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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Dependence of the neutron-freeze-out temperature

What is the physical explanation for the dependence for the freeze-out temperature $T_\text{F0}$ of the neutrons as a function of the number of degrees of freedom $g^\star$? ...
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65 views

Is it true that the sun will cause very high temperatures on Earth long before the red-giant-phase?

I heard at least three claims about the development of the heat of the sun. In an old book, I read, that nothing dramatically will happen in the next few billion years. Wikipedia states, that the ...
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1answer
141 views

Hubble time and its derivation? [duplicate]

I know the derivation of Hubble time goes something like this (I am an a-level student so this may not be the actual derivation): Two galaxy that is moving away from each other at speed v are now D ...
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1answer
54 views

The initial conditions of the CMB spectrum

The CMB spectrum shows the intensity of fluctuation at a certain angular scale: The achievement is the correspondence between the predicted power spectrum and the observed one. My question is as ...
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1answer
36 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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1answer
83 views

Could the Earth be ejected when the sun burns out?

My younger brother came home from school today and told us at the dinner table that when the sun burns out the Earth could be ejected from its orbit. Skeptical, I asked his source. He quoted his ...
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31 views

Are combined masses in space, such as galaxies, considered to be uniform bodies? *In addition, a related question about force

If we were to calculate the force that one galaxy exerted onto another, would we consider the individual masses within the galaxies, or the masses of the galaxies as a whole? Do the individual stars ...
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1answer
73 views

Consequences of inverse square law with vast distances (Gravity); (in addition, is light speed broken)? [duplicate]

As is well known, the gravitational force between two masses is dependent on the spatial distance between them. Therefore, even at vast distances, the masses exert equal and opposite forces on one ...
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50 views

Why are other universes in the picture of Dark Flow?

It seems to me that the popular suggestive explanation of dark flow is another universe interacting with ours. Isn't there other alternative explanations?
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2answers
120 views

Smolin on Cosmological selection and neutron stars

Regarding the cosmological selection hypothesis and testable predictions, Lee Smolin asserted the following: "Smolin: I did make two predictions which were eminently checkable by astrophysical ...
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1answer
44 views

Pulsars with accreting disk in binary system

Following this line, I am wondering about the following question. Accreting pulsars in binary systems are usually thought to accrete from a prograde disk, so increasing their spin in the process. ...
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1answer
89 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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5answers
236 views

Is dark matter around the Milky Way spread in a spiral shape (or, in a different shape)?

Dark matter doesn't interact with electromagnetic radiation, but it, at least, participates in gravitational interactions as known from the discovery of dark matter. But does dark matter exist in a ...
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349 views

Neutron stars and black holes

The official limits for a neutron star is $1.4 - 3.2\;M_\odot$. But I read that the limit depends on the particular structure of a star to estimate which mass it must have. I also read that neutron ...
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1answer
33 views

Galaxy velocity curve

When deriving the relation between velocity of a star and its distance $r$ from the center of the galaxy, one assumes that the masses outside $r$ have no contribution and those inside $r$ give a force ...
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3answers
149 views

Is the solar energy Infinite?

Is the solar energy coming from the sun infinite and will continue to be radiated to our earth forever? (discarding any outer factors) what's the sun's fuel?
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6answers
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Why are some galaxies flat?

What is the explanation for the flatness of some galaxies? (If it's due to their rotation then why they are rotating, why some other galaxies are not flat etc., I would like to hear a nice and ...
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1answer
56 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 ...
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2answers
2k views

What will happen if the solar system leaves the milky way galaxy? [closed]

Is the milky way galaxy meaningful to us? Will any bad thing happen if the solar system just leaves the galaxy?
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3answers
339 views

What keeps a gas giant from falling in on itself?

There is not enough gravity at the center to start nuclear fusion, but it seems that there would be plenty enough to collapse the planet.
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107 views

Stellar Power(Luminosity) Flux

So I was applying some mathematical techniques I learned to physics, and one thing that captured my interest, is the power or luminosity flux of a star. So modeling the situation, taking the scalar ...
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3answers
232 views

How many stars within 5 parsecs?

I've ran some scripts on some star catalogs, which counted the number of stars with a parallax greater than 200 mas, which should mean everything closer than 5 pc. The results are as follows: ...
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1answer
70 views

Why are no CMB experiments able to measure the whole range of multipole moments?

Have a look at the following diagram: This shows measurements of the CMB by various experiments, with multipole moment $l$ on the x-axis and the temperature of the corresponding moment on the ...
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1answer
95 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
61 views

How can we know that a black hole exists? [duplicate]

How can we know that a black hole exists if it attracts both matter and light, and to prove that it exists would need to observe it? It's a very common question but I don't know the answer.
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3answers
3k views

If I were standing on the surface of the sun, how far could I see?

Assuming it would be possible to handle the intense heat... Is the surface even? If not, would average surface elevation be an appropriate definition of "surface"?Does the sun have an atmosphere that ...
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5answers
5k views

The final death of a black hole

What are the different death scenarios for a black hole? I know they can evaporate through Hawking radiation - but is there any other way? What if you just kept shoveling more and more mass and ...
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2answers
8k views

Is Jupiter a failed star?

In my physics lessons, my teachers have always been keen to tell my class that Jupiter is considered a 'failed star' by scientists. Is this true? In my own effort I wondered if maybe this could just ...
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1answer
410 views

Would atmosphere of Jupiter blow up if I launched a atomic bomb there? [duplicate]

This is based upon an question I had, since in Jupiter there is no oxygen a simple fire cannot be started let alone even happen due to its temperature. Anyway that being said, If I somehow used a ...
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1answer
131 views

Supernovae and black holes?

I think i am correct in saying that a supernova ($Type$ $II$) is caused by the collapse of the core of a giant star. This contraction of the core is stopped by the Pauli exclusion principle and the ...
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41 views

B-mode and circular polarization in BICEP2

In the case of the B-mode detected in BICEP2, at first they assumed there was no circular polarization and then the Q and U stocks parameter of the CMB is given by E-mode and B mode. If we have ...
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2answers
158 views

Does the term “dark matter” apply to nonluminescent bodies which still interact electromagnetically?

On the new Astronomy.SE site, I was having a short discussion on one of my answers. The basic discrepancy was; can MACHOs like black holes/brown dwarfs/neutron stars be termed "dark matter"? My ...
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1answer
34 views

Temperature of WIMPs

As a dark matter candidate, what should be the temperature and kinetic energy (or also the speed) of the WIMPs (weakly interacting massive particles) to agree with the observed distribution of dark ...
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1answer
60 views

Vanishing Planets?

If we put a solid sphere in space, it will lose some molecules which will form a sort of an atmosphere around it so that we have the required vapour pressure for solid-vapour equilibrium (Temp. of ...
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2answers
107 views

Universal gravity at small distance

Could it be that there is simply a maximum gravitational force that two bodies of finite mass can exert on one another? This would occur at $r=0$, so maybe there is some really really really small $a$ ...
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450 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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1answer
89 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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1answer
80 views

Why does $H_2$ form on such a long time scale?

If we were trying to figure out the time scale for a gas-phase reaction between two hydrogen atoms in a molecular cloud (which has density $~10^4/$cm$^3$), apparently the reaction would happen on a ...
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142 views

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

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
84 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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1answer
99 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
42 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
103 views

Cosmic radiation cutoff at LOW energies?

The energy spectrum of the cosmic radiation (not CMB) is limited to both sides. I know about the GZK-cutoff at high energies. Basically the interaction probability for photons of energies above 10^20 ...
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2answers
97 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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3answers
177 views

Magnetic Pole Emission from Pulsars

I've been reading up on some pulsar emission theory (admittedly written in the 70's and 80's, but I figured that's a good place to start), namely this review by Curtis Michel as well as the book Black ...
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2answers
65 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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40 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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2answers
109 views

How accurate are our calculations about distant stars keeping in mind their enormous distances?

Since many stars are hundreds of light years away from the Earth and therefore, what we observe of them today is really their distant past, how can we say anything with certainty about their ...
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2answers
277 views

What are the demographics of stars visible to the naked eye?

Of the stars that can be seen with the naked eye, what are the distributions of each type? For example, how many are main sequence? how many are Super Red giants/white dwarfs/neutron stars...etc. ...
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41 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 ...