Questions tagged [astrophysics]

The application of physical theory to celestial systems such as stars, planets, galaxies, supernovae, and black holes. Astrophysics proper is concerned with explaining phenomena more so than making observations, the latter falling under the purview of astronomy.

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5answers
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What would the collision between a (large) solid planet and a gas giant be like?

Assuming a Jupiter-like planet and an Earth-like planet (Except, say... half the mass of Jupiter), what would happen when the two collide? For clarification: What would the actual collision be like? ...
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272 views

gauss-bonnet gravity constraints from string theory

recently there has been advances in observational constraints of gravity theories that contains scalars coupled to the gauss-bonnet topological term: http://arxiv.org/abs/0704.0175 http://arxiv.org/...
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1answer
233 views

What's the cause of this gap in this simulation of the Nice model?

A previous question brought me to this video (which has a spectacular change at about 0:34). It shows the orbits of Jupiter, Saturn, Uranus, Neptune, and what appear to be trans-Neptunian objects. ...
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0answers
335 views

View of the sky from inside a black hole

Consider an observer located at radius $r_o$ from a Schwarzschild black hole of radius $r_s$. The observer may be inside the event horizon ($r_o < r_s$). Suppose the observer receives a light ray ...
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110 views

Entropy and equilibrium concepts at astronomic scales

I am always puzzled to read here and there discussions dealing with thermodynamic concepts applied to astronomic scales where gravity matters. To my opinion, there is a certain carelessness to go into ...
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321 views

What is the Chandrasekhar-Friedman-Schutz (CFS) instability, exactly?

I am confused as to what the Chandrasekhar-Friedman-Schutz (CFS) instability is, exactly. It seems to partially refer to this paper (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.24.611) ...
8
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1answer
195 views

How does adiabatic cooling make a nebula colder than the CMB?

According to measurements, the gas from the Boomerang Nebula is expanding so fast that it's colder than the cosmic microwave background radiation... how does such a simple mechanic work on such a ...
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171 views

Relativistic rotational squeezing?

I would like to consider a sphere rotating at very high angular speeds, such that the speed in its equator would be relativistic. This is very similar to Ehrenfest paradox situation, except that ...
6
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1answer
140 views

Is there linear 'frame dragging'?

Very massive objects cause the so called 'frame dragging' that can increase the speed of a beam of light to a total aggregate speed faster than the speed of light in normal circumstances so my ...
6
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1answer
248 views

How does Planet 9's perihelion and aphelion affect the Kuiper belt objects?

Assuming that the existence of Planet 9 is confirmed and it really exists, how does it affect the orbits of the Kuiper belt objects? My understanding is that the discovery came in part from the ...
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92 views

Status of large-scale structure formation within cosmology today

Since the CMB results of the past decade, would it be fair to say that the consensus among cosmologists is that cosmic strings are no longer considered as a (major) source for density perturbations? ...
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0answers
167 views

Relationship between 3 quantities: Density Matter power spectrum, Density Matter angular power spectrum and Temperature angular power spectrum

Summary: I would like to go deeper in the relationship between Matter power spectrum and Angular power spectrum. From a previous post about the Relationship between the angular and 3D power spectra, I ...
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0answers
114 views

Is the average temperature of a big accretion disk colder than a small one?

Suppose we have a Shakura and Sunyaev style accretion disk between $r_1$ and $r_2$. What is the average temperature? We can calculate it by integrating the local temperature $$T(r)=\left [ \frac{3GM\...
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Could the recently discovered “Dancing Black Holes” be the experimental signature of Hawking Radiation?

Science just reported two gravitationally bound massive black holes that are slowly receding from each other. Under normal circumstances two such bound black holes would be approaching each other as ...
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228 views

What are the applications of hyperbolic $3$-manifold theory to cosmology?

I am a pure mathematician specialized in hyperbolic $3$-manifold topology. That has been an incredibly active field of research in the past few decades due to the seminal work of Thurston, as many of ...
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Experimental tests of Cluster Decmposition

How tight are experimental and astrophysical tests on whether Cluster Decomposition is satisfied at various space-like separations? Is there a review paper or a standard reference on the question? I ...
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0answers
411 views

Magnetic flux ropes - are they stable for topological reasons like fluxon/Abrikosov vortex?

In images of sun's corona there are clear bright lines - nearly 1D stable structures interpreted as magnetic flux ropes, often suggested to have a topological nature e.g. (source): They have some ...
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0answers
46 views

Do stellar model luminosities include neutrino losses?

I have had a sudden crisis in my understanding of the published outputs from stellar evolutionary model calculations. Usually these models output a "luminosity" that one can then use, along with the ...
4
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68 views

What happens in the event that the cooling radius is shorter than the virial radius of a Cold Dark Matter Halo?

The cooling radius of a cold dark matter halo is defined to be the time at which the cooling time $t_{cool} = t_{free fall}$ where $$t_{cool}=\frac{\rho \varepsilon }{\Lambda \left ( T \right )n_{H}...
4
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1answer
242 views

Mass-luminosity relation for a fully convective star

I'm trying to show that the mass-luminosity for a fully convective star is $$L \propto M^{\frac{113}{66}}$$ I know that the energy generation is via pp-chain and the opacity is due to $H^{-}$ ions ...
4
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1answer
1k views

Relationship between oscillator strength and cross-section

In the context of absorption of photons by atoms, I have come across two seemingly very related quantities, cross section and oscillator strength. In the book Physics of the Interstellar and ...
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88 views

Why do some stars actually produce “Gamma ray bursts”?

I looked it up but I haven't found any explanation as to why some stars produce them, I understand that collapsing and merging stars produce them, but my question is why is the energy concentrated in ...
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306 views

What is the theoretical geometry of bubble universes?

My research has led me to look into the idea of bubble universes which I don't know very much about. The first thing that I am looking for is understanding or visualising how could many bubbles co-...
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70 views

squeezed radiation astronomy

Squeezed electromagnetic vacuum does have a renormalized energy density smaller than the vacuum. So it makes it in my opinion a inconspicuous candidate for a dark energy carrier. Are there ...
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0answers
67 views

A formula that seems to involve the variation coefficient

A friend walking in Minsk, Belarus, found this mural from the Soviet period and sent me an image (see below) of it, asking me what the formula in it was referring to. From my naive reading (which ...
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32 views

Are planets without large moons getting weaker magnetic fields?

Recently I read a study according to which in order for a terrestrial planet to have a magnetic field, a sufficiently large satellite is needed, since the satellite's gravitational influence melts the ...
3
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0answers
54 views

Formula to get scales of the two components (radial and perpendicular) of Baryonic Acoustic Oscillations (BAO)

In an astrophysics context about BAO (Baryonic Acoustic Oscillation), it is written on the following paper BAO paper : "BAO are much smaller in amplitude than the CMB acoustic peaks, and are ...
3
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1answer
67 views

Will pyrolytic carbon/graphite repel the Sun's solar wind?

Since a moving charged particle has a magnetic field, as well as an electric field, and pyrolytic carbon/graphite repels an external magnetic field, would this mean that an object made out of ...
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48 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 ...
3
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0answers
138 views

Total number density of galaxies and problematic expression in practise

I am asked to give the formal expression of the total number density of galaxies and explain why is this expression problematic in practice. From what I saw from my research and into my lectures, I ...
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186 views

What is the density profile of Sagittarius A*'s accretion disk?

It is thought that Sagittarius A*, the supermassive black hole at the center of the Milky Way, is surrounded by an accretion disk. Can anyone provide a resource for the general density profile of ...
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0answers
72 views

Which theory do we use when dealing with the center of a star?

The center of a black hole is known as being one of the locations in spacetime that needs an unification theory between quantum physics and general relativity, because of having a large mass in a ...
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What is the acceleration of expansion?

If me and my pet duck, who weighs next to nothing, were placed far away from any gravity source and in such a way that our mutual attraction was negligible, would I see my duck accelerate away from me?...
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77 views

Neutron star merger: large scale dynamics

Yesterday was announced the first detection of a neutron star merger via gravitational and electromagnetic waves. On the announcement one of the speakers is showing a simulation of the dynamics of ...
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0answers
601 views

Gamma-Law equation of state

The gamma-law equation of state is very useful in studies of astrophysics. It is basically an equation that connects specific internal energy (internal energy per unit mass), pressure and mass density ...
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0answers
332 views

How to plot spectral indices $n_s$ and $r$ from Planck data?

From a certain inflationary model I have obtained the theoretical values for the spectral indices $n_s$ and $r$ as functions of $N$, the e-folds number of inflation. At this point it would be ...
3
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0answers
362 views

Pressure inside a typical white dwarf

Does any one know the order of magnitude of pressure inside a typical white dwarf (better with reference)? Thanks! I think it should be $m_e^4c^5/h^3$ (may be multiplied by $\pi$), which is $10^{22} \...
3
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1answer
442 views

Misbehaving singular isothermal sphere potential

The singular isothermal sphere (SIS) is a useful simple model often used in astrophysics. It has density profile: $$\rho(r) = \frac{\rho_0 r_0^2}{r^2}$$ This is well known to have some quirks (...
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0answers
150 views

Dependence of the neutron-freeze-out temperature

What is the physical explanation for the dependence for the freeze-out temperature $T_\text{F0}$ of the neutrons as a function of the number of degrees of freedom $g^\star$? $$T_\text{F0}=\left(\frac{...
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0answers
87 views

Modeling the formation of a stellar system and matter accretion

I am trying to figure out what do I need to know to properly simulate the creation of a solar system from a particle cloud with random distribution of hydrogen atoms. Being more of a programming ...
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0answers
56 views

What are some values for the scale factor $a(t)$ in cosmology?

In cosmology, the scale factor is given in $D(t)=a(t)D_0$, where $D(t)$ is the proper distance at epoch $t$ and $D_0$ is the distance at reference time $t_0$. What are some of the values that $a(t)$ ...
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0answers
80 views

Computation of standard deviation on $C_\ell$ with fraction of sky $f_{\text{sky}}$ and $\Delta\ell$

I try to demonstrate the expression of error on a $C_{\ell}$. Below the expression of standard deviation on a $C_{\ell}$ that I would like to get: $$\sigma_{C_{\ell}}=\sqrt{\dfrac{2}{(2 \ell+1) f_{\...
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0answers
32 views

Neutron star - Relativistic mass continuity equation

Neutron stars are relativistic objects. Let's suppose that it's static and spherically symmetric, therefore the metric is: $$ \mathrm{d} s^{2}=c^{2} \mathrm{~d} t^{2} \mathrm{e}^{2 \Phi}-\mathrm{e}^{2 ...
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1answer
23 views

Difference in the precession regarding the gravitational gradient

Let say a less massive star acts on a planet causing it to precess.Does a more massive star cause less precession of this planet if in both cases the gravitational force applied to the planet center ...
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0answers
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Evaporation of a gravitationally-bound spherical cloud of a warm ideal gas in a vacuum

It seems that for any gravitationally bound system with a very large number of atoms/particles, there will always be some particles that manage to achieve escape speed and ultimately leave the cloud. ...
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0answers
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What is the current status or resolution of Greisen–Zatsepin–Kuzmin (GZK) cosmic-ray paradox?

The Greisen–Zatsepin–Kuzmin limit (GZK limit) is a theoretical upper limit on the energy of cosmic ray protons traveling from other galaxies through the intergalactic medium to our galaxy. A number of ...
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0answers
31 views

Is Jupiter really protecting us from collisions with objects?

Many documentaries I see on the Solar System or the planets or the threat of collisions with meteorites talk about how lucky we are to have Jupiter, with its immense gravity attracting asteroids to it ...
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0answers
24 views

Silk damping in CMB power spectrum

Can someone explain Silk-damping in a conceptual way? I understand that it is the effect causing the power spectrum to decrease in amplitude at smaller angular sizes because the effect is apparently ...
2
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0answers
38 views

How efficient can an astrophysical jet be when it's driven by a black hole of maximal spin?

Some astrophysical jets appear to be driven by black holes. As far as I know, a detailed understanding of how black holes produce jets is still under development, but one general possibility is that ...
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0answers
37 views

Is an exact description of accretion flow possible very close to a Schwarzschild black hole?

While modelling accretion disks around Schwarzschild black holes, various pseudo-Newtonian potentials are prescribed and then compared to general relativistic results. Though the pseudo-Newtonian ...

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