Event Horizon Telescope image

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

p1t1o
Posts: 949
Joined: Wed Nov 10, 2010 4:32 pm UTC
Location: London, UK

Event Horizon Telescope image

Postby p1t1o » Thu Apr 11, 2019 9:22 am UTC

Cool isnt it.

My question is - why is it an image of M87* and not Saggitarius A*, which is 1000times closer?

Is there something that makes M87* a better candidate for observation, even though its very much more distant?

Or did M87* just happen to be first on the list of projects and it was their turn with the telescope and they happen to produce this groundbreaking image?

Tub
Posts: 472
Joined: Wed Jul 27, 2011 3:13 pm UTC

Re: Event Horizon Telescope image

Postby Tub » Thu Apr 11, 2019 9:37 am UTC

Some members of the EHT team did a reddit AMA yesterday, where they answered many common questions (including yours) in great detail.
https://www.reddit.com/r/askscience/com ... s_here_to/

User avatar
elliptic
Posts: 34
Joined: Tue Aug 14, 2012 2:21 pm UTC
Location: UK

Re: Event Horizon Telescope image

Postby elliptic » Thu Apr 11, 2019 10:08 am UTC

That's a great thread to dig down into, the tl;dr is they did also look at Sagittarius A* and will be releasing that data next.

M87* is much, much bigger than Sagittarius A* so its angular size in the sky is nearly the same, and we actually have a clearer view of it (surprisingly) because we're looking sideways out of our own galaxy instead of through all the dust and gas between us and its centre.

User avatar
Xanthir
My HERO!!!
Posts: 5413
Joined: Tue Feb 20, 2007 12:49 am UTC
Location: The Googleplex
Contact:

Re: Event Horizon Telescope image

Postby Xanthir » Mon Apr 15, 2019 11:10 pm UTC

Specifically, M87* is about 1600 times bigger than Sag A*, but only about 2000 times further away, so it's about 80% the size in the sky. So yeah, the much clearer image we get from not having to look thru our galactic disc is very worthwhile.
(defun fibs (n &optional (a 1) (b 1)) (take n (unfold '+ a b)))

ijuin
Posts: 1112
Joined: Fri Jan 09, 2009 6:02 pm UTC

Re: Event Horizon Telescope image

Postby ijuin » Tue Apr 16, 2019 12:26 am UTC

Due to General Relatively stuff, the radius of a black hole’s event horizon increases in linear proportion to its mass as opposed to the cube root of its mass (as applies to ordinary “solid” matter that is not under compression). That means that, for example, a black hole that is a thousand times more massive than another one will have a thousand times the radius instead of ten times the radius.

User avatar
Heimhenge
Posts: 373
Joined: Thu May 01, 2014 11:35 pm UTC
Location: Arizona desert

Re: Event Horizon Telescope image

Postby Heimhenge » Tue Apr 16, 2019 11:52 pm UTC

Xanthir wrote:Specifically, M87* is about 1600 times bigger than Sag A*, but only about 2000 times further away, so it's about 80% the size in the sky. So yeah, the much clearer image we get from not having to look thru our galactic disc is very worthwhile.


The other factor is the variability of Sag A* (compared to Powehi). They could get a longer "exposure" on Powehi and that means better sensitivity. But by my calculations, using reputable online sources for size and distance, the apparent sizes are:

Powehi: θ = tan-1[(2.8×10^10 km)/(5.2×10^20 km)] = 3.1×10^-9 degrees
Sag A*: θ = tan-1[(1.5×10^8 km)/(2.5×10^17 km)] = 3.4×10^-8 degrees

So Sag A* is about 10 times larger than Powehi.

For comparison, the closest stellar mass black hole is in the constellation Monoceros at a distance of “only” 3000 LY (2.8×10^16 km), but its diameter is a mere 30,000 km. This yields an apparent angular size of θ = 6.1×10^-11 degrees, far too small to image with today’s technology.

User avatar
Eebster the Great
Posts: 3465
Joined: Mon Nov 10, 2008 12:58 am UTC
Location: Cleveland, Ohio

Re: Event Horizon Telescope image

Postby Eebster the Great » Wed Apr 17, 2019 5:51 am UTC

Instead of "Powehi," can we please keep calling it Messier 87*?

User avatar
Heimhenge
Posts: 373
Joined: Thu May 01, 2014 11:35 pm UTC
Location: Arizona desert

Re: Event Horizon Telescope image

Postby Heimhenge » Wed Apr 17, 2019 6:37 am UTC

I get what you're saying about Powehi ... the translation of the name makes no sense to me either.

User avatar
Soupspoon
You have done something you shouldn't. Or are about to.
Posts: 4060
Joined: Thu Jan 28, 2016 7:00 pm UTC
Location: 53-1

Re: Event Horizon Telescope image

Postby Soupspoon » Wed Apr 17, 2019 8:09 am UTC

M87 is the galaxy designation, M87* the designation of this specific object/phenomenon within it.

SFAIK, nobody has really though of naming the host galaxy (just other designations, like NGC something and Virgo A?), but I'm not sure whether that's even required or expected.

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26767
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: Event Horizon Telescope image

Postby gmalivuk » Wed Apr 17, 2019 2:13 pm UTC

Eebster the Great wrote:Instead of "Powehi," can we please keep calling it Messier 87*?

Why?
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

DavidSh
Posts: 214
Joined: Thu Feb 25, 2016 6:09 pm UTC

Re: Event Horizon Telescope image

Postby DavidSh » Wed Apr 17, 2019 2:52 pm UTC

Soupspoon wrote:SFAIK, nobody has really though of naming the host galaxy (just other designations, like NGC something and Virgo A?), but I'm not sure whether that's even required or expected.

Virgo A is probably as much of a name as it will ever get. Elliptical galaxies aren't very photogenic, so don't get assigned names like Whirlpool Galaxy or Pinwheel Galaxy. For that matter, even the second most well known galaxy, the Andromeda Galaxy, is just named after the constellation in which it appears.

p1t1o
Posts: 949
Joined: Wed Nov 10, 2010 4:32 pm UTC
Location: London, UK

Re: Event Horizon Telescope image

Postby p1t1o » Wed Apr 17, 2019 4:14 pm UTC

Soupspoon wrote:M87 is the galaxy designation, M87* the designation of this specific object/phenomenon within it.

SFAIK, nobody has really though of naming the host galaxy (just other designations, like NGC something and Virgo A?), but I'm not sure whether that's even required or expected.


Then how come ours is Sagitarrius A* and not Milky Way*?

Is there a more formal name for our galaxy?

User avatar
Xanthir
My HERO!!!
Posts: 5413
Joined: Tue Feb 20, 2007 12:49 am UTC
Location: The Googleplex
Contact:

Re: Event Horizon Telescope image

Postby Xanthir » Wed Apr 17, 2019 5:34 pm UTC

The Milky Way is the formal name, just like The Sun is the name of our star, and The Moon is the name of Earth's moon. (Sol and Luna also being commonly-used names notwithstanding.)

Heimhenge wrote:
But by my calculations, using reputable online sources for size and distance, the apparent sizes are:

Powehi: θ = tan-1[(2.8×10^10 km)/(5.2×10^20 km)] = 3.1×10^-9 degrees
Sag A*: θ = tan-1[(1.5×10^8 km)/(2.5×10^17 km)] = 3.4×10^-8 degrees

So Sag A* is about 10 times larger than Powehi.

Huh, your numerators (the black hole radius, I presume) only show a 200x difference in size, but from what I'd heard it should be ~2000x. Are you sure your exponents are right? With that correction, we return to M87* being a similar size to Sag A*.

Heimhenge wrote:The other factor is the variability of Sag A* (compared to Powehi). They could get a longer "exposure" on Powehi and that means better sensitivity.

Yup. To be more specific here, Sag A*, being smaller, spins a lot faster (or rather, its accretion disc does, which is what we're imaging), while M87* is much bigger and slower - at the scale we're capable of seeing, Sag A*'s image "changes" on the order of hours, while M87* takes days to achieve the same degree of change. "Exposure" is the right analogy here, like requiring a fast shutter speed to capture someone running (and missing a lot of finer details) versus being allowed to use a slow shutter speed that captures a lot of nuance of lighting when taking a portrait. The mechanics are different, but that's just physics-talk. ^_^
(defun fibs (n &optional (a 1) (b 1)) (take n (unfold '+ a b)))

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26767
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: Event Horizon Telescope image

Postby gmalivuk » Wed Apr 17, 2019 5:50 pm UTC

I don't know where that 28 billion number is from. The event horizon is more like 40 billion across and the shadow 100 billion.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

User avatar
Pfhorrest
Posts: 5450
Joined: Fri Oct 30, 2009 6:11 am UTC
Contact:

Re: Event Horizon Telescope image

Postby Pfhorrest » Wed Apr 17, 2019 5:58 pm UTC

Is there a Sagittarius A without the *? Is that just a star? Is Sagittarius A* near the Sagittarius A star in our night sky, and that's why the black hole gets that designation?
Forrest Cameranesi, Geek of All Trades
"I am Sam. Sam I am. I do not like trolls, flames, or spam."
The Codex Quaerendae (my philosophy) - The Chronicles of Quelouva (my fiction)

User avatar
Eebster the Great
Posts: 3465
Joined: Mon Nov 10, 2008 12:58 am UTC
Location: Cleveland, Ohio

Re: Event Horizon Telescope image

Postby Eebster the Great » Wed Apr 17, 2019 6:05 pm UTC

The ratio of radii should equal the ratio of masses, which I find to be about 800-1800, depending on which estimates you use. So yeah, 200 is definitely low, though 2000 is a bit high.

In any case, his measurement for the diameter of Sgr A* seems to be about 10 times too high. Using masses of 4.1 * 106 for Sgr A* and 6.5 * 109 for M87* give Schwarzschild diameters of about 2.4 * 107 km and 3.8 * 1010 km. Using distances of 2.4 * 1017 and 5.0 * 1020 give angles of about 1.0 * 10-10 rad = 0.020 mas for Sgr A* and 7.6 * 10-11 rad = 0.016 mas for M87*.

User avatar
Sizik
Posts: 1257
Joined: Wed Aug 27, 2008 3:48 am UTC

Re: Event Horizon Telescope image

Postby Sizik » Wed Apr 17, 2019 6:08 pm UTC

Pfhorrest wrote:Is there a Sagittarius A without the *? Is that just a star? Is Sagittarius A* near the Sagittarius A star in our night sky, and that's why the black hole gets that designation?


https://en.wikipedia.org/wiki/Sagittarius_A

Doesn't seem to be a link to it from the Sgr A* page.
she/they
gmalivuk wrote:
King Author wrote:If space (rather, distance) is an illusion, it'd be possible for one meta-me to experience both body's sensory inputs.
Yes. And if wishes were horses, wishing wells would fill up very quickly with drowned horses.

User avatar
Heimhenge
Posts: 373
Joined: Thu May 01, 2014 11:35 pm UTC
Location: Arizona desert

Re: Event Horizon Telescope image

Postby Heimhenge » Wed Apr 17, 2019 6:41 pm UTC

gmalivuk wrote:I don't know where that 28 billion number is from. The event horizon is more like 40 billion across and the shadow 100 billion.


Well I was just trying to get a better idea of what the media meant by "Sag A* and Powehi are of comparable angular size", so I just searched for the relevant numbers on various sites. Can't relocate the one that said 28 billion, but here's one that claims 24 billion (at least for the size of the shadow): https://futurism.com/the-byte/big-m87-b ... -the-earth

Even in the so-called science media, the numbers varied from site to site. Still the reasons for imaging Powehi first were compelling, however many times larger Sag A* really is (in apparent diameter).

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26767
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: Event Horizon Telescope image

Postby gmalivuk » Wed Apr 17, 2019 7:03 pm UTC

Are you doing this in miles?

That site gives the correct EH diameter (38bn km), and the shadow is 2.6 times that.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

User avatar
Heimhenge
Posts: 373
Joined: Thu May 01, 2014 11:35 pm UTC
Location: Arizona desert

Re: Event Horizon Telescope image

Postby Heimhenge » Wed Apr 17, 2019 7:45 pm UTC

gmalivuk wrote:Are you doing this in miles?

That site gives the correct EH diameter (38bn km), and the shadow is 2.6 times that.


Shit. Indeed I was. Read the wrong number. Did the same for Sag A*. Corrected calculations should be:

Powehi: θ = tan-1[(3.5×10^10 km)/(5.2×10^20 km)] = 3.8×10^-9 degrees
Sag A*: θ = tan-1[(6.0×10^7 km)/(2.4×10^17 km)] = 1.4×10^-8 degrees

So Sag A* would appear about 3.8 times larger than Powehi. More in tune with their description of "comparably sized".

I realize the size of the shadow is larger than the actual event horizon, but for detection purposes it's the shadow that will be imaged.

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26767
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: Event Horizon Telescope image

Postby gmalivuk » Wed Apr 17, 2019 8:01 pm UTC

I think you're still mixing up the numbers you're using.

3.8e10 km is the event horizon diameter of Pōwehi. My understanding is that 6.0e7 km is the shadow diameter of Sag A*.
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

User avatar
Eebster the Great
Posts: 3465
Joined: Mon Nov 10, 2008 12:58 am UTC
Location: Cleveland, Ohio

Re: Event Horizon Telescope image

Postby Eebster the Great » Wed Apr 17, 2019 8:42 pm UTC

Yeah, it might be the same mistake made in the comic.

User avatar
Heimhenge
Posts: 373
Joined: Thu May 01, 2014 11:35 pm UTC
Location: Arizona desert

Re: Event Horizon Telescope image

Postby Heimhenge » Wed Apr 17, 2019 8:45 pm UTC

I guess my confusion is whether a particular source is citing the EH diameter or the shadow diameter. Hard to tell from the wording in some cases. Is the shadow diameter always 2.6 times the EH diameter? Or does it scale somehow with mass?

User avatar
gmalivuk
GNU Terry Pratchett
Posts: 26767
Joined: Wed Feb 28, 2007 6:02 pm UTC
Location: Here and There
Contact:

Re: Event Horizon Telescope image

Postby gmalivuk » Wed Apr 17, 2019 9:27 pm UTC

The maximum distance from the center that parallel light rays can avoid falling into a Schwarzschild black hole is exactly 3/2*sqrt(3) times the Schwarzschild radius, which is just shy of 2.6.

The Schwarzschild radius itself scales with mass, so a 6.5 billion solar mass black hole has a radius 6.5 billion times the Sun's Schwarzschild radius, and a 4 million solar mass black hole has a radius 4 million times the Sun's Schwarzschild radius.

Because the distances are so large, both of your fractions are really close to zero. For small x, we can approximate tan x = x, and if we just care about the ratio of angular size, we can just use the masses themselves because the difference is a factor of the Sun's Schwarzschild radius (times 2.6) on both sides, which cancels out.

Sag A* angular size / Pōwehi angular size
≈ (4.0e6 / 2.4e17) / (6.5e9 / 5.2e20)
≈ 1.3

(This ratio is of course the same if you use parsecs or light-years or anything else for the distances, and AU or light-seconds or anything else for the radii, so long as you're using the same units for both black holes.)
Unless stated otherwise, I do not care whether a statement, by itself, constitutes a persuasive political argument. I care whether it's true.
---
If this post has math that doesn't work for you, use TeX the World for Firefox or Chrome

(he/him/his)

User avatar
Heimhenge
Posts: 373
Joined: Thu May 01, 2014 11:35 pm UTC
Location: Arizona desert

Re: Event Horizon Telescope image

Postby Heimhenge » Thu Apr 18, 2019 5:34 pm UTC

Yep. I just redid the calculations after re-verifying the numbers. I came up with an apparent angular size ratio of Sag A*/Powehi = 1.4 ... differs from your 1.3 probably because of rounding, which was two sig figs all around.

But I now see where most of my confusion came from. Depending on the source, some were quoting the calculated EH diameter and others were quoting the shadow diameter. I hate it when my calculations don't work out so this was as frustrating as it was puzzling to me. I guess the distinction between EH and shadow diameters was just too subtle for most media sources to pick up on. Even some of the "science media".

Much appreciate your help on this one gmalivuk.


Return to “Science”

Who is online

Users browsing this forum: Google [Bot] and 5 guests