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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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 ...
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2answers
47 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 ...
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19 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 ...
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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??
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Why are galactic centers always brighter than the edges?

As you can see the image below and other galaxy images, the center is generally much brighter. Why is that? Is there a very big star? A very big gravitational field?
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Electron/positron annihilation lines in astrophysics

I have a reasonable understanding of electron/positron annihilation, in that it is a collision between a pair of particles, one matter and one antimatter, that generally produces gamma radiation. ...
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117 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 ...
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Is the magnetic field of a white-dwarf merely residual?

Follow-up to my other question How does Sol's magnetic field continue to exist at such high temperatures? Assuming Sol's magnetic field is generated by convective currents in it's plasma, how is ...
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1answer
146 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 ...
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9answers
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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?
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1answer
63 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)
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189 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 ...
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2answers
128 views

How does the Sun's magnetic field continue to exist at such high temperatures?

The temperature at the surface of the Sun is apparently well above 5000 C; I'm assuming the layers beneath the surface may be even hotter. At school, we learned that heating a metal beyond a certain ...
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1answer
206 views

Why do galaxies “disappear?”

So, this 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. However, while watching a movie ...
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0answers
47 views

Angular momentum in an accretion disk

I need to plot the time evolution of the total angular momentum in an accretion disc. This confuses me because I thought this should be constant, since angular momentum has to be conserved? I'm given ...
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1answer
54 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 ...
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262 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 ...
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1answer
26 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 ...
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2answers
164 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 ...
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2answers
64 views

Distinguishing Gamma-rays and stars from each other in nebulas

How do you tell the difference between a gamma-ray burst and a star just from a picture of a nebula, in which it cannot flash on and off here and there?
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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 ...
33
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5answers
6k views

Why aren't there spherical galaxies? [duplicate]

According to the Wikipedia page on Galaxy Types, there are four main kinds of galaxies: Spirals - as the name implies, these look like huge spinning spirals with curved "arms" branching out ...
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1answer
25 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?
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1answer
58 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 ...
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0answers
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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 ...
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1answer
24 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 ...
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1answer
29 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 ...
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2answers
129 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, ...
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What happens to the neighboring star of a type Ia supernova?

Supernovae of type "Ia" are those without helium present, but with evidence of silicon present in the spectrum. The most accepted theory is that this type of supernova is the result of mass accretion ...
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2answers
646 views

What would happen to a teaspoon of neutron star material if released on Earth?

I've read on NASA's page on neutron star that one teaspoonful of that star would weigh over 20 billion tonnes on Earth. If it was somehow possible to bring it to earth would it: Burn and disappear ...
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2answers
80 views

How much of a galaxy's mass is in stars?

I have been trying to find an answer for this question for a while without a success, so I guess it might not have a specific answer. But to make things easier, let's take the Milky Way galaxy as an ...
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3answers
162 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 ...
11
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1answer
268 views

Do celestial objects experience drag from the near vacuum of space/does the near vacuum have a mean velocity?

For instance do the planets around the sun experience drag from the near vacuum of space? Or do the (hydrogen) atoms in interplanetary space have a mean velocity near orbital speeds, such that object ...
3
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1answer
49 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 ...
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5answers
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Can there be Electron and/or Proton Stars?

What happens to all of the electrons and protons in the material of a neutron star? Could there ever be an electron star or a proton star?
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42 views

According to the initial mass function, should there be more brown dwarfs than red dwarfs?

According to the IMF and the stellar mass distribution, stars become more abundant the less massive they are. And while objects must have a mass > 0.075 solar mass to become a star, brown dwarfs with ...
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0answers
56 views

Electric and magnetic field in a black hole

I have many questions about this topic: Does the electric field of a charged black hole look like this? I mean how can it have an electric field if nothing can escape from a black hole, and what is ...
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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
636 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 ...
4
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1answer
57 views

What is the distribution of Population I and II stars in the Milky Way galaxy?

I have been trying to find out the distribution of Population I and II stars in the Milky Way. The distribution I mean is the percentage of each population to the total stars in the galaxy. So in ...
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2answers
478 views

What is the luminosity of the Milky Way galaxy?

The luminosity of the Milky Way galaxy according to this is $5\times10^{36}$ Watts, but this number suggests that there are about 10 billion stars with Solar luminosities in the Milky Way, which ...
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0answers
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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 ...
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5answers
77 views

Can a Coronal Mass Ejection (CME) cause a blackout on Earth and why?

Can a Coronal Mass Ejection (CME) cause a blackout on Earth and why is it so what's the relation between electro magnetic radiations and electrical and electronic appliance.what exactly does it do to ...
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2answers
268 views

How much does electromagnetic radiation contribute to dark matter?

EM radiation has a relativistic mass (see for instance, Does a photon exert a gravitational pull?), and therefore exerts a gravitational pull. Intuitively it makes sense to include EM radiation ...
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2answers
70 views

Stellar Activity Cycle versus Metallicity

Our Sun exhibits sunspot max/mins on about a 11 year period. It's a G2 spectral class on the HR Diagram. We know that some stars exhibit "starspot" cycles of various intensities and periods. These ...
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2answers
24 views

How velocity dispersion changes with change of inertial frame

I'm analysing a bunch of simulated galaxies, and one of the properties I'm looking at is their velocity dispersion (which is the same thing as the standard deviation of their speeds as far as I know). ...
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2answers
142 views

Is there a binary black hole system in the middle of the galaxy?

We have observed gravity effects from black holes in the center of galaxies, but galactic centers are dusty so we can’t tell if it’s one black hole or two black holes in a binary system in there. A ...
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1answer
56 views

Formation of supermassive black holes

Scientists have found very bright source of light which they call quasar and the are found to be supermassive black holes. So these black holes are so massive that they cannot be formed by a ...
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5answers
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Age of the Earth and the star that preceded the Sun

One of the great unheralded advances made in the history of science was the ability to determine the age of Earth based on the decay of isotopic uranium. Based on the apparent abundance of uranium in ...
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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 ...