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Re: Astronomy and Space

Posted: Thu May 12, 2022 2:38 pm
by Gfamily
The black bit of the shadow* is about the size of the orbit of Mercury - but it contains as much material as 4,000,000 Suns.


* I roughly recall that the shadow is 1 point something times the size of the black hole itself,but I can't remember whether it's the black hole or the shadow that's the size of orbit of Mercury (60m km)

Re: Astronomy and Space

Posted: Thu May 12, 2022 2:39 pm
by Grumble
lpm wrote:
Thu May 12, 2022 2:29 pm
It's not as big as I expected, etc etc etc
How do we even describe the size of a black hole? Is the event horizon a useful measure? We can know the mass.

Re: Astronomy and Space

Posted: Thu May 12, 2022 2:43 pm
by Gfamily
Grumble wrote:
Thu May 12, 2022 2:39 pm
lpm wrote:
Thu May 12, 2022 2:29 pm
It's not as big as I expected, etc etc etc
How do we even describe the size of a black hole? Is the event horizon a useful measure? We can know the mass.
Not sure whether it's interesting, but although we can calculate the volume enclosed by the event horizon, the volume inside the event horizon is indeterminate.

Re: Astronomy and Space

Posted: Thu May 12, 2022 2:47 pm
by dyqik
Gfamily wrote:
Thu May 12, 2022 2:38 pm
The black bit of the shadow* is about the size of the orbit of Mercury - but it contains as much material as 4,000,000 Suns.


* I roughly recall that the shadow is 1 point something times the size of the black hole itself,but I can't remember whether it's the black hole or the shadow that's the size of orbit of Mercury (60m km)
The ratio of the radius of the ISCO (innermost stable circular orbit for massive particles) to the Schwarzchild radius (radius of the event horizon) is 3 for a non-rotating black hole, but there are unstable circular orbits down to 2 times the Schwarzchild radius - which is where the inner edge of an accretion disk lies.

For spinning black holes, those radii get smaller for prograde orbits, and can reduce down to the same as the Schwarzchild radius for a maximally spinning black hole.

Re: Astronomy and Space

Posted: Thu May 12, 2022 2:48 pm
by dyqik
Grumble wrote:
Thu May 12, 2022 2:39 pm
lpm wrote:
Thu May 12, 2022 2:29 pm
It's not as big as I expected, etc etc etc
How do we even describe the size of a black hole? Is the event horizon a useful measure? We can know the mass.
The Schwarzchild radius is the usual measure - that's the distance from the singularity within which photons cannot escape - i.e. the event horizon.

The Schwarzchild radius is just GM/c², so directly proportional to the mass.

Re: Astronomy and Space

Posted: Thu May 12, 2022 2:48 pm
by Grumble
Gfamily wrote:
Thu May 12, 2022 2:43 pm
Grumble wrote:
Thu May 12, 2022 2:39 pm
lpm wrote:
Thu May 12, 2022 2:29 pm
It's not as big as I expected, etc etc etc
How do we even describe the size of a black hole? Is the event horizon a useful measure? We can know the mass.
Not sure whether it's interesting, but although we can calculate the volume enclosed by the event horizon, the volume inside the event horizon is indeterminate.
That’s the sort of statement that breaks my brain

Re: Astronomy and Space

Posted: Thu May 12, 2022 2:50 pm
by Grumble
dyqik wrote:
Thu May 12, 2022 2:48 pm
Grumble wrote:
Thu May 12, 2022 2:39 pm
lpm wrote:
Thu May 12, 2022 2:29 pm
It's not as big as I expected, etc etc etc
How do we even describe the size of a black hole? Is the event horizon a useful measure? We can know the mass.
The Schwarzchild radius is the usual measure - that's the distance from the singularity within which photons cannot escape - i.e. the event horizon.
Yeah, but is the black hole itself the singularity, which presumably doesn’t really have a volume?

Re: Astronomy and Space

Posted: Thu May 12, 2022 2:50 pm
by dyqik
Grumble wrote:
Thu May 12, 2022 2:50 pm
dyqik wrote:
Thu May 12, 2022 2:48 pm
Grumble wrote:
Thu May 12, 2022 2:39 pm


How do we even describe the size of a black hole? Is the event horizon a useful measure? We can know the mass.
The Schwarzchild radius is the usual measure - that's the distance from the singularity within which photons cannot escape - i.e. the event horizon.
Yeah, but is the black hole itself the singularity, which presumably doesn’t really have a volume?
I think it's undefined. You'd need to integrate over a infinitesimally small point of infinite curvature of spacetime.

Re: Astronomy and Space

Posted: Thu May 12, 2022 2:54 pm
by Grumble
Going to stop thinking about this now :lol:

Re: Astronomy and Space

Posted: Thu May 12, 2022 2:55 pm
by dyqik
Grumble wrote:
Thu May 12, 2022 2:54 pm
Going to stop thinking about this now :lol:
"Undefined" is just the mathematicians way of saying that.

Re: Astronomy and Space

Posted: Thu May 12, 2022 2:56 pm
by Gfamily
dyqik wrote:
Thu May 12, 2022 2:50 pm
Grumble wrote:
Thu May 12, 2022 2:50 pm
Yeah, but is the black hole itself the singularity, which presumably doesn’t really have a volume?
Depends on how you measure it. You can also say it has infinite volume.
Happy to be corrected, but my understanding is that a Black Hole is generally understood to be everything inside the Event Horizon (the Schwarzchild Radius). I am not sure whether that should include the Event Horizon itself.

Re: Astronomy and Space

Posted: Thu May 12, 2022 2:58 pm
by dyqik
Gfamily wrote:
Thu May 12, 2022 2:56 pm
dyqik wrote:
Thu May 12, 2022 2:50 pm
Grumble wrote:
Thu May 12, 2022 2:50 pm
Yeah, but is the black hole itself the singularity, which presumably doesn’t really have a volume?
Depends on how you measure it. You can also say it has infinite volume.
Happy to be corrected, but my understanding is that a Black Hole is generally understood to be everything inside the Event Horizon (the Schwarzchild Radius). I am not sure whether that should include the Event Horizon itself.
I edited that post, but the interior of the event horizon includes the singularity, so all bets are off.

Re: Astronomy and Space

Posted: Thu May 12, 2022 3:33 pm
by Gfamily
My bad
Gfamily wrote:
Thu May 12, 2022 2:38 pm
The black bit of the shadow* is about the size of the orbit of Mercury - but it contains as much material as 4,000,000 Suns.
The Accretion disc is about the size of Mercury's orbit
Sgr A star.jpg
Sgr A star.jpg (24.99 KiB) Viewed 217 times

Re: Astronomy and Space

Posted: Thu May 12, 2022 7:28 pm
by Sciolus
Gfamily wrote:
Thu May 12, 2022 3:33 pm
The Accretion disc is about the size of Mercury's orbit

Sgr A star.jpg
That's rubbish, surely. That picture implies we're looking at the accretion disk exactly face on, which is extremely unlikely. The disk is most likely roughly parallel to the rest of the galaxy, which means we'd be seeing it edge-on. Have they done some jiggery-pokery to create an image as if we were looking at it face on? Or is it some gravitational lensing thing? Or what?

Fake edit: OK, it's sort of the last of those. I was forgetting that the light gets bent so much that we are seeing behind the black hole as well as/more than classical line-of-sight.
https://www.eso.org/public/blog/spot-th ... ius-a-m87/

Re: Astronomy and Space

Posted: Fri May 13, 2022 12:31 am
by dyqik
Whatevs. You're just jealous because Krispy Kreme donuts didn't celebrate your last paper with free donuts for everyone*.

https://twitter.com/krispykreme/status/ ... AXY_g&s=19

Re: Astronomy and Space

Posted: Fri May 13, 2022 8:02 am
by jaap
The EHT got a picture 3 years ago of a different black hole which was in the centre of a whole different galaxy. Can anyone explain why that was easier than this picture of a black hole that is so much closer to us?

Well, actually I'm just assuming the other one was easier cause they did it first - I mean why wouldn't you do the Milky Way one if you had a choice?

Re: Astronomy and Space

Posted: Fri May 13, 2022 8:19 am
by Gfamily
jaap wrote:
Fri May 13, 2022 8:02 am
The EHT got a picture 3 years ago of a different black hole which was in the centre of a whole different galaxy. Can anyone explain why that was easier than this picture of a black hole that is so much closer to us?

Well, actually I'm just assuming the other one was easier cause they did it first - I mean why wouldn't you do the Milky Way one if you had a choice?
I've seen an explanation that the M87 BH is 1000 times bigger, so material in its accretion disc orbits in a matter of days, whereas the material orbiting Sgr A* does so in minutes, so the timescale for capturing data is much shorter.

The date of the observations is the same (i think) so it's the processing that has taken so much longer.

Re: Astronomy and Space

Posted: Fri May 13, 2022 11:13 am
by shpalman
jaap wrote:
Fri May 13, 2022 8:02 am
The EHT got a picture 3 years ago of a different black hole which was in the centre of a whole different galaxy. Can anyone explain why that was easier than this picture of a black hole that is so much closer to us?

Well, actually I'm just assuming the other one was easier cause they did it first - I mean why wouldn't you do the Milky Way one if you had a choice?
There's a whole sh.t load of dust between us and the centre of our own galaxy. That's why you can't actually see it optically but have to use the radio wavelengths.

Re: Astronomy and Space

Posted: Fri May 13, 2022 11:41 am
by Gfamily
Interesting - a suggestion that the appearance may be an artefact of the processing.
https://twitter.com/apontzen/status/1525027622874398720

associated paper preprint
https://arxiv.org/abs/2205.04623
The ring-like structures of the EHTC can be created not only from the public data, but also from the simulated data of a point image

Re: Astronomy and Space

Posted: Fri May 13, 2022 1:37 pm
by dyqik
Gfamily wrote:
Fri May 13, 2022 11:41 am
Interesting - a suggestion that the appearance may be an artefact of the processing.
https://twitter.com/apontzen/status/1525027622874398720

associated paper preprint
https://arxiv.org/abs/2205.04623
The ring-like structures of the EHTC can be created not only from the public data, but also from the simulated data of a point image
This isn't peer reviewed, and this scenario is tested in the original papers, as well as in the new papers - the analysis includes Bayesian model selection among multiple source models, and a non-ring like structure only works with very low probability.

This is shown in the first image in yesterday's special issue of The Astrophysical Journal Letters - https://iopscience.iop.org/journal/2041 ... _A_Results

See also Fig 6. and associated text (particularly section 7) in https://iopscience.iop.org/article/10.3 ... 213/ac6429

Re: Astronomy and Space

Posted: Mon May 16, 2022 1:00 pm
by IvanV
Gfamily wrote:
Thu May 12, 2022 2:56 pm
dyqik wrote:
Thu May 12, 2022 2:50 pm
Grumble wrote:
Thu May 12, 2022 2:50 pm
Yeah, but is the black hole itself the singularity, which presumably doesn’t really have a volume?
Depends on how you measure it. You can also say it has infinite volume.
Happy to be corrected, but my understanding is that a Black Hole is generally understood to be everything inside the Event Horizon (the Schwarzchild Radius). I am not sure whether that should include the Event Horizon itself.
Whilst that is the "size of the black hole", we have an astronomical object which is arguably more than a black hole. That very extensive accretion disk is, in a sense, part of "it". The accretion disk might be considered as being a bit like an "atmosphere" of a planet, in that it surrounds it from the very "surface" of the event horizon, without a gap.

There is a degree of inconsistency here in saying where the edge of an object is, to say how big it is. When asking "how big is the earth", we tend to give the diameter of solid and liquid part of it, and ignore the atmosphere. But when asking how big are the ice and gas giant planets, we assuredly include the atmosphere. Indeed we may be uncertain of the precise size of the solid/liquid part. And it is possible that the lower atmospheres of some of them are supercritical so that gas gradually becomes liquid without a defined surface. But we wouldn't include Saturn's rings, or the satellites. Saturn's rings are at a distance from the top of the atmosphere of the planet. The accretion disk of a black hole is a bit like an atmosphere, in that it is right up against the event horizon, albeit not in spherical distribution around it.

Re: Astronomy and Space

Posted: Mon May 16, 2022 3:05 pm
by dyqik
IvanV wrote:
Mon May 16, 2022 1:00 pm
Gfamily wrote:
Thu May 12, 2022 2:56 pm
dyqik wrote:
Thu May 12, 2022 2:50 pm

Depends on how you measure it. You can also say it has infinite volume.
Happy to be corrected, but my understanding is that a Black Hole is generally understood to be everything inside the Event Horizon (the Schwarzchild Radius). I am not sure whether that should include the Event Horizon itself.
Whilst that is the "size of the black hole", we have an astronomical object which is arguably more than a black hole. That very extensive accretion disk is, in a sense, part of "it". The accretion disk might be considered as being a bit like an "atmosphere" of a planet, in that it surrounds it from the very "surface" of the event horizon, without a gap.

There is a degree of inconsistency here in saying where the edge of an object is, to say how big it is. When asking "how big is the earth", we tend to give the diameter of solid and liquid part of it, and ignore the atmosphere. But when asking how big are the ice and gas giant planets, we assuredly include the atmosphere. Indeed we may be uncertain of the precise size of the solid/liquid part. And it is possible that the lower atmospheres of some of them are supercritical so that gas gradually becomes liquid without a defined surface. But we wouldn't include Saturn's rings, or the satellites. Saturn's rings are at a distance from the top of the atmosphere of the planet. The accretion disk of a black hole is a bit like an atmosphere, in that it is right up against the event horizon, albeit not in spherical distribution around it.
No, the accretion disk is much more like Saturn's rings or the asteroid belt than an atmosphere. It's gravitationally attracted into an orbit about the BH, with an orbital plane that's determined by the net angular momentum of the infalling material - much like the formation of the Solar System as the Sun collapsed and ignited. In contrast, an atmosphere is supported entirely by gas pressure - you can't go ballooning in an accretion disk.

The accretion disk stops at around twice the size of the Event Horizon, and inside that is virtually empty until you get to the Event Horizon.

Re: Astronomy and Space

Posted: Mon May 16, 2022 3:20 pm
by IvanV
dyqik wrote:
Mon May 16, 2022 3:05 pm
The accretion disk stops at around twice the size of the Event Horizon, and inside that is virtually empty until you get to the Event Horizon.
Thank you. You've just completely changed the image I've always had of a black hole being surrounded by the stuff that was just outside and soon to fall in.

Re: Astronomy and Space

Posted: Mon May 16, 2022 3:50 pm
by dyqik
IvanV wrote:
Mon May 16, 2022 3:20 pm
dyqik wrote:
Mon May 16, 2022 3:05 pm
The accretion disk stops at around twice the size of the Event Horizon, and inside that is virtually empty until you get to the Event Horizon.
Thank you. You've just completely changed the image I've always had of a black hole being surrounded by the stuff that was just outside and soon to fall in.
That's the difference between the ISCO (3 Rs), the unstable orbits within the ISCO (down to 2 Rs), and the Schwarzchild Radius (Rs).

The first two numbers get smaller for rapidly spinning black holes with a prograde accretion disk.