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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120 views

Is it known what causes the “knee” in the observed Cosmic Ray spectrum?

I've seen many versions of the figure shown below -- the famous Swordy plot. They tend to explicitly point out two features in the CR spectrum, the knee and the ankle. I know that the source of ...
-1
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0answers
40 views

Hydrogen is the ONLY Element?

Let's say you have Hydrogen and it's isotopes, Can you fuse them together to create all the 118 elements, by fusion or something else, Can anyone give me any research paper or anything which gives ...
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0answers
26 views
+50

Origin of Jet production: Active Galactic Nuclei

According to Netzer's book on Active Galactic Nuclei (AGN), pair production ($\gamma \gamma \leftrightarrow e^+ e^-$) which causes AGN jets presumably happens in "the corona of the central accretion ...
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1answer
27 views

Good values for gravitational potential [on hold]

In the context of a project, I had to solve numerically Poisson equation with cylindrical coordinates. I put here results for z = 0 on a 3D mesh 256x256x256. Below 1 figure representing the final ...
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0answers
13 views

What is the definition of the “stellar angular diameter” in stellar astronomy?

(Following the definitions here: http://arxiv.org/pdf/astro-ph/0509535.pdf ) What is the "stellar angular diameter", as measured by astronomers specializing in stellar astrophysics? Using the ...
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1answer
45 views

Does spaghettification really happen with conditions found close to a black hole?

This website tries to explain why the term spaghettification doesn't actually occur when something gets close to a black hole. The argument of the author is that the equations we use to predict the ...
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0answers
15 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
8 views

Do the arms of spiral galaxies differ based on the speed of which the galaxy is rotating?

If I were to take a circle with strings attached to it and spin it, the strings would behave differently based on how fast the circle is rotating. Do galaxies behave the same way?
14
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0answers
502 views

Do intergalactic magnetic fields imply an Open Universe?

According to a paper on the arXiv (now published in Phys Rev D), they do. How credible is this result? The abstract says: The detection of magnetic fields at high redshifts, and in empty ...
1
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1answer
25 views

The Thomson scattering optical depth for a photon ar radius r

I am looking to understand some more about the physics of gamma ray bursts. In particular I am looking at the origin of the "prompt emission". Some of the energy associated with this prompt emission ...
3
votes
0answers
68 views

Would a rocky planet orbiting so close to a very hot star eventually vaporize?

I was thinking about the physics behind a hypothetical scenario where a planet the size and the mass of the Earth is orbiting so close to a very hot star and what the long-term fate of such a planet ...
4
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4answers
644 views

What happens if a black hole sucks up a massive amount of oxygen and hydrogen?

Suppose a black hole were to suck up a massive amount of surrounding oxygen and hydrogen. Would this ignite inside the black hole and turn it back into a star?
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2answers
57 views

What causes the death of the Sun?

In a previous question I learned that in each second only a miniscule portion of the total hydrogen in the Sun is converted to helium and that the number is 1/10^18 of its mass converted each second. ...
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2answers
52 views

Density of the Sun

Being either on the surface or somewhere inside; where is the density of the gases of the Sun equal to the density of the ground we stand on here on earth?
1
vote
1answer
92 views

Crab pulsar radiation

Regarding the radiation from the Crab pulsar and nebula, some reading leads me to the following conclusions. The rotation energy of the neutron star is the source of the non-thermal electromagnetic ...
2
votes
3answers
128 views

Are there more photons than nuclei within the Sun?

Are there more photons than nuclei within the Sun? Is there a good way to estimate what the ratio would be?
0
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1answer
30 views

Will climate change cause the Moon to move away from the Earth at a faster rate?

The Moon is moving away from the Earth at a rate of 38.08 ± 0.04 mm/year. This is caused by the tidal bulges being pushed ahead of the Moon as the Earth rotates and the landmasses interact with the ...
0
votes
1answer
65 views

Where is the Hydrogen the Sun consumes?

I understand the Sun consumes 600,000,000 tons of Hydrogen each second. Where is this Hydrogen? Is all the Hydrogen the Sun will ever consume inside it or are there vast amounts of Hydrogen in space ...
1
vote
1answer
34 views

Origin of Stellar Nurseries

What is the mechanism thought to cause huge stellar nurseries to form e.g. eagle nebula?
12
votes
4answers
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 ...
1
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3answers
84 views

Can there be eternal stars?

the question is quite straightforward: Can there be stars that shine forever without ever collapsing nor growing? Do we know some really, really old stars? (whatever age that might be) I hope to ...
2
votes
2answers
43 views

Are there hypothetical processes which allow the existence of a fission powered star?

I know this is impractical given the rarity of heavy fissile elements in the universe (contrary to the abundance of fusion friendly elements like hydrogen), but is there any process via which a ...
3
votes
1answer
126 views

Why is metallic hydrogen degenerate matter?

Why is metallic hydrogen considered a form of degenerate matter, akin to neutronium and electron-degenerate matter? I can understand that for the other two, degeneracy pressure is the only force ...
1
vote
1answer
25 views

Looking for current review article on the Oort Cloud

I'm sure someone will tell me just to Google, but I'm struggling to find a truly detailed and informative article explicating our current understanding of the Oort Cloud. Could someone please ...
2
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0answers
40 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 ...
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vote
0answers
37 views

Signal-to-Noise ratio given pixel intensity map [closed]

I have a question regarding calculing the SNR of a signal given the pixel intensities. Say I'm given the pixel intensity values below: $\begin{array}{ |c|c|c| } \hline 3&3&3&3&3 \\ ...
0
votes
2answers
33 views

In stellar astrophysics, what is the difference between protostellar disk and circumstellar disk?

I have noticed both the terms "protostellar disk" and "circumstellar disk" in the stellar astrophysics and exoplanet literature. What exactly is the difference?
1
vote
1answer
24 views

Can the lithium test be applied to differentiate brown dwarfs from other substellar objects?

The lithium test is often used to differentiate brown dwarfs from low-mass M-type dwarfs (see e.g. Martin et al. (1994)), because brown dwarfs (at least the lower-mass ones) do not burn lithium, ...
0
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0answers
26 views

How do inert helium cores in sub giant stars create a pressure force?

I'm reading about the sub giant branch (SGB) and the evolution to the red giant branch (RGB). On the SGB stars have burned all hydrogen into helium, as a result, they have an inert helium core. The ...
3
votes
3answers
100 views

Is it possible to calculate the radius of star?

Suppose I know the Luminosity $L$, temperature $T$ and Mass $M$ of star. Assuming the star is very heavy so that we can treat it to be radiation-dominated star. This would imply that pressure inside ...
2
votes
4answers
65 views

Navigating to a distant star

Suppose I have a space ship that can travel at $0.9c$, and I'm going to a star located at 20 light years or so from the Sun. From a practical point of view, if I keep pointing the nose of my space ...
3
votes
1answer
86 views

Is it possible that Mercury collided with Earth, thus creating the Moon?

I recently heard a theory that Mercury might have been bigger before a massive collision that shrunk it and sent it on its current orbit. I also heard that a possible explanation for the formation of ...
9
votes
1answer
526 views

Do supernovae produce an appreciable amount of lithium?

David Z's answer to this question got me wondering - is any appreciable amount of lithium produced as the result of a supernova explosion, either by fusion (which seems unlikely to me, but I don't ...
31
votes
3answers
3k views

Why the galaxies form 2D planes (or spiral-like) instead of 3D balls (or spherical-like)?

Question: As we know, (1) the macroscopic spatial dimension of our universe is 3 dimension, and (2) gravity attracts massive objects together and the gravitational force is isotropic without ...
0
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1answer
21 views

Help reading power spectrum graphs?

How would I go about figuring out how long a source was measured when given a graph of the power spectrum? From notes I'm following, they just state the total observation time as being roughly ...
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0answers
23 views

Lorentz factor correction for luminosity

I am looking at the physics behind gamma ray bursts, working through a comprehensive review Kumar & Zhang (2014) (arXiv link). On page 92, Eq (111) the luminosity or a relativistically expanding ...
1
vote
1answer
32 views

Short Gamma Ray Bursts from black hole mergers

I have read in passing that short gamma ray bursts can be caused by the merger of 2 black holes in a binary system. I have Googled but can't seem to find any good sources describing the phenomenon ...
2
votes
2answers
3k views

Artificial planetary magnetic field

I wonder how difficult it is to create an artificial planetary magnetic field with generators? What power they would need? The question is inspired by thinking about possible colonization of ...
1
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2answers
42 views

How can contact binaries persist?

This answer discusses contact binaries, which I did not even know existed. I can understand how they could exist for a short time (maybe) as gravitational waves carry off energy that causes the stars' ...
2
votes
3answers
207 views

What happens to a white dwarf star if it has mass higher than the Chandrashekhar limit?

What will happen if a white dwarf star has mass higher than the Chandrasekhar limit, i.e. 1.4 times the mass of the Sun?
2
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0answers
24 views

Why do TiO bands dominate M dwarfs?

I'm new at understanding stellar classification and the spectral classification of stars. What is the exact reason TiO molecules (titanium oxide) dominate the spectrum for M dwarfs? How did this TiO ...
2
votes
1answer
90 views

What is the largest possible mass for a stable planet made of iron?

Consider a large spherical planet made of pure iron. Think of something similar to Mercury or Earth without its mantle, only much bigger, though those planets have elements other than iron mixed in ...
7
votes
2answers
1k views

Do we know what event caused the Sun and Solar System to form?

Some stellar formation theories suggest that stars are formed by shock waves from trigger events such as supernovae. This excerpt from Star Formation basically gives the background to my question: ...
13
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2answers
914 views

Would a submarine float in the atmosphere of Jupiter?

Would a submarine float in the atmosphere of Jupiter, or would it get crushed?
3
votes
2answers
62 views

Do gamma ray bursts play a role in cosmic evolution?

If gamma ray bursts were to interact with gas clouds in the early universe, might it be a relevant factor in star production?
0
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1answer
42 views

Quantum gravity- Vacuum catastrophe

Why does the predicted mass of the quantum vacuum have a very little effect on the expansion of the universe?
0
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1answer
39 views

Is it possible that a satellite once was a small planet?

Very common fact that a planet has satellite which revolves around it and has an impact on that planet too.Is it possible that a small planet had been bombarded with a big planet and transformed ...
0
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0answers
8 views

Region of resonance and overlap

In planetary dynamics what does a region of resonance (mean motion) between two bodies mean and how to quantify the region? How does resonance overlap occur and what are its consequences? What is ...
3
votes
1answer
62 views

Shoemaker-Levy 9 impact on Jupiter

When the comet Shoemaker-Levy 9 impacted with Jupiter it created a ball of fire 3000 Km high and it left a mark on the surface of the planet that was visible for weeks (source: Wikipedia). How was the ...
1
vote
1answer
31 views

How was hydrogen gas (H) obtained by spectroscopists? Why is there more H than H2 in space?

Introductory quantum mechanics lessons talk about emission and absorption spectra for the hydrogen gas, and then give you an explanation as if this gas were pure $H$ atoms, and not the $H_2$ molecule ...