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.

learn more… | top users | synonyms

0
votes
1answer
44 views

Do I have what it takes [on hold]

I am a freshman in college, and I love physics but I took high school as a joke and now am taking beginner algebra for my freshman year. Is it still possible if I work hard to be a physics major. Has ...
1
vote
1answer
32 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
vote
1answer
46 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 ...
2
votes
0answers
16 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. The four coloured circles indicate the orbits of ...
1
vote
1answer
62 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.
0
votes
0answers
20 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 ...
0
votes
1answer
26 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
votes
0answers
62 views

if the universe is expanding because of big bang, then please some body explain for me mass center and gravity in this universe ? [closed]

it was proved that the universe is expanding ( moving away from it's center in all directions ) because of the big bang . So it means that the center of mass is the same center of expanding , and this ...
24
votes
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
votes
1answer
34 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 ...
3
votes
1answer
49 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). ...
1
vote
1answer
55 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 ...
1
vote
1answer
81 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 ...
4
votes
1answer
91 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 ...
6
votes
1answer
87 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, ...
0
votes
0answers
23 views

How is optical depth (opacity) related with gas pressure?

In stellar atmospheres, if we are given the gas pressure on the surface level of a star $P_{0}$ and we are expected to calculate the gas pressure on a deeper level (say 10000km) of the star, what kind ...
2
votes
1answer
65 views

Can a star with a constant density profile be possible?

The equation of states for a star is given by a polytropic equation, where density depends on the $n$th power of $\theta$. Please refer to the literature First, what is this $\theta$? It can't be a ...
2
votes
1answer
62 views

What would a very massive rocky body look like?

I have a basic understanding of how gaseous bodies behave according to their mass: "Low mass" bodies are gas giants (or brown dwarfs), Beyond a certain mass, hydrogen fusion starts, making a star, ...
2
votes
2answers
46 views

Position of Neutron Stars in H R diagrams

Why is that neutron stars are never depicted in a Hertzsprung-Russell diagram? They can be placed in the bottom left corner but you will never find any diagram in literature showing neutron stars.
0
votes
0answers
28 views

Boltzmann equation (Number density)

I'm trying to understand the Boltzmann equations use in the early Universe. The derivation is somewhat tedious, but in the end I end up with: $$a^{-3}\frac{d}{dt}\left(n_1a^3\right) = ...
0
votes
0answers
26 views

Gravitational collapse (artificial). Is it possible? [duplicate]

I am rather a beginner to this field, so please forgive me if this is a very meaningless question. If I were to somehow increase Jupiter's mass by adding more hydrogen to its atmosphere, can I ever ...
0
votes
1answer
23 views

What is the relation between Rössler attractor and thin accretion discs (like in the movie Interstellar)?

Is there any relationship between the Rössler attractor and thin accretion disks, like the accretion disk(s) in the movie Interstellar?
1
vote
1answer
37 views

Type II supernovae explosions

I'm quite confused about the explosion of a Type II supernova. As far as I understand, when the Fe-56 core has been created, and the star has all the other layers around it, it starts to collapse, ...
2
votes
1answer
55 views

Exoplanet Mass-Radius Diagram

I'm currently studying the following diagram: But I'm not entirely sure I understand what's going on. Is it just, that most exoplanets discovered, is pretty much made up of Hydrogen and Helium ? ...
0
votes
1answer
94 views

A Theory of Almost Everything? [closed]

Before I present this theory, I ask that you take the time to read this disclaimer. I do not claim to have an excellent knowledge of physics, it is actually rather limited. Therefore, I am not ...
2
votes
1answer
37 views

Electron degeneracy and helium flashes in stars

I have a question regarding the above mentioned. When a star have a mass of about 3-8 it does not go through the so-called helium flash phase, but instead just run along as nothing had happened, turn ...
0
votes
0answers
13 views

Does the density of matter increase as we approach the big bang? [duplicate]

I am interested in knowing whether it is clear (undisputed) that the density of matter/energy increases as we approach the time of the big bang? Does this follow from the FLRW metric?
0
votes
1answer
27 views

Planetary nebulae, thermal pulses and mass loss

I'm reading about planetary nebulae and how they are formed, but as is sometimes the case, I've gotten a little confused. So, I have a star, let's say 5 times the mass of the sun. At some point, when ...
0
votes
2answers
69 views

Can we solve most of the cosmological questions using the Illustris universe simulation?

The Illustris project (http://illustris-project.org) attempts to simulate the universe in its most accurate form according to their website. With the simulation, they were able to predict the neutral ...
1
vote
0answers
31 views

Why is the Great Dark Spot so shortliving unlike the Great Red Spot?

The Great Dark Spot is an anti-cyclone in Neptune. But unlike the Great Red Spot of Jupiter which lasts for more than hundred years, the Great Dark Spot exists for only one year or so. Why is it so??
4
votes
1answer
131 views

Chemical and stellar evolution in stars

I'm having an exam soon where I have to discuss/describe the follow figure: It's the first figure I have to explain, and it was on the same slide as the one below, so I'm thinking they are somewhat ...
3
votes
1answer
177 views

What is the average mass of galaxies according to Hubble Deep and Ultra Deep field observations?

It is very widely known among people interested in astronomy that there are 100-400 billion stars in the Milky Way galaxy and there are ~ 100 billion galaxies in the observable universe, which is ...
2
votes
1answer
78 views

The mass of universe at the point of the Big Bang

The density of universe at the time of the Big-Bang was infinitely high. Does that mean that the mass was also infinitely high? ( the universe was extremely small at that time)
0
votes
4answers
202 views

What is the largest number applicable to the universe? [closed]

That is, what is the largest number one could possibly come across in the physics of the universe? I would assume it might have something to do with the maximum number of combinations of possible ...
0
votes
1answer
59 views

How can we be Certain that Dark Matter Exists if we Cannot See it or Directly Detect It? [duplicate]

It seems very certain that dark matter exists and that it makes up 26.8% of the Universe today (along with 68.3% Dark Energy and 4.9% Atoms), but how can this be if we cannot see it or directly detect ...
5
votes
3answers
275 views

What does this stellar mass distribution mean?

According to this pie above and for the "Red Dwarfs" part, which of these is correct : 1) 41% of the stellar mass of a galaxy is in stars with masses < $0.25$ $M_{\odot}$ or 2) 41% of the ...
1
vote
1answer
33 views

Magnetar field energy density

According to Wikipedia a Magnetar... Earth has a geomagnetic field of 30–60 microteslas, and a neodymium-based, rare-earth magnet has a field of about 1 tesla, with a magnetic energy density ...
10
votes
2answers
196 views

Where did the energy released due to gravitational binding energy of the Earth go?

The gravitational binding energy of the Earth is $2×10^{32} J $, so the same amount of energy must have been released during the Earth's history. According to this and this, the current internal ...
37
votes
9answers
5k views

How would night sky look like if the speed of light was infinite?

Would it be brighter? Different color? Gravitational lensing? Would black holes exist?
0
votes
1answer
64 views

Temperature of the surface of the sun? [closed]

I recently had an exam question that asked for the temperature at the surface of the sun. The question is The equation I believe you have to use is The Q/t is the radiant power produced by the ...
0
votes
1answer
30 views

Difference between astronomy and astrophysics [duplicate]

In my university, the department for astronomy and astrophysics are distinct. I want to know what's basically the difference between the two fields?
0
votes
1answer
63 views

Where does gravity originate?

Does it emanate strictly from energy dense regions of space? What does that mean? Is it possible to, say, arrange clumps of matter in such a way as to create a virtual gravity well in space where ...
2
votes
0answers
19 views

How can comets have sand dunes without atmospheric erosion? [duplicate]

This article here talks about dunes visible on comet 67P: In August, the European Space Agency (ESA) achieved a major success when its Rosetta probe rendezvoused with comet 67P. The spacecraft ...
3
votes
1answer
33 views

Why is the rotation curve of a galaxy lower if measured with CO rather than H?

I have a simple question regarding galactic rotation curves. I know they are studied mostly using 21 cm emission of Hydrogen in the outer regions and with CO emissions in the inner regions, but why do ...
3
votes
2answers
143 views

What is the origin of CMB fluctuations?

I have read somewhere that CMB (cosmic microwave background radiation) fluctuations in temperature are linked to mass distribution fluctuations in the early universe (at ~350000 years after Big Bang, ...
3
votes
2answers
49 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 ...
1
vote
1answer
61 views

Why does orbiting matter form an accretion disk? [duplicate]

Why does matter in orbital motion around a central body tend to form an accretion disk, as opposed to some other configuration like a sphere? I know this has something to do with angular momentum ...
3
votes
1answer
51 views

Could the estimated stellar mass for the Milky Way galaxy include brown dwarfs?

Trying to find an estimate for the stellar mass of the MW galaxy, I found this paper and the estimated stellar mass is $~6.5 \times 10^{10} M_{\odot}$. I was also trying to understand the methods used ...
10
votes
1answer
661 views

What happens to Protons and Electrons when a Neutron star forms?

What happens to Protons and Electrons when a Neutron star forms? At some point gravity overcomes the Pauli Exclusion Principle ( I assume) and they are all forced together. What happens in the ...
1
vote
0answers
44 views

Where do ultra-high-energy cosmic rays come from?

Physicists have detected an amazing variety of energetic phenomena in the universe, including beams of particles of unexpectedly high energy but of unknown origin. In laboratory accelerators, we can ...