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Questions tagged [astrophysics]

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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moving singularities

I am aware that black holes can move through space-time. Is there a way to move a naked gravitational singularity artificially? Sorry if this is a little stupid, trying to figure out the feasibility ...
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What is angle between ray to center of the sun and its perimeter?

If I took a ray from a point on the earth P to the center of the sun. And another ray from the same point on the earth to the perimeter of the sun. What would the ...
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Do only 'giant stars' fuse large amounts of elements other than hydrogen and helium?

Do only giant stars initiate 'full-on' carbon (and higher element) fusion, even though red giants on the larger end of the spectrum create carbon and oxygen via producing helium per the CNO cycle?
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Why does a star die once it has iron?

I found out that iron is the death element for stars, but i couldn't find why can anyone knowledgeable on stars explain why iron causes the star to die?
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Why does a tidal bulge result in a net torque on the Moon?

I understand that a tidal bulge is caused because the side of the earth facing the moon experiences a stronger gravitational force than the other. This results in the oceans on that side slightly ...
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Do supernovae push neighboring stars outward?

I know that a supernova can mess up the heliosphere of nearby stars, but I'm wondering if it could physically push neighboring stars off their trajectories. It's fun to imagine all the stars ...
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How to make a planet out of krypton? [on hold]

Let's suppose there is a star that explodes and releases a large mass of krypton. What is the smallest mass of krypton necessary for a planet to form made entirely of krypton under the attraction of ...
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What is the least energy trajectory in Astronomy?

Hi I wanted to know what the least energy trajectory is in relation to astronomy as I can't seem to find a good answer on Google. I want to know because I am trying to answer this question: A ...
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How to calculate the corotation radius?

I am studying this paper: P.Ghosh & F.K.Lamb, APJ, 232, pag.259 (1979) I do not understand how the authors calculate the corotation radius, $r_{co}$, where the matter of the disk has the same ...
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How is black hole formed after a supernova explosion?

Does all the mass of a core collapsing star get scattered during a supernova explosion? If it does, then how can a black hole be formed without any remaining mass? Or are there remnants of the star ...
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1answer
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Doubts about the formation of the bullet cluster

The bullet cluster is formed by the collision of two clusters of galaxies. After the collision, the stars and galaxies in those two clusters passed through each other. But the intergalactic gas clouds ...
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1answer
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Spacetime drag and galaxies' radial velocity mismatch

We know - or theorize - that a spinning black hole drags spacetime in its surroundings, producing measurable effects on the accretion disk (the minimum stable orbit radius). What I cannot grasp is why ...
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If we know the distance of distant light emitting object, how could we know the age without knowing how long the light has been here?

I have been contemplating the age of our observable universe, and I was unsure as to how we could tell the age by only dividing the distance by the constant without knowing how long the light has been ...
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1answer
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How is the energy density of radiation relatedd to the energy density of particles and the critical density?

In a solution set I was reading for cosmology I saw that the energy density of radiation is defined as $\epsilon_r=\epsilon_c-\epsilon_p$. Where $\epsilon_p$ is the energy density of particles at ...
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number density of photons

My lecturer put up as a solution to the number density of photons to be $n_{o\gamma}=\tfrac{8\pi}{c^3}\int^{\infty}_0(\tfrac{kT}{h})^3\tfrac{x^2dx}{e^x-1}$. The integral on the right hand side being ...
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What is the correct value of the matter density of the universe?

I'm studying for an exam right now and I came across a solution my lecturer gave that is really confusing me. So in finding the density of matter in the universe we can use the density parameter $\...
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1answer
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How is the co-moving co-ordinate defined mathematically in cosmology?

The relationship between proper distance and co-moving distance in cosmology is given by: $$R(t)=a(t)r$$ Where $R(t)=$ the proper distance which corresponds to where a distant object would be at a ...
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Unmagnetized plasmas in space

Please, I need some information about high temperature plasmas in astrophysical environments. To my knowledge, plasmas in astrophysical environments are magnetized (is it true?). Are there ...
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Where are the high mass White Dwarfs that are going to create type 1a SN

The mass distribution of WD stars is explained at Why is the white-dwarf mass distribution highly peaked?. However this mass distribution shows very few stars that are about to exceed the WD limit of ...
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What are the starting units to perform the Gauss method?

To calculate the Gauss method, we need : the respective geodetic latitude the equatorial radius of the body (here earth) the respective local sidereal time the respective declination the respective ...
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How would gray dust work, on a microscopic level?

I came across an article on something called gray dust, a hypothetical substance explored theoretically during the early 2000s. It attempted to explain cosmological supernova observations by ...
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2answers
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Are there super-super black holes in the center of super clusters?

If a galaxy contains a super-massive-black hole at it's center. Is there anything likely to be at the center of a super-cluster of galaxies? Or to put it another way, are there likely to be any ...
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Why do macro black holes take ridiculous amounts of time to evaporate—unlike micro black holes which dissolve in even less than a second?

Why do macro black holes take ridiculous amounts of time to evaporate, considering that micro black holes dissolve in even less than a second? Does this mass-based behavior imply that the matter ...
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How could someone calculate the coulomb barrier transparency in 3 dimensions [duplicate]

Transparency is also called probability of transmission, or probability of quantum tunneling.
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1answer
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Naive question about Gravitational collapse

Suppose that you have a probe orbting a $10M_{\odot}$ star in the final moments before gravitational collapse. In a time $t$ the collapse event occurs. So do you really would see all the matter "...
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Can mass still be observed in our universe after moving from a black-hole to a white-hole?

This question is based on the recent papers on LQG black-holes by Abhay Ashtekar. Quantum extension of the Kruskal spacetime and Quantum Transfiguration by Kruskal Black Holes When mass moves from ...
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Looking for redshift database or data

I am looking for data wich contains the redshift values(z) of observed supernova or galaxies with their observed distanced(so the distance between the object and earth how we observe it now and not ...
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1answer
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What do “broad” and “narrow” mean in the context of spectral lines?

For example, at this page: http://pages.astronomy.ua.edu/keel/agn/spectra.html it talks about some lines being broader than others. But they all look the same width to me. I feel like this is ...
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Schwarzschild Radius and Gravitational Acceleration Discrepancy

When I use online calculators or resources to determine the Schwarzschild radius of the sun, they all agree and return the same answer of just under 3000 meters. When I use various online calculators ...
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1answer
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Could a sufficiently massive Dyson Sphere under very specific conditions depend solely on electron degeneracy pressure for structural integrity?

Stuck in an argument. I claim this is theoretically possible: Provided you could build a type 1 Dyson Sphere, made from a supposed material that rivals the structural integrity of a white dwarf, i.e. ...
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Would a Dyson sphere face physical limits faced by white dwarves?

During a recent online discussion, one of the participants made the claim that a Dyson sphere would be limited in mass by the Chandrasekhar limit. Attempts to solicit evidence for this claim were ...
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1answer
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How does Sun's fusion output compare to its gravity?

How does the total solar energy output compare to the total energy required to keep it contained (i.e. Sun's total gravity). In other words, is Sun's output "positive" if its gravity is factored in ...
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2answers
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Energy requirements for relativistic acceleration

I want to visualize the energy required to continue accelerating near the speed of light. If I use some amount of energy to accelerate to 0.5c, what speed will I be going if I use the same amount of ...
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Total number density of galaxies and problematic expression in practise

I am asked to give the formal expression of the total number density of galaxies and explain why is this expression problematic in practice. From what I saw from my research and into my lectures, I ...
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1answer
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What condition needs to be met before a type of matter dominates the density of the universe

If we're given some theory about the expansion of the universe such as particles being created at some time t and having a density related to their critical density at that time for example and we ...
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1answer
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Steady solution for flow in Boussineq approximation in star [closed]

I am trying desperately to find the solution indicated in this question : If I compute the equations on the 3 axis, I can't get the flow to be directed along $\vec{e_y}$ I have only : $$\dfrac{\...
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2answers
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Rocket speed (Is the equation my choose is the correct one and is my (asumption) and calculation is correct with regard to Hawking's sentences

Hawking wrote, Exhaust speed of chemical rockets is 3 kilometer/second. By dropping 30 percent of their mass, they can achieve speed of about 0.5 kilometer per second and then slow down again. ...
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Can the gravitational field be considered conservative despite the existence of singularities?

Assuming singularities are physical objects as opposed to mathematical artifiacts, can the gravitational field still be considered conservative? And if not, does this open a possibility of breaking ...
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1answer
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What is the propeller phase of a pulsar?

I am studying this paper: Isolated thermal neutron stars, soft gamma-ray repeaters and anomalous X-ray pulsars: propellers and early accretors with conventional magnetic fields? arxiv:astro-...
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1answer
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Are the masses of merging black holes correlated?

LIGO/VIRGO recently released data on observed "gravitational wave transients", mostly BH-BH mergers. The data includes a plot of the primary and secondary mass (the primary mass is defined as the ...
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Hawking said virtual particles can almost become real particles, following closed trajectories. What does that mean?

Here's hawking quote from his latest book. Brief Answers to the Big Questions "When space-time gets warped almost enough to allow travel into the past, virtual particles can almost become real ...
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1answer
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When a star undergo collapse in a binary system what effect it has on its companion star? [closed]

when a star in its lifetime fuses up all its hydrogen and then collapse under gravitational force, till the temperature inside become high enough to restart the fusion of helium and radiation pressure ...
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Why does the Drake equation never account for the abundance of the elements in the universe?

The Drake equation which is used to describe a rough estimate of possible ET civilizations that is able to communicate with us does not take into account the abundance of the chemical elements in the ...
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2answers
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Why do we say the three-dimensional space is flat (in Physics)? [closed]

This is quote from Hawking's book: The surface of the Earth is what is called a two-dimensional space. That is, you can move on the surface of the Earth in two directions at right angles to each ...
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1answer
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Milky Way Density

It seems to be a simple question, but I wasn't really able to find an appropriate answer: How dense is the Milky Way? I am certain that there are reliable statistic, maybe even new ones from the GAIA ...
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1answer
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Dark matter relative velocity

In Dark Matter Indirect Detection literature, it is usually said that the relative velocity of dark matter particles in the galactic halo is about $$ v \sim 10^{-3}c$$ See for an example this lecture ...
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1answer
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Does the sun have nonzero electric charge? [duplicate]

The sun is a star, meaning it's a ball of gas undergoing nuclear fusion in its core. The sun, like any other star, experiences line-driven stellar winds due to the transport of ions from its interior ...
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1answer
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Bekenstein bound and black hole

I'm confused with this sentence from Wikipedia: "Upon exceeding the Bekenstein bound a storage medium would collapse into a black hole." Does that mean "the storage medium will become a black ...
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1answer
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Was a black hole formed by only one star or many stars? And can a black hole be formed by other materials (non-stars).

Although I know that a black hole can be formed by gravitational collapse of a massive dead star, I'm not sure whether a black hole can be formed by collapse of many stars.
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Rayleigh Scattering and Red Giants

Rayleigh scattering is responsible for the color of the sky. Consider a planet with an atmospheric composition similar to Earth's but orbiting a red giant. Suppose further that the planet is in the ...