepheids and RR Lyraes), this is noteworthy purely because some stars oscillate at multiple frequencies and some don't. Typically we see the fundamental radial mode, and sometimes harmonics thereof. How ...



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