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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Oldest population 1 star system?

While reading Stanislaw Lem's essays on advanced civilizations, I had a question: When did the earliest generation of population 1 star systems form? How much older could they reasonably be than our ...
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1answer
117 views
+100

Is there an exact formal definition of the Universe?

I've read several articles about observable Universe, Universe and Hubble volume, including Wikipedia article and references on it, and I wondered: is there exist a formal, rigorous definition in ...
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4answers
216 views

What happens during gravitational collapse to cause the formation of a star?

I know that stars are formed from dense regions in large gas clouds. I know that when gravity causes the mass of the clump to get so big that its internal pressure can't sustain it, it collapses and ...
3
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1answer
33 views

What was the significance of the ionization caused by the Population III stars?

I am reading that the ultraviolet light that radiated from the first stars would ionize the surrounding gas and apparently, all of the matter in the universe would eventually become ionized. So, ...
6
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1answer
95 views

What types of fusion reactions happened in population III stars?

I have read that, in smaller stars, such as our Sun, the fusion reaction that takes place is a proton-proton chain, or PP chain for short. From what I have learned, in larger stars, a different ...
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1answer
35 views

Experimental Data for Mass Distribution of a Galaxy

My goal here is not to discuss dark matter in general. I know there are many other observational clues that hint us towards Dark matter. My goal is simply to understand this argument here a little ...
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2answers
2k views

Is there a limit as to how fast a black hole can grow?

Astronomers find ancient black hole 12 billion times the size of the Sun. According to the article above, we observe this supermassive black hole as it was 900 million years after the formation of ...
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5answers
2k views

Hubble's law and conservation of energy

If all distances are constantly increasing, as Hubble's law say, then lots of potential energies of form ~$\frac{1}{r}$ changes, so how is the total energy of the Universe conserved with Hubble's ...
4
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1answer
45 views

No tremendous neutrino-flux for SNIa?

Why do neutrino account for 99% of the energy release for a SN II, while it is not expected to be the case for SN Ia? Is it because the densities are not high enough to induce inverse beta-decay? ...
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4answers
2k views

How do neutron stars burn? Is it decay or fusion or something else?

What makes a neutron star burn, and what kind of fusion/decay is happening there? What is supposed to happen with a neutron star in the long run? What if it cools, then what do the degenerated matter ...
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1answer
19 views

Woltjer's Theorem, partial or total derivative?

Morning everyone.. I am currently studying Plasma Physics on a recent book named "Basics of Plasma Astrophysics" by Claudio Chiuderi and Marco Velli. While demonstrating Woltjer's Theorem they state ...
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0answers
33 views

For gravitational wave from twin stars, how was the tidal effect counted?

As the primary indirect evidence, the work on calculating the rotational slow down earned the 1993 Nobel prize. However, I cannot find any where mention how the work deal with the tidal effect. Are ...
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0answers
129 views

The Solar System explosion in the Nice model

This video depicts one variant of the Nice model (pronounced "neese", like the city in France). I'll briefly describe it in case the link ever dies. Here is the initial configuration: The four ...
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1answer
52 views

Does all the theoretical work of astrophysicists have to be confirmed by the observations of astronomers?

I am a chemist an I have some doubts about the work of astrophysicists. I know that astrophysicists do a lot of theoretical calculations based in other theoretical work and also based in real ...
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0answers
20 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. ...
10
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2answers
1k views

How can a gas giant be about the same size but six times more massive than Jupiter?

I've just read this article: http://www.sci-news.com/astronomy/science-kepler-432b-new-super-jupiter-exoplanet-02490.html And I wondered how this could be possible? Maybe it's because this gas giant ...
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0answers
14 views

Solar System Snow Line: 5AU or 2.7AU?

I am trying to update the Wikipedia article "Frost Line (astrophysics)". During my last update (by QuantumShadow), I noticed that different sources cite different values for Solar System water ice ...
9
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2answers
235 views

What is the Sun's core made of?

The obvious answer is hydrogen and helium plasma but the nuclear fusion can also create heavier elements. Are these heavier elements a significant portion of the core? Do the heavier elements "sink" ...
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0answers
23 views

Density of an astrophysical ring of fluid around a star

*An equilibrium ring of isothermal fluid orbits a star at radius R. In the plane of the ring, mechanical equilibrium results from a balance of centrifugal force and the gravitational force of the ...
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0answers
20 views

Cosmological and astrophysical bounds on physics beyond SM [closed]

There are many cases when cosmological and astrophysical observations make restrictions on particle physics models. For example, some axion models are rejected by white dwarf luminocity functions, ...
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2answers
76 views

Why has the amount of star formation in the Universe decreased over time?

If you like, refer to my old question from the last year, about star formation rates and their declining, answered by Rob Jeffries. I'm now examining why this process happens. It appears that in ...
14
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4answers
390 views

What mechanism is responsible for the creation of these dunes on Comet 67P/Churyumov-Gerasimenko?

What mechanism is responsible for the creation of these dunes on Comet 67P/Churyumov-Gerasimenko? The following high resolution picture from ESA's Rosetta mission shows the dunes: At a distance ...
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0answers
21 views

How much cheaper and smaller on average is SiPM technology over conventional PMTs?

I've heard it said that some of the major advantages of silicon photomultipliers are its low cost and compactness when compared to widely used photomultiplier tubes, but haven't found much information ...
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0answers
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Modeling atmospheric reentry with respect to the rotation of a planet and its atmosphere in a simulation

I am creating a hard science fiction flight simulator. I am a civil engineering student, so its a little out of my area of study. Currently I model air resistance on the velocity difference between ...
4
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1answer
52 views

Galaxy rotation curve and dark matter

I am reading "The Essential Cosmic Perspective" by Jeffrey O. Bennett, Megan O. Donahue, Nicholas Schneider, Mark Voit. In Chapter 14, it is stated that an evidence of the presence of dark matter in ...
0
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1answer
35 views

What does multi-periodicity mean in stellar pulsations?

How can there exist multi-periodicity in stellar pulsations? http://www.kitp.ucsb.edu/sites/default/files/kitp/preprints/moskalik2.pdf How can one visualize a multi-periodic pulsation or oscillation?
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1answer
428 views

Kinetic energy of an expanding sphere [duplicate]

In the study of Newtonian stellar structure, Weinberg (1972) writes The uniform dilation of a sphere with uniform density will give it a kinetic energy $$U=\frac{3}{10}M\dot R^2$$ I don't ...
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0answers
24 views

How to estimate the chance of cosmic radiation reaching Earth?

Say some cosmic radiation like X-ray generated by some distant star is towards Earth, what is the chance that it reaches Earth successfully without being blocked or deviated or absorbed?
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1answer
40 views

Are the gravitational redshift and blueshift factors inverses of each other? [closed]

at a point in gravitational field assuming swcharzschild metric and the exact analysis. The other point in context is infinity. It would be helpful if you can provide citation/source of the ...
0
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1answer
65 views

What is the total matter equivalent of the Sun's output per year?

Say we can collect all the energy from the sun's output and all the particles from the solar wind. If we had an energy to mass converter and turned everything into say, carbon, how many kilograms of ...
1
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1answer
37 views

What makes red giants big? [duplicate]

I read some about red giants and so far I understand red giants become exhausted of burning hydrogen in the core, so then start hydrogen burning at shell and may or may not be burning helium in the ...
1
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1answer
54 views

Confusion with the meaning of CMB

We say that CMB is the radiation leftover from big bang. When we measure the radiations i.e., the flux of photons in a given microwave range (say 0.1cm to 70cm, for example), in deep sky, there are ...
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1answer
77 views

What is an 'S-factor' in nuclear physics?

I have seen the "S-factor" in many places, but I've never read an explanation of what it actually is. I have read that it is related to the cross-section of a reaction, but that's about it.
11
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5answers
396 views

Interstellar dust/matter distribution

It is known that one of the main problems of interstellar flight is a presence of matter between stars in form of very fine dust and huge asteroids. Which can slowly (or fast) destroy any ship. What ...
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1answer
30 views

Are the large moons of Jupiter, Saturn, and Neptune still cooling and does this give and indication of their age?

The core temperatures and the rate at which they emanate heat should correspond to their estimated age. Other forces may create some heat such as the tidal locking that is supposed to be the source of ...
0
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0answers
23 views

What is a threshold/subthreshold state and resonance?

I understand what a 'state' is for a quantum mechanical system, but upon reading a paper on reducing the error for a particular nuclear reaction rate I saw the following sentence. "The extrapolation ...
3
votes
2answers
50 views

Typical energy of a solar flare

I read that solar flares are customarily viewed in H-alpha light, as a temporary brightening of a small portion of chromosphere. What all can be interpreted from this? Is it because, energy of the ...
24
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2answers
3k views

Why would a black hole explode?

It is common in popular science culture to assume that Hawking radiation causes black holes to vaporize. And, in the end, the black hole would explode. I also remember it being mentioned in A Brief ...
2
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1answer
38 views

r-process: Is it correct to talk about “primary” and “secondary” r-processes?

This website, written in 1994, makes a differentiation between the "primary" and "secondary" r-processes in the context of astrophysics. As far as I can tell the main difference between the primary ...
8
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4answers
695 views

Do neutron stars reflect light?

The setup is very simple: you have a regular ($1.35$ to $2$ solar masses) evolved neutron star, and you shine plane electromagnetic waves on it with given $\lambda$. Very roughly, what shall be the ...
7
votes
3answers
252 views

What is the origin of spin of celestial objects?

In an older question from June 2011, Why does each celestial object spin on its own axis?, apparently revived by the system, a user is asking about the origin of the rotation of celestial bodies. The ...
11
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3answers
1k views

Can black holes actually merge?

If time stops at the event horizon, can we ever detect two black holes merging? In other words, if you are a short distance away, would you encounter a spherically symmetric gravitational field, or a ...
3
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1answer
53 views

Difference between collisional and collisionless Boltzmann equations?

Reading an excellent answer, I've read about there are different Boltzmann statistics for a collision-less system (f.e. stars in a galaxy) and in a system with collisions (f.e. gas in a closed box). ...
5
votes
1answer
111 views

Has the number of new stars born decreased over time?

Has the number of new stars being formed decreased at all over the age of the universe? Would this be because the average density of the universe is decreasing due to the expansion of the universe, it ...
6
votes
1answer
96 views

Do the stars in a galaxy have a thermal kinetic energy distribution?

I think, there is practically everything given to that: many point-like masses, able to exchange energy pseudo-randomly, and far long enough time to reach a thermodynamical equilibrium. Of course, ...
4
votes
1answer
1k views

How does a star ignite?

I remember reading that X-Rays are generated by 'braking' electrons in a Coolidge tube. Is it fundamentally a matter that the extreme gravity immediately before a star ignites is so strong that it ...
1
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1answer
100 views

Galactic Rotation Speeds - Ehrenfest Paradox, Gravitational time dilation, Dark Matter - all of the above?

The observed paths and speeds of objects, part of some distant galaxy, do not match up with speed vs distance curves it seems - the observed speeds are not falling off in fact they're trending as ...
2
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3answers
431 views

Redshift of light in dark matter

Following Edwin Hubble, it is widely believed that the universe is expanding, which is based on the red-shift of light from distant objects. Can dark matter cause light to be red-shifted and make it ...
4
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1answer
99 views

What would happen to the universe if dark energy started “disappearing”?

Terribly naive question, I know. Obviously, not simply "disappearing", but if it could, theoretically, be absorbed or "used" somehow, what would happen to the universe? Would it stop expanding, would ...
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3answers
3k views

Why are stars, planets and larger moons (approximately) spherical in shape (like, the Sun, the Moon, the Earth, and other planets)?

Why are stars, planets and larger moons (approximately) spherical in shape (like, the Sun, the Moon, the Earth, and other planets)?