# 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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?
$$\rm {}^{12}C +{}^{12}C\rightarrow {}^{20}Ne + {}^4He$$ why doesn't this reaction occur on earth? but occurs in big stars?
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 ...