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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33 views

Intensity mapping of the 21 cm line

I'm currently reading Astrophysics for physicists by A.R. Choudhuri. Section 6.5 of his book he presents the following plot: and tries to explain how it was generated. I'm having a hard time ...
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847 views

When we are talking about black hole evaporation - what exactly happens?

According to Wikipedia: Hawking radiation reduces the mass and energy of black holes and is therefore also known as black hole evaporation. Because of this, black holes that do not gain mass ...
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Does the direction of Earth's axial tilt change over time? In relation to the sun? What about the 'pointy ends' of our ellipse?

Does the direction we are tilted change in relation to the background stars? And the points where our ellipse is pointiest? (most elongated?) Does this change the timing of our aphelion and ...
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29 views

Stellar Aberration

In Stellar Aberration Experiment how do I know there is a star above my head before doing the experiment? Reference: Special Theory of Relativity - Resnick (Section-1.7)
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Can a planemo turn into a star?

I read this question Why is planet CFBDSIR2149-0403 hot? and wonder what will happen to this "planemo". Will it attract more mass as it flows around in the gas clouds in space and eventually light ...
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42 views

How hot can the surface of a planet be?

I just read this article https://edition.cnn.com/2018/07/02/world/newborn-planet-image-study/index.html and noticed that the astronomers estimates the surface temperature of the planet to 1000 °C. ...
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Relation between oscillator strengths $\log{gf}$ and the Einstein $A$-coefficient, rotational quantum number, …

I'm analyzing the spectra of M dwarfs. Some authors characterize the lines with the oscillator strengths $\log{gf}$ while others follow different paths with the Einstein $A$-coefficient. I would like ...
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34 views

Can Thorne–Żytkow object (heavenly body) exist in universe?

Can Thorne–Żytkow object (heavenly body) exist in universe? It's a new concept I came across. Are there such heavenly bodies in universe? It's a heavenly body in which a neutron star is present inside ...
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50 views

Where did Nitrogen come from in the Universe?

Does nitrogen come from nucleosynthesis or spallation?
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Relation between distance, sizes and angular diameter?

If we pick up a ruler and look at it and at the sky at the same time we can have the moon be completely inside the ruler or if we add distance between the ruler and us the moon will become apparently ...
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36 views

Sun’s corona is so much hotter than Chromosphere [duplicate]

What makes Sun’s corona so hot? I tried to google it but can’t find satisfactory answer.
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42 views

Is time relevant to mass of the planet or the star the planet is part of?

For instance, if we travel from a planet in one star system to another where there is a huge difference in mass(neutron star) for both the planet and the star compared to the other system (planet and ...
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58 views

Why sky is blue? [closed]

The major factor of different scattering is the ratio of wavelength to the size of particles which are working as microscopic scattering mirrors. In a sparse particle medium like air, the longer the ...
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Which is the most imminent doomsday? [closed]

By "doomsday" I define something which is caused by nature, given that the technological advancement of humanity stays the same as today, and also disregarding possible resource shortages. For ...
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1answer
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How do I transform flux density into temperature?

I have flux density data from Planck mission (in Jy) - measure in a certain frequency - and I would like to transform it to temperature data (Kelvin). I'm not sure if I should use Stefan Boltzmann's ...
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Is the expansion and contraction of the Universe an endless repeating cycle due to Newton's 3rd Law of Motion? [closed]

If the creation & expansion of the Universe was an action, then the reaction to this action is that the Universe will contract and be condensed back into a singularity. If the contraction of the ...
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36 views

Database of Galactic Mass?

I'm currently working on a paper investigating any relationship that might exist between the mass of the AGN, or more specifically the relativistic mass at its center, and the mass of the host galaxy. ...
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146 views

Black Holes for Beginners

I’m studying a PhD in Theoretical Physics (quantum field theory, to be specific), but I would like to learn more about black holes (and related areas. Does anyone have any suggestions for textbooks/...
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Spiral galaxy stability

Is there a limit of baryonic masses $M_{B}$ ($M_{B}=M_{\star}+M_{g}$), beyond which a spiral galaxy is no longer rotationally supported? Like for example: Could spiral galaxies of baryonic masses $M_{...
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What do fringe patterns have to do with VLBI?

I think I have fundamental confusions with how VLBI works. I don't know why: Resolution increases when incident rays meeting to a focal point from a farther separation along an axis perpendicular to ...
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112 views

Schwarzschild Black Holes

Page $190$ of “Tensors, Relativity, and Cosmology”: Consider a particle falling radially into a black hole with a radial velocity $u^1=dr/ds$. As the particle is falling radially, we have $u^2=u^3=...
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What is the importance of sub-Keplerian component in accretion flows around compact objects?

In advective accretion disks around compact object, the flow is assumed to have both Keplerian as well as sub-Keplerian component. If the flow is perfectly Keplerian, then $v_r=0$ and $v_{\phi}=\sqrt{...
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Role of sub-Keplerian component in accretion flows around compact objects

In the study of accretion flows around compact objects, particularly in the Two Component Advective Flow (TCAF) model as proposed in this paper, the authors differentiates three distinct components of ...
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Am I right in thinking that black holes are not infinitely dense since they grow in size? [duplicate]

Something that is infinitely dense does not need to grow in size since they can pack an infinite amount of matter in a single point. So the fact that black holes grow in size means they are not ...
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Percentage of Neutron Stars that leave the Galaxy

When a neutron star(NS) is born the supernova kick gives the newborn NS a certain boost in a supposed isotropic direction and, depending on how strong the boost is and on the position of the NS at ...
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How can I deduce a probability using a Kroupa IMF and a set of observed stars along a model isochrone?

I’m plotting a colour magnitude diagram for an open cluster and setting it against a few model isochrones. I want to deduce the probability that the cluster belongs to one or the other isochrone, but ...
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Intermediate product of CNO cycle

CNO cycle of stellar nucleosynthesis involves several steps in which different isotopes of Carbon, Nitrogen, and Oxygen are transmuted into each other when hit by a ...
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1answer
50 views

How can a gas be in circular orbit when centrifugal force is dominant in an accretion disk around a black hole?

In the textbook "Black Holes, White Dwarfs and Neutron Stars: The Physics of Compact Objects" by Shapiro and Teukolsky, the author makes the following statement while discussing the standard thin disk ...
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1answer
55 views

How can a non-rotating black hole or singularity be created?

Every star or other massive body in the universe rotates, if only a little. If such a body collapses, its spin, any spin at all, and thus, angular momentum approach infinity as r approaches 0. Angular ...
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29 views

Texts on Hoag's Object

Are there any good texts online that someone can recommend on Hoag's Object? It is a very weird galaxy and I would like to learn more.
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How can Sun burn in space?

If there is no oxygen in space how can sun burn in space. (even though I know fire is caused by nuclear reaction inside sun)
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Do solar energetic particles follow the Parker spiral?

I understand that the solar wind follows the Parker spiral shape of the Sun's magnetic field, thus it is possible for solar wind from a point on the opposite side of the Sun to reach Earth. I have ...
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16 views

Which “straight line” should I travel to maximize the number of temperature gradient reversals I experience?

Fire a neutrino from whatever starting point and in whatever direction you like in the universe. As the neutrino travels, it will traverse many temperature gradients in its ambient environment. What ...
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Disk-like structures spectra vs ring spectra?

In the introduction to this paper (https://arxiv.org/abs/1803.06423) it is mentioned that disk-like spectra was observed in black hole X-ray binaries, when models would expected to be unstable (...
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61 views

Would the Doppeler effect be incorrect to use as evidence to support an expanding universe?

In our science class we have a question to answer, which is 'explain how redshift supports the idea of an expanding universe'. I am not sure how to answer this question, as I know that there are '...
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Rotating and precessing magnetic dipole

I am considering the problem of a rotating and precessing magnetic dipole (my final aim is to study the effect of the magnetic field on the production of gravitational waves in a binary neutron star ...
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38 views

Are wet mergers good for star formation?

In my astrophysics lecture my professor said wet mergers are bad for star formation since it often results in gas being fed to the black hole. In the wiki however it says the opposite. What is the ...
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28 views

Why are most of the stars in the central bulge of a galaxy?

Just wanted to know if this is because of gravity initially clumping most of the matter at the centre of the galaxy during its formation or some other reason? It was never really discussed in my ...
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39 views

Why do stars start off burning deuterium?

Given hydrogen is a lighter and more abundant element, why do most baby stars start off burning deuterium?
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31 views

Is there any time difference when we see distant a object? [duplicate]

I was just wondering that We see objects thru our eyes and light has to reach our eyes so that our brain can process that light. But then there must be a time difference between that object which I ...
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Adiabatic and isothermal astrophysical flows

Astrophysical flows around black holes (like accretion and winds) can be adiabatic as well as isothermal. In adiabatic flows, the flow is non-dissipative except at the shock location. However, in case ...
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122 views

Why physical / cosmological models that have been falsified weren't abandoned? [closed]

Before anything, I'd like to say that I'm a layman (non physicist) and english is not my main language; I apologize if my choice of words make me sound rude / arrogant, that is not my intent. I often ...
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Deriving equations for Equatorial -> Galactic transformation via Euler Angles

As the title suggests, I'm looking for a derivation of the transformation equations between the equatorial and galactic coordinate systems via Euler angles. $\cos{b}\cos{(l-l_{0})} = \cos{\delta}\...
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What affects the propagation of secondary cosmic rays?

Primary cosmic rays produce, upon entering the Earth's atmosphere, a whole load of secondary particles. These primary particles are necessarily stable particles such as protons, electrons, and ...
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What is the relationship between the colour of a star and its mass to light ratio

We were talking about methods to measure the mass of a galaxy and one way was to measure the mass to luminosity ratio and multiply by the luminosity. That seems like a trivial equation but apparently ...
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Why is most of the star formation that goes on in the milky way occurring in the spiral arms?

This fact was mentioned by our astrophysics lecturer and I can't seem to understand why. Isn't the densest and hottest part of our galaxy the central bulge? I would've thought star formation would ...
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1answer
147 views

Can a gas cloud of pure helium collapse and ignite into a star?

Assuming there could be a giant gas cloud with negligible amount of hydrogen and metal (elements with atomic number $Z\geq3$), could it collapse gravitationally and form a pure helium star that would ...
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59 views

Was Titan once warmed by a hot Saturn, & therefore much more Earth-like than it is today?

Is it possible that 4 billion years ago, Saturn was sufficiently hot to act as a second sun to Titan, so that conditions on Titan were then similar to those on Earth?
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108 views

Explain to me why the following reaction can happen in big stars but not on earth

$$ \rm {}^{12}C +{}^{12}C\rightarrow {}^{20}Ne + {}^4He $$ why doesn't this reaction occur on earth? but occurs in big stars?
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Are / Why are the H II regions positively charged?

One of the questions in our recent astrophysics course homework was to find the general opacity $\kappa$ of H II regions. We know that the H II regions are almost entirely ionized hydrogen and I ...