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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What is the minimum amount of heat required to produce radiation? [closed]

What is the minimum heat required to produce radiation from the stars, planets, or small objects?
Janu P's user avatar
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Relativistic Fluid Equation - Density Perturbations in Cosmology

While I am deriving the relativistic fluid equation for the radiation component I am getting an extra term like $\vec{v} \cdot \vec{\nabla}P$ where $\vec{v}$ is the velocity and $P$ is the pressure. ...
seVenVo1d's user avatar
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Neutron Star formation Entropy

When a neutron star forms, is its entropy lower than when it was in the form of the star core. In the case of a black hole the entropy becomes the surface area, so what happens in a neutron star ...
MiltonTheMeme's user avatar
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Can the integrated Sachs-Wolfe and the Rees-Sciama effects have any influence on matter?

CMB photons can be affected by the expansion of the universe through the linear integrated Sachs-Wolfe effect (ISW) 1 and the non-linear ISW effect or also called Rees-Sciama effect 1. In particular, ...
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(Astrophysics) How to calculate photons detected by a radiometer over a period of 10 seconds?

I know the Flux (calculated from flux density), and frequency but i dont think I have area. This is the question, it is part d and ive done all other parts: "Consider three widely separated ...
jedijessop's user avatar
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2 answers
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Are black holes XXXL stars?

Am I right to think that black holes are stars that are so dense and heavy that radiation and matter cannot leave from it's surface?
Joel Joseph Johnson's user avatar
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Estimating the radius of saturn [closed]

I have a question on how to approach such a problem. In this task, we'll focus on the Saturn mission Cassini. The spacecraft reached its final orbit through a fly-by at Titan and captured one of its ...
Enkt Enktson's user avatar
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1 answer
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Atmospheric Pressure? [closed]

I have been struggling with this question in particular. To establish a new environment on the celestial body Orcus with mass $M = 6.35 × 10^9*10^9*10^2) $ kg and radius R = $455$ km, a plan involves ...
Enkt Enktson's user avatar
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Angular Momentum of a Dust Grain Orbiting the Sun

I am working on a problem involving a dust grain of surface area $\sigma_g$ orbiting the Sun at a distance $r$. The dust grain absorbs 100% of the Suns radiation and re emitts it in the direction of ...
kodel's user avatar
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3 answers
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What would the collapse of a neutron star into a black hole look like from the center?

I have read this question (unfortunately this mentions supernova and not black hole): If it it the latter, then the instabilities that lead to the collapse of a neutron star would begin near the ...
Árpád Szendrei's user avatar
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2 answers
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Luminosity of stars

Good afternoon, I am attempting to calculate the ratio of temperature and luminosity between two stars, one entirely made of iron and the other of hydrogen, with the same volume. To do so, I have ...
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Bunch-Davies initial condition

I have the Mukhanov-Sasaki equation in terms of $Q(t)$ \begin{align*} Q''(t)+3HQ'(t)+k^2Q/a^2+\left(3\phi'(t)^2-\phi'(t)^4/2H^2+2\phi'(t)V_\phi/H+V_{\phi\phi}\right)Q=0,\ u=aQ \end{align*} and also ...
Jules Alvarez's user avatar
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Holsapple simple scaling law [closed]

Could someone explain Holsapple's simple scaling law? Furthermore, is Holsapple simple scaling law able to be used on Earth in the context of dropping an object and measuring the impact crater size? ...
Imstrugglingomd's user avatar
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New models for extreme scales? [duplicate]

Physicists struggling to explain too-fast spinning galaxies with standard models of gravity is weird to me. If we can accept that normal physics don't apply on a quantum level why shouldn't the same ...
Fayjakin's user avatar
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How do we know that the Sun is 71% Hydrogen by mass from emission/absorption spectra?

In my Intro to Astronomy ("intro" is very important, please keep responses as simple as possible) course, we're currently learning about light and electron orbitals and such, and I came ...
Sami Hanna's user avatar
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Deep learning in astrophysics reference request

I am looking for an introductory and possibly comprehensive book/review article for deep learning techniques in astrophysics. I don't know much of the argument, so it should start from the very basis ...
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Is there a galactic "goldilocks" region in the galaxy

I'm wondering if there's a region where the star density in the galaxy create the conditions in outer space where the galactic temperature is between 0 and 200°F. This may cause a ring shaped where ...
Matt Staab's user avatar
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How do we realistically calculate the monochromatic flux of an object in space?

If the collecting area of a telescope is ∆A, and it collects light for a time interval of ∆t, do we just build a telescope with a very small ∆A and make ∆t and ∆λ very small as well? Also, how do we ...
Maxine's user avatar
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Strong Equivalence Principle and Milgrom's Law

I am reading this paper by Milgrom(1983) that suggests a modification to Newton's Second Law in order to do away with the requirement of dark matter on astrophysical scales. My question is regarding a ...
Ambica Govind's user avatar
2 votes
1 answer
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Einstein's derivation of stellar aberration formula: replacing the direction of the ray with a difference in angles justified?

I was going through the translation of Einstein's 1905 paper "On the Electrodynamics of Moving Bodies"(See https://einsteinpapers.press.princeton.edu/vol2-trans/175). In deriving the stellar ...
JKrsl's user avatar
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4 votes
1 answer
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Is the amount of dark matter per galaxy the same as you look back through time (further away from earth)?

In the hope that it may inform us about the development/evolution (if any) of dark matter over time, are there any differences (eg. in structure or concentration) in the dark matter at large radial ...
Zinn's user avatar
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6 votes
1 answer
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What is the albedo of our Sun?

I think the Sun is the closest to a perfect natural black body we can get in the Solar System, if so then what happens to the light if I point a laser pointer at the Sun?
user6760's user avatar
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What mass fraction of a main sequence star produces energy?

Only some fraction of the total mass of a main sequence star produces energy in the star's centre through the pp-chains, or in heavy stars, the CNO-cycle. My question is about the mass fraction where ...
gamma1954's user avatar
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Is there any tools for observing magnetic fields and forces of black holes and galaxies?

I want to know about how we can find magnetic forces and fields in space around stars, planets, galaxies, pulsars especially pulsars. How (by what means) did determine magnetic fields around a pulsar?
QQQ's user avatar
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Is cosmological redshift associated with recession velocity when the light left or when it arrived?

Is the cosmological redshift $z$ associated with the recession velocity when the light left, when it arrived, or something in-between?
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Gravity formula inside a planetary core

I am trying to work through this problem so that I can understand how to convert from pressure values to radius values inside a planetary core in a code. The core has variable density depending on ...
Priya Bose's user avatar
10 votes
2 answers
4k views

Where is there visible light inside the Sun?

The solar core has a temperature of 15 million K, but the visible color temperature is only between 1000 K and 10000 K. Also, the plasma is very dense at the core, so it won't be able to travel there. ...
Jostein Trondal's user avatar
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Finding wave vector using Data

I have 50 data for each velocity components ux, uy in vector form in two columns. so $\mathbf{U} = u_x\hat{x}+u_y\hat{y}$. Moreover, $\mathbf{U} = \mathbf{U_0} + \delta \mathbf{u}$ where the ...
Tasnim's user avatar
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Broad and narrow line regions of active galaxies: is the matter orbiting a massive black hole in the state of plasma?

The broad and narrow line regions (BLR and NLR) of an active galactic nucleus (AGN) are commonly described as regions of gas emitting atomic spectral lines whose width is broadened - via the Doppler ...
cosimoNigro's user avatar
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1 answer
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Gravitational waves and EOS in neutron stars

What is/are the gravitational wave observable/s that makes possible to measure the parameter/s of the equation of state of neutron stars?
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What is the difference between granulatiion, mesogranulation, supergranulation and giant cells in sun's photosphere?

Help me understand the differences between granulation, mesogranulation, supergranulation, and giant cells and at what depth they are created.
Devesh Sharma's user avatar
9 votes
5 answers
6k views

How can micro black holes exist?

How could a micro black hole exist, when there is so little matter to produce the intense gravitational force required to crush matter to that extent? It takes the collapse of a supermassive star to ...
Peter R. McMahon's user avatar
1 vote
1 answer
27 views

Spectral line in the gamma range of the gamma-ray burst

I would like to know about any research that is conducted to get spectral lines in the gamma range of gamma-ray burst. I try to find data or information about these typical spectral lines. Is it ...
Edy A.'s user avatar
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Does the Earth's magnetic field lag behind the Earth as Earth orbits the sun?

I imagine that when the earth orbits the sun, the earth's magnetic field is also subject to the sun's gravity, since photons and light are subject to gravity. As a result, the magnetic field does not ...
garmichaels's user avatar
1 vote
1 answer
77 views

Measured Value of Hubble's Constant [duplicate]

As we know, Hubble's Constant is not exactly a constant, but its value varies with time. However, we also speak of measuring its present-day value. The measurements by cosmic distance ladder would ...
V Govind's user avatar
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What is a simple way to estimate $σ_{RV}$ assuming photon noise in radial velocity measurements?

Hello there i have been trying to figure out what is wanted in the question "Simple way to estimate σ_RV assuming photon noise" for a presentation. But i can not seems to find much, but i ...
GodOfDemoting's user avatar
1 vote
1 answer
36 views

Why can we not describe matter as an ideal gas when the gas is degenerate

When a low mass stars core mostly consists of helium (after years and years of fusion reactions) its thermal pressure decreases and so the core gets denser due to gravitational pressure. Why is it ...
selin's user avatar
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Numerical solution for the Mukhanov-Sasaki equation with Bunch-Davies vacuum state

I have the Mukhanov-Sasaki equation in terms of $u_k$ \begin{align*} u''_k(\tau)+\left(k^2-\frac{a''(\tau)}{a(\tau)}+f(\tau)\right)u_k(\tau)=0 \end{align*} and also the initial condition from Bunch-...
Julian Yussef's user avatar
6 votes
2 answers
110 views

Thermal Radiation from Neutron Stars

I'm a little bit confused over the essence of thermal radiation. The thermal radiation is electromagnetic waves, and EM waves are generated from the acceleration of charged particles. So I feel that ...
tomas feng's user avatar
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0 answers
43 views

How to select proper values to solve Bunch-Davies initial condition?

I am trying to solve the Mukhanov-Sasaki equation for a Starobinsky potential $V(\phi(t))=3/4m^2M_P^2(1-e^{-\sqrt{2/3}\phi(t)})^2$. I could change the equations so that $m^2$ does not appear and chose ...
Julian Yussef's user avatar
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0 answers
46 views

Ejected bodies, dynamical friction and dark energy?

I have a question after reading a couple of papers (https://arxiv.org/abs/1707.06220 and https://www.arxiv-vanity.com/papers/1102.0007/). Here, the authors seem to indicate that when bodies like stars ...
vengaq's user avatar
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3 votes
2 answers
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Could Earth's Zircon (used in geological dating) have formed in extraterrestrial events before the Earth was formed?

This is a follow up question asked and answered (Radio-dating and the age of the earth) The answer was given that the mineral Zircon is formed under high pressures and temperatures. How do we know, ...
Lucas's user avatar
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1 vote
2 answers
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Is Each Day the Same Duration?

Since Earth's orbit is elliptical surely each day does not have the same duration. My understanding is that at the perigee the Earth would have to rotate more than at the apogee due to its larger ...
Nullity's user avatar
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GRB 221009A Comparison with solar flare NOOA scale

I'm trying to found a comparison of the energy received on Earth by GRB 221009A in comparison of various solar flares based on NOAA Space Weather Scales (or other). I failed to find any comparison (...
Vincent ISOZ's user avatar
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Is differential number density the same as flux density?

I'm trying to convert the differential number density of photons to luminosity (using slide 17 of https://www.astro.rug.nl/~sctrager/teaching/OA/Photons.pdf) and ...
John's user avatar
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How much radiation on the surface of the moon during a Coronal Mass Ejection (CME)?

I'm writing a book set on the moon. A Coronal Mass Ejection (CME) as large as the Carrington Event hits it. Three astronauts are caught out during an EVA. They will cover their rover in regolith to ...
L.R. Lam's user avatar
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1 answer
32 views

Can a receding body due to the accelerated spacetime expansion be "rescued"?

Once a body "crosses" the limit where dark energy wins over gravitational forces (Is there a distance from a gravitational source where the influence of gravity and dark energy are balanced ...
vengaq's user avatar
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How can I calculate the column density of Hydrogen of a galaxy?

I have the following data: Mass of HI of the galaxy and radius. I thought using the following equation: $N_{HI} = \int n_{HI} ds \quad$ where the number density would be $n_{HI} = \frac{M_{HI}}{m_H V}$...
MJ_'s user avatar
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2 votes
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Would ocean tides on the moon of a gas giant really be kilometers high?

I was reading an article on exomoon habitability constrained by illumination and tidal heating. The article imagines an Earth-like exomoon around Jupiter-like host planet. That got me thinking about ...
Boreal Stars's user avatar
9 votes
1 answer
1k views

Nuclear fission in the Sun

The Sun's energy comes primarily from fusion of light elements in its core. It is estimated that a very small fraction of mass of the Sun (~$10^{-12}$ times the abundance of hydrogen) is uranium (both ...
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