28 votes
Accepted

What is it exactly about these flares of infrared light from Sgr A* that "confirms" it is a supermassive black hole?

We have reasonably good measurements of the mass of Sagittarius A*, thanks to measurements of the movements of stars like S0-2 over several decades. It's been well-established that the mass of the ...
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23 votes

Did the earth form outside the solar system?

The chance happenstance that Earth happened to be floating along and got captured is minuscule. How did Earth wind up floating through space? There's no established mechanism for terrestrial planets ...
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16 votes

Did the earth form outside the solar system?

I think the Earth's orbit is by far the strongest argument you have that the Earth formed around the Sun. The orbit is nearly circular and almost in the Sun's equatorial plane, similar to the other ...
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14 votes
Accepted

How did they make a video of the center of the galaxy, and what is it exactly that's flashing there?

Question: How are these images obtained? Later in the video the narrator says they took the images using ESO's VLT. 03:40 [Narrator] 14.​ Making these measurements pushed the power of ESO’s Very ...
13 votes
Accepted

Why didn't all the iron and heavier elements find their way to the center of the accretion disc in the early solar system?

The process of differentiation is how metals like iron and nickel can become separated from less dense substances. It generally occurs in large, partly solid bodies in the early solar system, whose ...
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12 votes
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Quasar mass and accretion rates

Tricky to say for sure, but I would imagine it comes about from measurements of the luminosity and inference of the black hole mass in such systems. The most extreme objects radiate at the Eddington ...
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  • 117k
10 votes
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How do rocky objects between 1cm and 1m accrete to form planetesimals?

This is indeed a tricky problem, and the accretion of pebbles to form planetesimals is a big question in planetary science. You are right to say that small particles can stick together through ...
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8 votes

How much matter was ejected when the Solar System formed?

It is actually the other way around: First a massive accretion disc can form, through which material looses angular momentum and accretes onto the star radially, hence being angular momentum poor. ...
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8 votes

What does using these units (c=G=M=1) in MHD simulations mean?

It just means you choose a set of units where $M=1$. What those units are would then depend on what the mass of the object is. For example, in geometrised units, the Schwarzschild radius is $2M$. In $...
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7 votes

Why would the merger of spinning black holes within the accretion disk of a supermassive black hole cause them to "shoot straight up" out of the disk?

Let me see if I can try answering both parts of your question. The key is a combination of two things: 1) Most of the binary BHs in an accretion disk will have their binary orbits in the same plane as ...
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7 votes
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Has gain in mass from accretion onto a black hole been observed?

Unfortunately, the answer is "No", because accretion rates are far too low -- and our ability to measure black hole masses is far too uncertain -- for this to be visible in reasonable times. ...
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6 votes
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What is the temperature of an accretion disc surrounding a supermassive black hole?

It depends on the distance from the central body. This gives the temperature $T$ at a given point as a function of the distance from that point to the center ($R$): $$T(R)=\left[\frac{3GM \dot{M}}{8 \...
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6 votes
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What defines the plane of an accretion disk around a black hole?

Let me give an answer based on my intuitive understanding of the thing... Initial conditions The initial accretion disk movement is defined by the objects falling into it, the black hole's mass and ...
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  • 1,402
6 votes

If space was n>3 dimensional, would clouds of particles still turn into 2D disks due to gravity?

It just so happens that this problem has been rigorously analyzed. Max Tegmark wrote an interesting paper on it which can be gotten from the arXiv. It is not super-technical, so I recommend you look ...
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  • 7,390
6 votes

If a black hole does not emit light, how can one take a picture of the black hole itself?

The picture is of the central region of M87, taken at a wavelength where the gas is "optically thin". The ring of bright light is pretty much exactly where it is expected to be for the synchrotron ...
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6 votes
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How will they know when to start taking the picture of the black hole at the center of the Milky Way?

The Milky Way's central supermassive black hole (SMBH) is feeding, albeit at a very low level. Radio emission from the accretion disk (and/or weak jets) is responsible for the long-lived "Sgr A*&...
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6 votes

Could a black hole's accretion disk be bright through natural means?

As I understand the quote, the "artificial" thing about your accretion disk is not that it is bright, but that it doesn't emit X rays. In real life, SMBH accretion disks are usually ...
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5 votes
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Can matter fly directly into a black hole and avoid the accretion disc?

In your analogy: A satellite stays in orbit because it has angular momentum. This always balances the force of gravity, thus when being in the satellite (or in the ISS) it seems as if you're ...
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5 votes
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Accretion disks - why are they disk-shaped, rather than spherical?

There's two different effects here, and they're both related to viscuous forces in the accreting matter. First, if the infalling matter has some nonzero angular momentum vector $\mathbf{L}$, then ...
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5 votes
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What is the light source in the black hole images?

The radiation being detected in this case is mostly synchroton radiation, caused by energetic electrons spiralling around magnetic field line, rather than thermal radiation, but it comes from the same ...
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5 votes

Why are accreting objects portrayed with a white flash at the point where the gas stream from the star enters the accretion disk?

When the stream of gas falls towards the disk it gains a significant fraction of the orbital kinetic energy (after all, it is falling from the top of the Roche lobe) which means that it is moving fast ...
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4 votes
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Luminosity of black hole accretion disc

Three times the Schwarzschild radius corresponds to the closest stable circular orbit around a black hole. The general idea is that as matters moves in towards the black hole it gets stuck in an ...
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  • 117k
4 votes

Did the earth form outside the solar system?

It would be really difficult for the Earth to end up in a nearly circular orbit if it came from outside the solar system. Effectively falling from infinity, it would have a hyperbolic orbit and make ...
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4 votes
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What enhances the capture and merge rates of pairs of small black holes orbiting around supermassive black holes?

I would think that the same reason that a gas disk enhances the growth of planetesimals. Drag from the disk enforces circular, co-planar orbits, which in turn means that objects that get close to each ...
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  • 117k
4 votes

How does gas in accretion disks of supermassive black holes create orbital torques on smaller black holes within the disks causing them to migrate?

Not really my area, but this question is probably related to the planetary migration in circumstellar disks. In this case, the migration is caused by gravitational interactions between the planet and ...
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4 votes
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Why are relativistic jets perpendicular to the accretion disk?

We can think of the magnetic field lines as part of its host gas in the jet, in the sense that when the gas moves, the magnetic lines of force must move with it - and vice versa. Magnetic field lines ...
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4 votes
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Why is synchrotron radiation from relativistic electrons low in energy?

Synchrotron radiation is emitted by charged particles (mostly electrons) executing helical motion, accelerated by the Lorentz force exerted by the vector product of their velocity and the magnetic ...
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4 votes
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How did astronomers "measure a large (>40 degrees) misalignment between the black hole spin and the orbital spin" of MAXI J1820+70?

See Figure 3 from https://arxiv.org/pdf/2109.07511.pdf: In this picture, the North and East directions are in the plane of the sky, and the $\hat{o}$ direction is in the direction of the observer. In ...
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3 votes

Did the earth form outside the solar system?

Occam's razor defeats the hypothesis. There are no astronomical anomalies about earth's obit or position that require the complication of earth's insertion from beyond the solar system. Unless some ...
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