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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Introduction of a factor $\Delta\ell$ when summing equal distants $C_\ell$

I have the following formula which is the error on a $C_\ell$ : $$\sigma_(C_{\ell})=\sqrt{\frac{2}{(2 \ell+1)\Delta\ell}}\,C_{\ell}\quad(1)$$ where $\Delta\ell$ is the width of the multipoles bins ...
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If black holes never form (like M87) then does this mean that theoretically we could decode light and information that is still coming out of them?

I have read this question: This isn't some accounting trick, it means we will never see an event horizon form. At this point someone will usually pop up and say that means black holes don't really ...
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Poisson noise for a squared contrast of density - expression of variance equal to $1/n^2$

I manage to compute the Poisson noise of a density field like this : If I take $N$ the density of galaxies and compute the Shot noise with a Poisson distribution, I get : $\langle N^2\rangle - \...
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When Sun was forming and before nuclear reactions inside the core what should have been its surface temperature?

When the Sun was forming and before all the nuclear reactions started inside its core, what should have been its surface temperature (only due to gas friction)?
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Is a fusion process possible inside an accretion disk?

Is a fusion process possible inside an accretion disk? As the speed of matter inside an accretion disk approaches the speed of light can an incoming object collide with the disk material and cause a ...
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Are there any subtle differences in the gravitational waves emitted from NSNS, NSBH and BHBH of identical masses?

LIGO has detected several NSNS and NSBH merger events. However, it’s difficult to tell their identities directly unless the neutron stars are very light in an NSNS merger (such as GW170817) or the ...
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What’s the maximum amount of angular momentum a neutron star can carry?

For Kerr black holes, the spinning rates can be described by a normalized spin parameter $a=cJ/GM^2$, which takes any values between 0 and 1 regardless of the masses of the black holes. If we use the ...
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2answers
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Trajectory of an asteroid hit by a micro black hole? [closed]

I am not asking about how long it would take for the black hole to acquire the mass of the asteroid. I am specifically asking only about the trajectory of the asteroid (or the merged object), how that ...
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The shape of stars that have anti-gravity [closed]

When a star has a negative acceleration, what will it look like? Can it get a spherical shape?
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70 views

Can the amount of heavier elements in the Sun's core be approximated?

Can the amount of heavy and other types of elements as impurities inside the Sun's core be approximated? If these elements are present, do they disturb the process of hydrogen fusion, since they ...
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What would be the likelihood that when the Milky Way formed, it would be composed entirely of sun-like stars? [closed]

So, what would be the likelihood that when the Milky Way formed, it would be composed entirely of sun-like stars? What was the configuration of the gas, the history of the primordial stars, for this ...
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If event horizons (black holes) never form in a finite time, then how can they merge to create a common event horizon in a finite time? [duplicate]

There is another question about this topic, but that is asking about gravitational waves, and I am not. I am specifically asking about the timescale of the merger being finite (actually quick), ...
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77 views

How do I calculate Earth's velocity?

How do I calculate Earth's velocity (on its elliptical orbital path, as a function)? I know how to calculate it assuming that Earth's orbital path is circular but I want to know what we would do if we ...
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How much of the suns radiation is infrared from compression heat, not fusion?

Preface: Im a college level dropout, please dont stomp on my ignorance too hard. As mass falls into a gravity well, it compresses and heats up. Even if fusion didnt exist/occur, in a body as large as ...
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Minimum mass of asteroid that could knock moon away [closed]

What is theoretically the minimum mass of asteroid, coming from deep space, that could knock the Moon away from Earth? The asteroid is not hitting the Moon, just interacting with it by gravity. I was ...
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Difference between physical, proper, and co-moving distances

Eq. (1.14) and (1.15) of David Tong's lectures (p.g. 12) writes: Consider a galaxy which, in co-moving coordinates, traces a trajectory $\vec{x}(t)$. Then, in physical coordinates, the position is $$\...
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Does the ringdown phase of a black hole merger ever stop?

When binary black holes merge they emit gravitational waves in three stages, the inspiral in which the two black holes shed angular momentum through gravitational waves at a rate that becomes ...
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What are some values for the scale factor $a(t)$ in cosmology?

In cosmology, the scale factor is given in $D(t)=a(t)D_0$, where $D(t)$ is the proper distance at epoch $t$ and $D_0$ is the distance at reference time $t_0$. What are some of the values that $a(t)$ ...
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Gravitational wave and 1st law of thermodynamics

Introduction: A prediction of the general relativity is that any moving mass produces fluctuation in the space-time fabric, commonly referred as Gravitational-Wave. This prediction was recently ...
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What would happen if an asteroid-sized primordial black hole encountered a very large star?

What would happen if an asteroid-sized primordial black hole encountered a very large star? I thought about this and Googled for answers but got no results. Maybe I'm asking the wrong question. I know ...
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1answer
148 views

Demonstration: Integral (discrete summing) of $C_\ell$ has a better variance than one single $C_\ell$

I am working in a cosmological context where I use the $C_{\ell}$ quantities coming from Legendre transformation. I am faced to a issue to prove the gain that we get by computing the variance of an ...
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1answer
88 views

Neutron star forms after merging of white dwarfs?

In the news report 'A White Dwarf Living On The Edge' (Keck Observatory), Ilaria Caiazzo writes about a $1.35 M_\odot$ white dwarf which was formed by the merging of two less massive white dwarfs: “...
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What is the size of the magnetosphere of a neutron star?

More precisely, what are the factors that influence that size (mass of the star, spin velocity, accretion disk around it, ... do any of those things matter?) I tried reading this. I cannot: [paper ...
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1answer
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Finding the dominant frequency of a stellar orbit

I was reading this paper on astro-ph (Sanders et al. 2020, "Models of Distorted and Evolving Dark Matter Haloes"). It's about modelling the gravitational potential in a cosmological ...
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Would we even notice the merger with the Andromeda Galaxy?

I have read this question: When galaxies collide it is not that their stars crash into each other, because their individual cross-sections are extremely small when compared to the space between them. ...
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Effect of the Sun on the motion of bodies on the Earth

Does a body at the surface of the Earth fall at a slower rate when the Sun is at its zenith (that is when the side of the Earth that the body is on is turned towards the Sun and the body is directly ...
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Lifetime of pulsars

According to many textbooks, pulsars have lifetime (the time it would take the pulses to cease if $d(P)/dt$ were constant). Lifetime is $P/[d(P)/dt]$, $P=$period Why do P/[d(P)/dt] means the lifetime?
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Relationships between G magnitude, Johnson V magnitude, and spectral type of stars?

I'm simulating a population of binary stars for a summer research project. I'm adding uncertainties to my simulated observables such as angular position and radial (line of sight) velocity. I'm using ...
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Has an object been observed falling inside the black hole in direction antiparallel to an astrophysical jet?

Has an object been observed falling inside the black hole in direction antiparallel to an astrophysical jet? I do not know if the jet is fighting against gravity or not so will that part around the ...
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1answer
49 views

Applying Gauss law to a disk-like distribution

In an paper titled: "Mass Distribution in Rotating Thin-Disk Galaxies According to Newtonian Dynamics" by Feng and Gallo: https://www.mdpi.com/2075-4434/2/2/199/htm the fourth paragraph of ...
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Electron particle density

I was solving an exercise in which I was asked to find the particle density for photons and electrons right after the big bang. I used the two following formulas: $(\frac NV)_{BE}=g(\zeta(3)/\pi^2)(KT/...
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How can supersonic accretion flows exist in astrophysical scenarios?

I often hear about supersonic accretion flows in astrophysical scenarios: for example, the inner parts of the accretion disk around a supermassive black hole or cold filaments accreting into galaxies ...
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Age determination of stars

In the report 'New observations of the most distant galaxies close in on cosmic dawn', an astronomer states: 'Using a well-understood age indicator based on the amount of hydrogen absorption seen in ...
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1answer
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What are the exclusion limits on WIMPs?

As I understand the way dark matter WIMP exclusion plots work, they are cross-sections (usually 2D) of the fuzzy boundary surface of a hyperdimensional property space, defining a fuzzy volume of that ...
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Computation of standard deviation on $C_\ell$ with fraction of sky $f_{\text{sky}}$ and $\Delta\ell$

I try to demonstrate the expression of error on a $C_{\ell}$. Below the expression of standard deviation on a $C_{\ell}$ that I would like to get: $$\sigma_{C_{\ell}}=\sqrt{\dfrac{2}{(2 \ell+1) f_{\...
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Is it correct that a sphere of water the size of Venus's orbit would instantly be a black hole? [duplicate]

Let's say I have a genie and wish for a sphere of water at earth density the size of Venus' orbit - would it be instantly indistinguishable from a supermassive black hole due to the schwarzchild ...
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What's the minimum possible size of a round astronomical body?

What's the minimum possible size of a spherical shaped rocky planet or satellite? Is it possible to occur a spherical shaped planet without having a molten core?
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What is the pressure that dark energy exerts on stars and planets while increasing the distance between them?

What is the pressure that dark energy exerts on stars and planets while increasing the distance between them? I can understand that the pressure should be at a volume scales lower than molecules ...
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35 views

Why are wide binary systems assumed to contain negligible dark matter?

I'm doing a project that simulates wide binaries to do gravity tests, specifically a dark matter vs MOND test. I've come across similar papers, and their justification for using wide binaries for this ...
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May a star explosion be in that way weak that the expelled material gradually recollapse and form the star again?

May a star explosion be in that way weak that the expelled material gradually recollapse and form the star again? My question is not about would that explosion leave a neutron star as a remnant but ...
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How does the density of main sequence star impact spectral lines and the depth/width of a spectral line graph?

My understanding is that a denser star will cause the width of a spectral line to broaden and it will cause the depth of the intensity plot to increase (dip lower). Would this be correct? Also, is ...
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Graph Interpretation of Gravitational Waves

In the image is the data recorded by the LIGO's 2 observatories in USA. What is its interpretation? I mean what does the zig zag lines represent? Similarly, what does the blond red and blue lines (...
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Relationship between 3 quantities: Density Matter power spectrum, Density Matter angular power spectrum and Temperature angular power spectrum

Summary: I would like to go deeper in the relationship between Matter power spectrum and Angular power spectrum. From a previous post about the Relationship between the angular and 3D power spectra, I ...
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What is the heaviest element being produced in Sun?

Helium is being produced currently in Sun. 3 questions. Other than Helium what other elements are currently being produced in Sun right now.? Looks like in 5 billion years, when Sun will become a ...
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3answers
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Why does relativity affect relativistic jets?

Relativistic jets are regions outside black holes where matter is spewed out. Wikipedia states that these jets will demonstrate relativistic effect such as relativistic beaming. I don't understand why ...
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Compute the variance of spectroscopic Shot Noise in cosmology context

I would like to know the right expression for the expression of variance of Shot noise in spectroscopic probe. Sometimes, I saw $$\sigma_{SN,sp}^{2} = 1/n_{sp}$$ with $n_{sp}$ the average density of ...
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Four Momentum in an Isotropic and Homogeneous Universe

We define the four-momentum of a particle by $p^\mu = (E, \mathbf{p})$, where $E$ is energy and $\mathbf{p}$ the three momentum of the particle. In cosmology, we conventionally use the Friedmann-...
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39 views

Interaction of supersonic wind with static Inter-Stellar Medium (ISM)

Recently I was studying about the interaction of supersonic flow (wind) with a slowly flowing medium. In a paper by Dyson, Falle, and Perry 1980 MNRAS stated that when a supersonic wind impinged upon ...
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Are the physical structures in our sun of comparable complexity to those in the human brain? [closed]

The writings of Rupert Sheldrake tend to provoke strong emotions, be they ridicule, curiosity, outrage, sympathy, disgust, or otherwise. While Physics SE is not an appropriate forum in which either to ...
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Cosmological models of the expansion of the universe for different curvatures

My question refers to page 38 of Tong's Notes on Cosmology. Here we consider a matter dominated universe and derive the equation $$2h^\prime + h^2 + k = 0$$ for $h = a^\prime/a$, where $^\prime$ ...

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