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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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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226 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
191 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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268 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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198 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
332 views

Is there a theoretical maximum “size” for galaxies?

I am aware of upper limits to the mass of burning stars and neutron stars, beyond which they degenerate into neutron stars and black holes respectively. And also, if I understand it right, there is a ...
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157 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 ...
7
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1answer
169 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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0answers
84 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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127 views

Rotational axes of galaxies

The disc shape is a preferred form for galaxies due to angular momentum, and the axis of rotation indicates a locally preferred orientation for the angular momentum vector. Why do galaxies in Hubble ...
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87 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
85 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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118 views

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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181 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 ...
5
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1answer
190 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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71 views

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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283 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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40 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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60 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}...
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495 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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85 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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1answer
189 views

What is the average temperature of mass in the universe?

That is, ignoring speculative concepts like Dark Matter and just summing across stars and gas clouds etc
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298 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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69 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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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 ...
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0answers
153 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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71 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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77 views

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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76 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
170 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 ...
3
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0answers
251 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 ...
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0answers
340 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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2answers
119 views

Is the fluctuation pattern of the Cosmic Microwave Background (CMB) identical in every location of the universe?

I know that the Cosmic Microwave Background (CMB) is the leftover radiation from the "surface of last scattering". Let's say an alien civilisation lives on andromeda or further away. Would they see ...
3
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1answer
225 views

How much of the Solar convection zone is completely ionized?

I was reading about the energy transportation in stars here, and I found this: "The outer portion of solar mass stars is cool enough that hydrogen is neutral and thus opaque to ultraviolet photons, so ...
3
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1answer
109 views

Is deep-space radiation greater in the Sun, or in the shadow of Earth?

https://space.stackexchange.com/questions/604/is-it-possible-to-get-pregnant-through-natural-means-in-space indicates cosmonauts are at risk of irradiation whilst in space. What little I know about ...
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83 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 ...
2
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1answer
61 views

Would the Doppeler effect be incorrect to use as evidence to support an expanding universe?

In our science class we have a question to answer, which is 'explain how redshift supports the idea of an expanding universe'. I am not sure how to answer this question, as I know that there are '...
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49 views

Are / Why are the H II regions positively charged?

One of the questions in our recent astrophysics course homework was to find the general opacity $\kappa$ of H II regions. We know that the H II regions are almost entirely ionized hydrogen and I ...
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Long-term fate of hydrogen atoms in a rogue planet

Imagine a rogue planet drifting in intergalactic space. For the sake of specificity, suppose that it is the size of Jupiter and composed of pure hydrogen. What will happen to it over the next $10^{...
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0answers
56 views

Can mass still be observed in our universe after moving from a black-hole to a white-hole?

This question is based on the recent papers on LQG black-holes by Abhay Ashtekar. Quantum extension of the Kruskal spacetime and Quantum Transfiguration by Kruskal Black Holes When mass moves from ...
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0answers
92 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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0answers
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Possible origins of the Antlia 2 galaxy?

At $32.3\,{\rm mag}\,{\rm arcsec}^{-2}$, the Antlia II dwarf galaxy is the most diffuse galaxy ever observed in terms of surface brightness. It was only discovered by pre-selecting stars based on ...
2
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1answer
84 views

What is an active region in sun's photosphere? Why sometimes there is an active region but no sunspots?

What is the theory behind an active region? If we check data in solar monitors, there are some cases where there is no sunspot but there is still an active region. Why is that the case?
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What portion of the total power emitted by the Sun comes from photons?

We usually talk about luminosity, which is the total power from emitted photons. The other sources are neutrinos and the solar wind (which includes protons and other particles). I assume that most ...
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52 views

On the conversion of neutron stars in deconfined quark stars

Imagine a neutron star, constituted of nucleons ($p$, $n$) and charged leptons (say $e$ and $\mu$), can decay into a deconfined quark star through some process. Given the Bodmer-Witten hypothesis, the ...
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0answers
54 views

Observational evidence for superfluidity in neutron stars

What is the earliest evidence for the existence of superfluidity in neutron stars? I'm about to present the subject in a seminar and I'm under the impression that at the moment there is no ...
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0answers
94 views

How can we evaluate the ratio of abundance of Carbon to Li, Be and B?

In stars' nuclear processes, if the triple $\alpha$ reaction explain why Carbon is much more abundant than Li, Be, B, how can we evaluate the ration of abundance to ~$10^5$ times ? Is it possible to ...
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0answers
55 views

On a Doppler-shift-induced radiative dissipation of angular momentum for the accretion disk of a black hole

A recent question explored the possibility that the accretion disk of a black hole could wind down and lose its angular momentum through radiative process, and Rob Jeffries' excellent answer there ...
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0answers
420 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
57 views

Frequencies of stellar g-mode oscillations in astroseismology

It's a simple derivation that in a gravitating fluid, if the density decreases with height then a perturbed fluid element should oscillate at the Brunt-Vaisala Frequency, $$N^2 = - \frac{g}{\rho_0} \...