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 can't Iron fusion occur in stars?

It is said that iron fusion is endothermic and star can't sustain this kind of fusion (not until it goes supernova). However star is constantly releasing energy from fusion of elements like Hydrogen ...
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1answer
89 views

What is the bigger number of particles crossing an area: the number of photons or the number of neutrinos? [closed]

Take an squared area with (10²)² m² in front of the sun. What is the bigger number of particles crossing an area: the number of photons or the number of neutrinos? Just for clarification: you can ...
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210 views

What would happen if all dark matter disappeared?

This is related to this question which provided evidence about dark matter, and this question which asked what would happen if dark energy started disappearing. This question assumes that dark matter ...
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1answer
80 views

What evidence is there that infinities exist anywhere around or within a black hole?

I've often seen black holes referred to as having a "singularity", which is described as a point of "infinite density", presumably due to the mass of the black hole occupying a point with zero volume ...
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Origin of elements heavier than Iron (Fe)

In all the discussions about how the heavy elements in the universe are forged in the guts of stars and especially during a stars death, I usually hear that once the star begins fusing lighter atoms ...
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355 views

Is there an exact formal definition of the Universe?

I've read several articles about Observable universe, Universe and Hubble volume, including Wikipedia articles and other references. After this I wondered: Is there a formal, rigorous definition in ...
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1answer
142 views

What is the curve that describes the Daytime line in a Day and Night World Map?

A Day and Night World Map shows which parts of the Earth are in daylight and which are in night at a given instant. At one side of the Daytime line they are in daylight and at the other side they are ...
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4answers
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Why does Venus spin in the opposite direction?

Given: Law of Conservation of Angular Momentum. Reverse spinning with dense atmosphere (92 times > Earth & CO2 dominant sulphur based). Surface same degree of aging all over. Hypothetical large ...
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1answer
158 views

Why are galaxies flat since it's an unstable configuration in 3d? [duplicate]

I'll probably have to delete this question because someone's already asked it, but what accounts for the stable 2-d structure of spiral galaxies in three dimensional space (assuming random starting ...
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4answers
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Why doesn't the solar wind disrupt the planets?

The sun creates this heliosphere by sending a constant flow of particles and a magnetic field out into space at over 670,000 miles per hour, which is also known as solar wind. If the speed of the wind ...
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1answer
95 views

What is the difference between a quasar and an Active Galactic Nucleus (AGN)?

In answering another Phys.SE question about quasars - Why no new quasars? - an issue arose about which object is the nearest quasar. That got me puzzled. To what is the label "quasar" attached, as ...
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2answers
107 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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3answers
131 views

Why doesn't the volume integral vanish?

I am reading Galactic Dynamics by Binney and Termaine, and I'm struggling with the following passage. According to Poisson's equation the resulting change in potential $\delta\Phi(\mathbf x)$ ...
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82 views

Does the motion of the particles within the star (radioactivity) create a deeper gravity well than the same size moon (solid rock)? [closed]

Compare two celestial body: a star and a moon. The diameter of a star is 864,948.7 mi. The star is mostly composed of the elements hydrogen (H) and helium (He). By mass the composition of the star is ...
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3answers
108 views

Why is the universe not uniform?

From my understanding, the Big Bang theory says the universe expanded from a state of infinite density and pressure. I feel that if this is true, this should lead to a universe where matter is ...
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3answers
149 views

How do shock waves form in space?

Today my aerodynamics professor mentioned that the equations we were learning for atmospheric shock waves can also be applied by astrophysicists to study black hole-related shock waves in space. How ...
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1answer
7k views

Why does the Moon or the Earth revolve anti-clockwise?

The question is very simple: Why does the Moon or the Earth or any another planet revolve anti-clockwise? And can any planet (or satellite) revolve clockwise? What is the physical law for this ...
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2answers
291 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 ...
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0answers
58 views

Bulge-Disk decomposition in galaxies

I'm starting a new project on dark matter astrophysics in which I have to extract from some data (I will write some below) the surface brightness, to later construct the mass density profiles of de ...
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1answer
65 views

It's a Hypothesis about the Beginning of the universe [closed]

Let's take a piece of paper and plot a couple of dots in it. If we shrink the paper infinitely, at some point it would reach a state of infinite density, that is; a Singularity. But even at that ...
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1answer
2k views

What is the theoretical lower mass limit for a gravitationally stable neutron star?

I ask here intentionally not for the size of the smallest possible observed size of neutron stars, which corresponds approximately to the well-known Chandrasekhar-limit for the upper limit of the ...
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2answers
111 views

Will the Sun ever get 100x powerful? If so, when?

I was doing a theoretical research regarding life on Titan. The temperature of Titan is so low, and it needed more sunlight, as a result, the Sun would require to get hotter. My question is, when will ...
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0answers
54 views

How much of the Solar convection zone is completely ionized?

I was reading about the energy transportation in stars here, and I found this: "The outer portion of solar mass stars is cool enough that hydrogen is neutral and thus opaque to ultraviolet photons, so ...
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21 views

What is Colour-Colour-Intensity Diagram?

What are color-color intensity diagrams? These are 3D plots usually used in high energy astrophysics ( particularly in relation to XRBs).
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2answers
112 views

Can less gravity speed up time? [closed]

We know more gravity slows down time per Einsteins Theory of Relativity. However will a significantly less gravitational pull speed up time? Or is "our time" aka earth standard of time the fastest ...
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1answer
42 views

Earth precession

I got a bit confused while studying the tropical and sidereal years. I noticed that the Earth's precession is in the opposite direction than that of a top's precession, when their rotational direction ...
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2answers
137 views

Would a collision of two black holes emit any electromagnetic radiation?

The video linked in this answer shows how space-time would be distorted during such an event. No doubt such an event would be an extremely strong source of gravitational waves. But would anything be ...
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3answers
499 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
62 views

Speed of light violation?

While reading this article Discovery of the companions of millisecond pulsars, it mentions a pulsar rotating multiple times a second, and I was wondering if this doesn't exceed the speed of light?
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0answers
127 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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2answers
125 views

Do the magnetic moments of neutrons mutually align to produce the magnetic field and emission beams of a pulsar and/or magnetar?

In this graphic of a pulsar the emission beams are at the north and south poles of the neutron star's magnetic field. I've read that the Earth's magnetic field is produced by a dynamo effect, whereby ...
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1answer
42 views

Does the stellar MK classification fix the apparent luminosity?

Introduction Let say we have some stars and their spectral classification in the MK index. (not all HD stars) HD xxxx - B3 HD yyyy - O7 HD zzzz - F0 .... If the stars share the same spectral ...
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1answer
6k views

Do all known planets and moons have magnetic field?

In this Wikipedia article it is stated, that magnetic field of Earth is caused by currents in her core. The same origin is for Jupiter magnetic field. For Moon (article) there is a magnetic field, ...
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1answer
127 views

What will be destroyed? The Moon or the Death Star?

The situation: Suppose Death Star passes by the Earth towards the Moon (near speed of light) and fires its laser (starting from the center of Death Star) against the Moon. If the laser gets to the ...
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2answers
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Why is the Sun almost perfectly spherical?

Relatively recent measurements indicate that the Sun is nearly the roundest object ever measured. If scaled to the size of a beach ball, it would be so round that the difference between the widest ...
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4answers
16k views

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
64 views

Why aren't Roche limit and the difference in gravitational acceleration the same?

The definition of the Roche limit is: "The distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the ...
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1answer
86 views

How do you project a Sersic profile into 3D?

I'm attempting to create a velocity profile for the M31. I have a Sersic profile from a paper by Sofue et al (2009), but I'm unable to project it into 3D space in order to get the actual 3D density ...
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1answer
77 views

What would a star look like that was the most oblate possible?

This question about why is the Sun so spherical and it's corresponding answer made me wonder: If some younger stars that are rotating faster than the sun are more oblate, then how oblate are they? ...
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4answers
233 views

Speed distribution of objects in the universe

If we pick a reference frame where, for example, our planet is standing still (although it should be irrelevant), what would the speed distribution of all objects in the universe look like? For the ...
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1answer
49 views

What do you see if you directly observe your destination while in warp? [closed]

Say you are traveling in a warp bubble towards Earth from 10,000 lightyears away with an ETA of 24 hours. On the front of your ship, you mount a high powered telescope pointed directly at Earth (your ...
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1answer
76 views

In what ways does the sun output energy?

I'm curious about the energy output of the sun, and how it breaks down. What fraction is from EM radiation, solar wind, neutrinos, et cetera? How does energy output vary over the EM spectrum? I was ...
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1answer
209 views

How to calculate the radius of a main sequence star based on mass?

What would I need in addition to the mass to figure out the radius of a main sequence star?
2
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3answers
523 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?
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2answers
2k views

Is there a limit as to how fast a black hole can grow?

Astronomers find ancient black hole 12 billion times the size of the Sun. According to the article above, we observe this supermassive black hole as it was 900 million years after the formation of ...
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2answers
373 views

Oldest population 1 star system?

While reading Stanislaw Lem's essays on advanced civilizations, I had a question: When did the earliest generation of population 1 star systems form? How much older could they reasonably be than our ...
4
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1answer
171 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 ...
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1answer
64 views

How does fusion work in the Sun if neutrons have more mass than protons?

According to my textbook, the next result of the fusion reactions in the Sun is: 4H -> He + neutrinos + gamma photons However, if hydrogen atoms are basically a proton and helium atoms are 2 protons ...
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4answers
5k views

Do solar systems typically spin in the same direction as their galaxy?

Is the net angular momentum vector of our solar system pointing in roughly the same direction as the Milky Way galaxy's net angular momentum vector? If yes or no, is that common for most stars in the ...
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2answers
221 views

What will happen to the Earth when Milky Way and Andromeda merge?

Hypothetically, if our solar system survived into the far future and human was still around, is the merge between 2 galaxies were a disaster for lives on Earth? Were the Sun get thrown away from the ...