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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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Moving moon explanation

I saw the moon moving in a fast pace several times with the naked eye, even during day time when its cloudy. How is that possible? I asked my teacher about it and he said its just an illusion, but ...
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Why are more massive white dwarfs smaller?

I understand that a white dwarf is supported by the Pauli Exclusion Principle and that the larger the gravitational force against them, the closer the electrons must pack. But I have two queries: ...
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Is a galaxy made up of dark matter really a galaxy?

So I am watching a video discuss Dragonfly 44. The galaxy is made up almost entirely of dark matter and is 100x dimmer than the Milky Way galaxy. Wouldn't this mean it is just a large clump of ...
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Query about the evolution of the sun, from hydrogen shell burning to helium flashpoint

My understanding of the transition from hydrogen shell burning to helium flash point: The hydrogen shell burning occurs because the hydrogen in the suns core during the main sequence has been used up,...
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1answer
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Currently the Universe is expanding so if Big Bang is true what caused our universe to shrink that size?

What we currently perceive is the expanding universe , if dark energy is the cause of it , which is overtaking gravity (expansion) what would have caused our universe to be size of singularity .
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Finding number of photons per second crossing an area with distance ${}d$ from a radiating point [on hold]

Estimate the number of photon per second entering one eye of the observer from 10 Watt red light source at 50m away form point source.
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1answer
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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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1answer
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Luminosity of a white dwarf

Why in an H-R diagram the white dwarfs are at the bottom left? I understand that they would have a high temperature when they are formed, and so at that instant shouldn't they have a high luminosity ...
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What imparts the linear motion or the tangential velocity to natural satellites like the moon to enable them to be in orbit around the earth?

From Newton's laws of motion & gravitation, it is clear that the force of gravitation provides the necessary centripetal force which acts along the line joining the centre of masses of the earth &...
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Strange quarks in neutron stars

Recently I have read that the pressure within neutron stars can be so extreme that it can cause the deconfinement of quarks to form a kind of quark 'soup' and that these extreme conditions allow for ...
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28 views

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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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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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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1answer
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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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1answer
355 views

If I observe a planet because of reflected sun-light, how does the radiant flux vary with distance?

I wish to figure out how the flux of the sun's reflected light from a planet varies with distance, so assume an observer is on the sun, looking at a planet, and the light originating from the planet ...
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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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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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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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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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1answer
308 views

Doppler shift and speed of rotating objects in space

I understand the concept of how we can use the doppler effect to know if an object is spinning, in the sense that the part of the object spinning towards us will exhibit a blueshift, and the part ...
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1answer
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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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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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Solar granulation pictures showing a wonderous spatial resolution

Recently, I came across spectacular pictures of solar granulation like this: (source) The photograph is impressive because it has a resolution of less than 0.1 arc seconds, which corresponds to ...
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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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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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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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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 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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What is the physical size of a black hole?

Something that's always confused me. How large is a black hole's physical size - not mass? From descriptions, it would seem that the 'singularity' is a single point, but is it really? Say for ...
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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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1answer
137 views

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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1answer
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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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Mass to Light Ratios

I know that the mass to light ratio for galaxies is between 2 and 10. I also know that the ratio changes primarily based on what kind of galaxy you have (elliptical, spiral, etc). Is there a table ...
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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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How can Quasars emit anything if they're black holes?

We've heard it many times, nothing can escape the gravity of a black hole, even light once it's past the event horizon. If this is true, how can a black hole emit anything? Quasars are massive black ...
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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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How Much Do Spiral Arms Affect Their Galaxies' Rotation Curves?

According to Jerry Sellwood of Rutgers University, in the article What process creates and maintains the beautiful spiral arms around spiral galaxies?: Fortunately, nearly every one agrees that ...
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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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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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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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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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1answer
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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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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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What is the evidence for a supermassive black hole at the center of Milky Way?

Black holes cannot be seen because they do not emit visible light or any electromagnetic radiation. Then how do astronomers infer their existence? I think it's now almost established in the scientific ...