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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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How much in the past are we viewing the accretion disk of a black hole?

Sine the gravity near a black hole is so high, the time moves slower than the observer(in our case we from the earth).In that case, imagine if are viewing a black hole's accretion disk(the innermost ...
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Stability of a 10km planetoid with a micro-black hole in its centre

Let's assume there is a tiny, 10km-radius planetoid with a micro-black hole in its centre of about 10^19kg, so the surface of the planetoid is at 1g. How it appeared (or was constructed) would be the ...
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Is the surface temperature of the sun constant?

I was just wondering that during the suns mains sequence would it maintain a constant temperature at the surface and only cool once the hydrogen in its core runs out, or is it always slowly decreasing?...
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What does the line strength mean in terms of spectral lines?

In my previous astrophysics lecture we were discussing the spectral types of stars. We were looking at the absorption features of dwarf stars in different spectral classes at a particular luminosity ...
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2answers
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Why did it take so many telescopes to capture the image of the black hole?

Recently, we got the first picture of a black hole. From what I heard, it took multiple telescopes to capture the image and combine the pictures captured by each telescope. Why did we need to use ...
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How is the local dark matter density in the Milky way measured?

The answer in this post gives the local dark matter density in the Milky way. How is this local density measured? What is the physics involved?
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What is the relationship between the absolute magnitude and power output of a star?

I can't seem to find a straight answer online. Is the absolute magnitude of a star equal to / linearly proportional to / or unrelated to the power output of the star. When I say absolute magnitude I ...
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Are pressureless, collisionless and self-interacting dark matter all synonymous?

Is there any distinction between pressureless, collisionless and self-interacting dark matter or does being one imply the other?
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Wind-fed Accretion and White Dwarf stars

Can a White Dwarf star in a binary system accrete matter via the Wind-fed accretion process?
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Do different neutron stars have different density?

When stars go supernova, a neutron star may be left over if there isn't enough mass to create a black hole. Based on the mass present, will a neutron star have a varying density or is the radius ...
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What is the current status of interpretation of $130$ GeV photons from the galactic center?

The observed spectrum of photons coming from the galactic center has a sharp line at $130$ GeV which for some time was thought to be a signature of WIMP annihilation. But the bounds on WIMP cross-...
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What molecular distribution models apply for nonequilibrium systems as cold stars?

Maxwell-Boltzmann distribution function describes well the distribution of ideal particles in thermal and chemical equilbrium (the concentration of the species is constant). Which models would apply ...
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Why was the imaging of M87 black hole easier than imaging the Milky way's Sagittarius A*? [duplicate]

In the recent EHT press release of the image of the super massive black hole at M87, I am curious to know why the super massive black hole at the centre of milky way has not been imaged yet.
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Orange ring in a black hole image

What exactly is the origin of the orange ring around M$87$? I understand that the image was not taken in the visible light range. The colors are therefore artificial. I also read that the image ...
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Can star such as our sun produce astrophysical jet?

Must there be an accretion disc for astrophysical jet to form? Does it have anything to do with how massive the star is? Since it is already a hot plasma and produce it's own magnetic field, can't it ...
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Is there a relatively painless way to calculate the radius of a neutron star, given its mass?

I know there are different models based on, as I understand it, different equations of state describing the neutron star mass-raduis relation. I'm not sure what to do with that, though. My math isn't ...
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The Magnetic Field Strength of Crab nebula

Consider the spectrum of the Crab nebula shown below. Assuming that the ~TeV photons are produced by Inverse Compton scattering of synchrotron photons from around the peak of the synchrotron ...
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Using the flux of Bremsstrahlung to analyse medium [closed]

An HII region at a distance of 400 pc is observed to emit thermal Bremsstrahlung across a wide range of radio wavelengths. Independent analysis of emission lines suggests that the characteristic ...
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The precision of a gravitational wave interferometer [closed]

The latest design of the space-based interferometer LISA will search for gravitational waves using three telescopes separated by arm lengths of 2.5 million km in a triangular configuration. The LISA ...
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1answer
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Light emission from the vicinity of a black hole [duplicate]

The photograph of the black hole, at the centre of galaxy M87, is now a part of human history. We are told that the red-colour is caused by hot gas, matter & radiation around the "edge", the Event ...
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2answers
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How do you calculate the black hole diameter of M87*?

The recent data from the EHT Consortium on the size and mass of the central black hole of M87, named M87*, are telling us that the diameter of the event horizon ...
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2answers
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How to estimate the temperature required for the 2nd energy level of hydrogen to be occupied?

In my atstrophysics lecture today we were covering atomic spectra and we saw that at both the hot and cool end, the atomic spectra of stars showed a very weak balmer series. The reason being at the ...
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Is Sun brighter than what we actually see?

I learned from that plasma can reflect radiations of frequency less than that of its own oscillations. If so, considering the plasma in Sun's atmosphere, it should also reflect solar radiations. That ...
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How can a black hole not be a perfect sphere?

As is stated or illustrated in many different articles, including this one, rotating black holes are oblate spheroids. It is likewise stated in some sites that this fact should be intuitive, as all ...
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First Black Hole Picture Takeaways [closed]

We just have obtained the first "picture" of a black hole, the supermassive blackhole at the center of galaxy M87. As spelled in this approved answer, there were multiple unsolved questions to be ...
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Messier 87: What am I looking at? [closed]

I'm sure in the last ten minutes you've already seen this very cool image of Messier 87: But what am I looking at here? In particular, is there a reason why it looks dark in the middle? Are we ...
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1answer
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Is it possible to form a toroidal star?

Perhaps a fast spinning star (not spherical) where a black hole forms in the center, but the event horizon is smaller than the equatorial radius but larger than the polar radius?
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2answers
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How can cosmic jets exist? [duplicate]

Cosmic jets are jets of matter ejected from super massive black holes. The gravitational pull of a black hole is so high that even light can't escape it. Yet particles with mass traveling at speeds ...
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Galactic or Extra-galactic Cosmic Rays?

If we want to show whether or not a specific Cosmic Ray particle is confined to our galaxy we must use the Larmor radius (relativistic version), $$r = \gamma \, \frac{ m c}{q B}$$ Considering a $10^...
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1answer
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Why does not dark matter gather and form celestial bodies? [duplicate]

since the only thing we know about dark matter that it "attracts" and affect our Baryonic matter's momentum and speed, which means that it does have mass of a sort. so why didn't we witness a ...
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1answer
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black dwarf stars and dark matter

Today we understand that a black dwarf star represents a hypothetical star that is the result of the complete consumption of the energy of a white dwarf which is the remnant of a star of little or ...
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1answer
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Luminostiy, Flux, Intensity definitions and meaning

I am taking astrophysics course and we are constantly talking about Luminosity,Brightness,Intensity,Flux etc. I thought that I understand the idea properly but I see now that I dont. Intensity is the ...
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3answers
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Why do we have to wait half a year to measure stellar parallax

To measure distance of astronomical objects, often we may use parallax, the angle it makes with the earth-sun and trigonometry to determine the distance. The thing I don't get is why do we need to ...
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How can CMB anisotropies be due only to inhomogeneities?

I'm studying from these lecture notes (Author: Wayne Hu, title: Lecture Notes on CMB Theory: From Nucleosynthesis to Recombination). At the very end of page 10 and beginning of page 11 he says: ...
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Why doesn't the centrifugal force cause rotating dark halos to flatten?

Consider a spherical cloud of dark matter like the spherical halo around our galaxy. Please see the diagram below Assuming that the halo is purely made up of dark matter which has only gravitational ...
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Interesting astro/particle/quantum/experimental physics investigations for gifted pre-calculus students? [closed]

TL;DR What are interesting applications that I could work through with pre-calculus level students? I'd like to go from pop science explanations to an actual building of physical intuition and ...
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1answer
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Does the scale factor evolution in a matter dominated universe allow for acceleration?

One probably very trivial issue which is bothering me. Standard cosmology says that in a FRW metric, in a matter dominated flat universe (Pressure $P = 0$ and $K = 0$), the scale factor, $a \propto t^{...
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Element creation in space

In my astronomy magazine I recently saw the periodic system revisited where the elememnts were color-coded according to their production site. For example, Hydrogen was created from the early Universe ...
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Broadening of SNIa lightcurves

Is the observed delaying of photons emitted by SNIa best evidence that space is expanding or should this be explained otherwise and how?
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For a given mass, how big can a Kerr black hole get?

We know that in a Kerr Black Hole the singularity is in the form of a 1 dimensional ring. If we have a 25 solar mass black hole, how big would the Kerr Ring be, width wise? Also, I read the Wiki on ...
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Behaviour of a star as it approaches Ultrarelativistc limit

By considering the potential energy of a degenerate star of mass $M$ and radius $R$, I can use dimensional arguments to show that the radius of the star depends on its mass as: $$ R ∝ M^\frac{2-n}{...
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How to express the Earth's orbital speed as a function of time?

I would like to create a function that tells me the orbital speed of Earth at any given day. The vis-viva equation says that v=√(GM(2/r-1/a) ), expressing speed as a function of distance from the ...
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1answer
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Gravity and dark energy in space

Ive been studying astrophysics and i read that our universe is still expanding because the gravity is not enough compared to the dark energy present. And right now we are not sure whether are universe ...
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1answer
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Does a black hole have thrust? Could they be acting as galactic engines? [closed]

I identify four possible kinds of thrust below (there may be more or less). I don't know how to do the math to test them though. Direct transfer of kinetic energy as thrust. The energy of objects ...
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Query about the ion tail of a comet

I understand that the solid gas coating the nucleus of a comet is melted and that this gas is ionised by the UV rays from the sun. This makes the gas particles charged. What I dont understand is why ...
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Why are stellar absorption line spectra passed through a cold gas?

I am currently studying spectral lines and saw in my notes that the way in which the absorbtion line spectra for a star was measured was by taking its light and shining it through a cold gas. The ...
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Can a star visually look like a black hole? [duplicate]

Amateur question incoming so please bear with me. Assumptions If a black hole is born from a star going supernova, intuitively it should have the same or less mass compared to the star. Black holes ...
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What is gravity shearing in molecular cloud rotation?

I have read that molecular clouds rotate due to gravitational shearing. What is this phenomenon?
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The Cosmic Microwave Background Paradox

I was reading an article on Olber's Paradox (why the universe isn't as bright as the sun) and the more I read on it, the more the same question came to mind... We know the observable universe is ...
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Why is the Sun approximated as a black body at ~ 5800 K?

Apparently spectral solar radiation is approximated by a black body at 5800 K. The spectral black body distribution (Planck distribution) is shown below (from Incropera, Fundamentals of Heat and Mass ...