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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Comets in a young star cluster, how are they affected by gas?

As I understand it from looking at some simulations of young star clusters they were held together by a gas that dissipated eventually. The gas is usually represented in models of star clusters as a ...
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1answer
47 views

The origin of the colour of the sun

I have learned so many concepts under astrophysics and unfortunately, I have muddled everything together... Let me try to illustrate my problem: When a star is in main sequence, it fuses hydrogen to ...
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0answers
42 views

What is the shape of the Sun's orbit around the Milky Way? [on hold]

Here is a question for you really bright folks. How does the motion of the sun along the midpoint of the plane of the galaxy look like? I suspect it to travel above and then below the plane but I ...
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1answer
56 views

Why can't we detect redshift of Galaxies/Clusters moving parallel to us?

If a moving Galaxy/Cluster doesn't have radially outward motion component from us, we are unable to detect its redshift. The answer may look obvious: There's no wavelength squeezing/expanding ju-ju. ...
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1answer
25 views

Experimental evidence for the relic neutrinos

What are the experimental (indirect) evidence for the cosmic neutrino background? Where can I read more about this? The discussion on the wikipedia page about the C$\nu$B seems to me to be more ...
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2answers
36 views

Behavior of Saha and Boltzmann

So I'm just wondering why the Saha and Boltzmann distributions behave differently as temperature increases? I know one is for ionization levels while the other is for energy levels but is that the ...
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2answers
53 views

When do stars become red giants?

I am a bit confused when do stars become red giants? Is it just after they have finished core H burning and the core contracts creating high temperatures which result in core He burning to occur which ...
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0answers
43 views

Why are cgs units the norm in astrophysics?

Other physics communities, e.g. the particle physics one, have their own set of units, custom-tailored to their own needs. Now, the astrophysics community is somewhat similar, in that a lot of ...
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1answer
85 views

Is there a connection between exclusion principle and the speed of light?

As far as I know exotic stars are composed of degenerate matter created by the balance between gravity and pressure of exclusion principle and as the mass of the star grows it goes into more ...
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26 views

Precession period matching solar year

Consider the case of a tidally locked planet: its axis of rotation must be perpendicular to the plane of its revolution around the parent star. Therefore, no precession. It is possible for a planets ...
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0answers
23 views

What's the shocks mach number in this specific problem?

The shock wave I'm interested in occurs for a special Riemann problem, known as 1D Noh's problem. It means basically that two streams of air (equal density) are colliding at $x = 0$ for $t = 0$; I ...
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1answer
37 views

Electron degeneracy pressure

Why is it that in stars undergoing gravitational collapse electron degeneracy kicks in? Why couldn't the electrons form energy bands like in semiconductors?
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1answer
70 views

When did the first carbon atom in the Universe come into existence?

I am a chemist with a passion for astrophysics and particle physics, and one of the most marvellous things I have learned in my life is the process of stellar nucleosynthesis. It saddens me how my ...
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2answers
191 views

Light formed by the sun?

This is an extract from the astrophysics chapter in my book: Hydrogen atoms fuse to form helium. At the same time, lots of of gamma photons and neutrinos are produced. The photons take thousands ...
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2answers
51 views

Studying Physics [closed]

I'm preparing for entry into a Physics degree course, and was planning on a double major in Physics (with a specialization in Quantum Information later) but am now considering a minor in ...
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1answer
86 views

Stress-Energy Tensor

As of recent, I've been doing a bit of self education in GR, equipped with a working knowledge of the key elements of the differential geometry in GR, and in looking at the Einstein-Rosen bridge, I ...
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2answers
51 views

Could Gamma ray bursts be caused by matter-antimatter annihilation?

As far as I know a collisions of matter and antimatter leads to the complete annihilation of both, whereby 100% of the rest mass of the particles is converted into gamma rays. Could this mechanism be ...
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1answer
44 views

Mass density of dark matter in solar system near us

What is the approximate mass density of dark matter in our solar system at the radius of the Earth's orbit? I would like some idea of the mass of dark matter going through each cubic meter of ...
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2answers
68 views

Are leptons, baryons and energy the only products of radioactive decay?

I recently visited my child's elementary school to speak to a science classroom about rocks and minerals. While trying to explain what a crystal is, I got sloppy and mis-spoke that an atom was the ...
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1answer
57 views

Could the Sun be partly powered by cosmic rays?

The Earth and the Sun has magnetic fields which shields us from cosmic rays, as a charged cosmic ray particle will loose kinetic energy when its direction is perpendicular to the magnetic field. So ...
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0answers
45 views

Inward Pressure of Gravity

We are looking at White Dwarfs in Quantum and specifically how they are stabilized between gravitation pressure inwards balanced by the Fermi pressure due to electrons. To start the problem off ...
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1answer
31 views

Is there a simple formula for calculating semimajor axis bounds of “habitable zone”?

I understand that an accurate determination of the bounds of the "habitable zone" for a given stellar system depends on a large number of factors, including many beyond characteristics of the parent ...
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2answers
87 views

Photons straight into black hole

What happens to a photon shot straight into a black hole? Does it gain infinite momentum before it crosses the horizon? If it has a finite momentum going in, then it would seem that a photon of the ...
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3answers
76 views

State of the Stars

What's the state of stars? I'm not pretty sure whether the stars are stationary or rotating. For instance, if they are not rotating what makes them to be stable?
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1answer
46 views

Are the known exoplanets representative of the population of exoplanets?

An article about the recent release of a large number of confirmed exoplanets states that most of the 715 newly announced exoplanets orbit very close to their host stars. Do the theories predict ...
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1answer
115 views

Why do galaxies “dissappear?”

So, this is a dumb question but a bit of information confused me lately. Before, I figured galaxies were no longer visible by us because their luminosity decreased in an inverse square manner. ...
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1answer
113 views

Would a collapsing Universe have the density of water?

I understand that the typical density of the super massive black hole is close that of the water. It is also my understanding that this density is not true matter density because the volume used to ...
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1answer
68 views

Why does a star beyond a certain mass limit (Chandrasekhar limit) only become a black hole?

Why does a star beyond a certain mass limit (Chandrasekhar limit) only become a black hole? A star is first made of hydrogen, it undergoes nuclear fussion reaction combining into helium and ...
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64 views

Modelling a matter dominated universe collapsing into a black hole

With the FLRW equations we can get solutions for a matter dominated closed universe in which the finale is an ultimate collapse, but this is only in terms of $a$ (the scale factor) and $t$ (time) and ...
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79 views

How fast do large asteroids usually travel?

I have just watched this video made by discovery channel, and you can hear the narrator at 0:51 saying that : "even though it is moving at 720 THOUSANDS kilometers per hour..." I stopped once I heard ...
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0answers
36 views

Mass of dark matter

The Alcubierre warp equation specifies that negative mass is required for said "warp drive" to function properly. Could dark matter provide the negative mass that's required?
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4answers
133 views

Observed composition of UHE cosmic rays

How much is known about the composition of ultra high energy cosmic rays (say $E>10^{20}\text{ eV}$)? I get the impression that the particles are often assumed to be protons or other heavier ...
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0answers
93 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 ...
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5answers
2k views

Why does a star collapse under its own gravity when the gravity at its centre is zero?

The gravity at the centre of a star is zero as in the case of any uniform solid sphere with some mass. When a massive star dies, why does it give rise to a black hole at it's centre? I know how to ...
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0answers
48 views

Do wormholes have a side to their path through space?

In theory do wormholes have a side to their path through space? What is there at a point in line with the entry and exit, would anything look different at that point in space? Could a space ant get ...
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1answer
46 views

Implications of the Jeans mass

The minimum mass for the spontaneous collapse of a gas cloud is the Jeans mass. For low temperatures and high densities, this jeans mass is greater than the mass of a typical star. What are the ...
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1answer
54 views

(Polar jets) Is it possible that black holes radiate substantial amount of the mass/energy they accrete?

Is the amount of energy a black hole emit through polar jets comparable to the mass it accretes or is it negligible? Here I am mostly concerned about the observational findings as opposed to the ...
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2answers
85 views

Earliest terrestrial planet?

If I've understood correctly, the heavier elements needed for terrestrial planets such as iron can only form in supernova. If that is indeed true, how long since the beginning of the universe would ...
2
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1answer
38 views

Cepheid variable light curves

Basically, given a set of noisy observations for the apparent magnitude of a Cepheid variable, how is this fit to a curve which allows the period, and therefore distance, to be found? Cepheids' ...
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2answers
131 views

What is a simple mathematical model of a star?

I had a discussion at work regarding a recent fusion experiment in China that resulted in temperatures five times hotter than the Sun. Someone mentioned that no one can know the temperature of the ...
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1answer
87 views

How long would it take for a galaxy to collapse without dark matter?

I am trying to understand the effects of gravity in the Cosmos without complications of Dark Matter/Dark energy issues. So my question is, assuming that a galaxy (for example, the Milky Way) does not ...
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0answers
64 views

How is dark energy calculated

This should be a very simple question. What would be the proper way to calculate Dark Energy in Joules at any point in history and that is consistent with the Standard Model? I'm thinking that ...
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2answers
72 views

Simulating how comets travel with Newtonian physics

Imagine that a comet enters the solar system from Oort cloud, what concepts and laws does one need to take into consideration to simulate how it travels? I'm about to start researching how to write ...
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2answers
102 views

Gravitational field has no curl? What about gas discs around stars, black holes, etc.?

So everybody says the gravitational field has no curl, and is not comparable to a liquid swirling around a drain. Observationally, of course, there are many examples of vector fields (which I think ...
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1answer
326 views

Why the galaxies forms 2D plane (or spiral-like) instead of 3D ball (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 ...
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3answers
144 views

Can the Cosmological Constant explain an accelerated expansion?

From what I've learned so far, it appears that all models that attempt to explain the expansion of the universe are either based on Lambda-CDM or quintessence. The former support a big bang with ...
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1answer
117 views

Is a Betelgeuse supernova able to neutralise earth's nuclear arsenal?

According to an article on newscientist.com, a neutrino beam could neutralise nuclear bombs by inducing a slow meltdown of the nuclear fuel. The neutrino generator would need to be more than a ...
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1answer
56 views

What is the intrinsic colour of the star? [closed]

Hey I have this question and I am wrestling with it all day and I am completely lost. The V magnitude of a star is 15.1, B-V =1.6, and absolute magnitude Mv= 1.3 The extinction in the direction of ...
2
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1answer
50 views

Relation of fraction of binary stars with spectral class (mass)

What is relation of fraction of binary stars with spectral class (mass)? For example, how many binary stars are among O,B,A,F,G,K,M stars separately?
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1answer
42 views

Would gravitons follow the same trajectory as photons through a gravitational lense?

Would gravitons follow the same trajectory as photons through a gravitational lense? would all other particles follow the same trajectory?