82

The fusion that occurs in the core of the Sun occurs in nothing like the conditions you might be thinking of in a bomb, or a fusion reactor. In particular, it occurs at much lower temperatures and at a much lower rate. A cubic metre of material in the solar core is only releasing around 250 W of power by fusion. The fusion rate is set by the temperature (...


16

If fusion were to proceed faster, the core would get hotter, it would expand and become less dense, and with less density, fusion would slow down. The main sequence in stars like the Sun does proceed much more slowly than other stages. This is because the p-p chain reaction starts with the fusion of two protons to form a diproton, or helium-2. The diproton ...


9

Fusion in stars requires enormous pressures and temperatures. Any body, including stars, are subject to their own gravitational field. At any point inside a spherically symmetrical body (which most stars approximate well) the gravitational force will be due to all the mass "below" that point - between that point and the center. That gravitational force ...


6

Stars live most of their lives (see main sequence) in a dynamic equilibrium. If the core gets extra hot because of increased heat production, the star expands and the rate of the fusion decreases. In most cases, the equilibrium is also pretty stable and the star does not oscilate its heat production. Well, some stars DO oscilate their luminosity, but that ...


4

The neutrinos are produced by nuclear reactions in the core of the Sun. About 2% of the mass lost in the conversion of hydrogen to helium ends up in neutrinos (almost entirely in the form of kinetic energy, since neutrinos have a negligible mass here). The neutrinos interact very weakly with matter and therefore they basically all escape from the Sun. The ...


3

Explanation: In the article the author is trying to calculate the speed of sound in the cosmological fluid that consists of radiation and baryons. Now the baryons are almost pressureless when compared to radiation while the radiation has a pressure $P=\frac{1}{3}\rho_\gamma c^2$. Thus for the speed of sound in the radiation+baryon fluid one has $$v_s^2=\frac{...


2

An explosion always requires a self-accelerating process. If you set a pile of conventional fuel on fire, it will not explode: it will quickly consume all the oxygen in the surrounding air and the process will slow down waiting for more oxygen to be available. If you want to make an explosive, you will need an oxidizer: a substance which releases oxygen ...


2

No. If the dark matter density is sufficiently high, a galaxy may remain gravitationally bound and thus there is no theoretical upper size limit for a galaxy. However, if we talk amount luminous mass, constraints on an upper mass limit exist, and they are unsurprisingly dependent on a mass-luminosity relation. As an example, radiation pressure which pushes ...


1

$E$ is monotonic like $t$. It isn’t limited to be between $0$ and $2\pi$.


1

For the answer to the question a), just use a Taylor expansion in the parameter $x= R/d \ll 1$, so that \begin{equation} (1-x^2)^{3/2} \simeq 1- \frac{3}{2} x^2 \end{equation} and then you obtain a inverse square law in $d$ \begin{equation} P_{rad}= \frac{2\sigma T^4}{c} \frac{R^2}{d^2} \end{equation}


1

1) If we can visualize angular momentum in 3D as a vector, how should we think about its counterpart in 4D, a tensor? And what about the law of conservation of angular momentum. In general, angular momentum is a geometric object called a rank $2$ differential form. To oversimplify a bit, you can think of it as a plane, along with a signed magnitude. In ...


1

To enlarge slightly on Rob's answer, for a neutrino to interact with (for example) a proton requires it to strike the proton essentially "head on" which greatly reduces the chance that any neutrino will interact with a given chunk of matter, even if traveling vast distances through solid rock. But if you collect enough matter together that reacts with ...


1

I would like to know if we have obtained any observations regarding the energy spectrum of solar energetic particles. Yes, there are several decades (>30 years) worth of observations of solar energetic particles (SEP) [e.g., Desai et al., 2016a,b; Reames, 2017]. And if so, what is the index of the power-law. Thank you! There is not one, but two for each ...


1

There cannot be any strong effect of gamma rays, photons, passing through magnetic fields, even if strong fields. This is due to the effect that photon photon interactions are highly improbable due to the coupling constant (1/137) entering the QED calculation. To illustrate how the lowest order Feynman diagram would look for photon-magnetic field ...


1

In addition to the other answers given, I should add that the energy we observe from the GC is that of either some very exotic mass like a boson star, or of a supermassive black hole. We know mathematically a black hole is much more likely to form than these sorts of objects. Here is a diagram showing the progress in nailing this down over time with ...


Only top voted, non community-wiki answers of a minimum length are eligible