<p>As Ingolifs says, Sgr A* and M87* are the obvious candidates. At the press conference, Heino Falcke explained why they got a picture of M87* first:  </p>

<blockquote>
  <p>But it would take some more time because Sagittarius A Star is 1000 times faster and smaller. Its like a toddler who is moving constantly. In comparison, M87 is much slower, like a big bear.</p>
</blockquote>

<p>&mdash; <a href="https://www.deccanherald.com/science-and-environment/astronomers-release-first-photo-of-black-hole-727987.html" rel="noreferrer">The Deccan Herald</a></p>

CC BY-SA 4.0
2019-04-12T12:04:52.700Z
That analogy doesn't really work. I can reliably confirm that a toddler runs slower than a bear.
Sneftel
CC BY-SA 4.0
2019-04-12T14:24:44.387Z
@Sneftel I'm so sorry for your loss
Michael
CC BY-SA 4.0
2019-04-11T09:10:20.737Z
2019-04-11T09:10:20.737Z
user24582
<p>There are a few criteria necessary to see a black hole with the <a href="https://en.wikipedia.org/wiki/Event_Horizon_Telescope" rel="noreferrer">Event Horizon Telescope</a>. They are, in importance:</p>

<ul>
<li>Active Feeding: you need a thick accretion disk with lots of matter accreting onto the black hole. M87 fits this criteria, and is a glut, <a href="https://arxiv.org/abs/astro-ph/0202238" rel="noreferrer">consuming about 90 Earth masses a day</a>. </li>
<li>Apparent size. Even though it is 53 million light-years away, M87 is 6.5 billion solar masses. Since the radius of the event horizon scales linearly with mass, its distance is made up for by sheer scale. </li>
</ul>

CC BY-SA 4.0
2019-04-11T07:40:44.283Z
Calculates quickly... the M87 BH consumes one Earth mass every 16 MINUTES!
Chappo Hasn't Forgotten
CC BY-SA 4.0
2019-04-11T18:38:30.140Z
Mmmmm, Earth-masses. :)
Barmar
CC BY-SA 4.0
2019-04-11T00:55:07.340Z
2019-04-11T05:27:21.053Z
2019-04-11T05:27:21.053Z
cms
<p>Another quick note - They are trying to get a photo of Sag. A*:</p>
<p>From <a href="https://www.space.com/first-black-hole-photo-by-event-horizon-telescope.html" rel="noreferrer">Space.com</a></p>
<blockquote>
<p>The project has been scrutinizing two black holes — the M87 behemoth, which harbors about 6.5 billion times the mass of Earth's sun, and our own Milky Way galaxy's central black hole, known as Sagittarius A*. This latter object, while still a supermassive black hole, is a runt compared to M87's beast, containing a mere 4.3 million solar masses.</p>
<p>Both of these objects are tough targets because of their immense distance from Earth. Sagittarius A* lies about 26,000 light-years from us, and M87's black hole is a whopping 53.5 million light-years away.</p>
<p>From our perspective, Sagittarius A*'s event horizon &quot;is so small that it's the equivalent of seeing an orange on the moon or being able to read the newspaper in Los Angeles while you're sitting in New York City,&quot; Doeleman said during the SXSW event last month.</p>
<p>...</p>
<p>And in case you're wondering about Sagittarius A*: The EHT team hopes to get imagery of that supermassive black hole soon, Doeleman said today. The researchers looked at M87 first, and it's a bit easier to resolve than Sagittarius A* because it's less variable over short timescales, he explained.</p>
</blockquote>

CC BY-SA 4.0
2019-04-11T14:48:55.590Z
2019-04-11T14:55:46.833Z
2020-06-17T09:47:40.653Z
BruceWayne
<p>The difficulty in imaging anything on the visible-light spectrum is dust.  Sagitarrius A is clouded by dust clouds which can be penetrated with infrared. The M87 fulfilled the criteria of being big and relatively close while at the same time enabling light to reflect off the event horizon and not being blocked by dust clouds.</p>

CC BY-SA 4.0
2020-10-13T08:31:41.173Z
They weren't imaging in visible light, or even anything we'd normally call IR. Very short radio waves, really. Also nothing reflects of an event horizon.
Steve Linton
CC BY-SA 4.0
2020-11-12T17:49:22.200Z
Well, if nothing did reflect off an event horizon there would be no photo to take.  QED
Danny F
CC BY-SA 4.0
2020-11-12T18:04:44.583Z
The photo is essentially a silhouette. A **very** crude approximation is that there is light coming from hot stuff *behind* the black hole, and we see it with a dark area in the middle corresponding to the black hole.  This is an approximation because a black hole is a distorted are of space and time, and light takes strange paths near to one. However what you see does look like a bright ring around a dark center from the right angle
Steve Linton
CC BY-SA 4.0
2019-04-13T22:46:34.420Z
2019-04-13T22:46:34.420Z
Danny F
<p>I was surprised too when I first heard they were trying to image M87's black hole.</p>

<p>The short answer is because it's really, really big. It is 1500 times bigger (diameter) than our Sagittarius A*, and 2100 times farther away. This makes its apparent size about 70% of that of Sgr A*, which they are also attempting to image.</p>

<p>A cursory search of wikipedia's <a href="https://en.wikipedia.org/wiki/List_of_most_massive_black_holes" rel="noreferrer">List of Largest black holes</a> shows that there's no other black holes with a combination of size and closeness greater than these two. </p>

<p>A couple of other candidates are not too far off. Andromeda's black hole is 50x the size of ours, and at 100x the distance, it would appear half the size of Sgr A*. The Sombrero galaxy is 380 times farther way than Sgr A*, and has a black hole estimated to be 1 billion solar masses, which is 232 times Sr A*, resulting in an angular diameter about 60% of Sgr A*.</p>

<p>There appear to be many other considerations to which black holes were chosen, as explained in <a href="https://astronomy.stackexchange.com/questions/30313/why-didnt-the-event-horizon-telescope-team-mention-sagittarius-a">this similar question.</a> At a guess these would include how obscured each black hole is with foreground dust/stars etc, how active (and therefore bright) the nuclei are, and their inclination w.r.t earth affecting which observatories could observe them at which times.</p>

<p>Edit: I've found another plausible candidate. <a href="https://en.wikipedia.org/wiki/NGC_1600" rel="noreferrer">NGC_1600</a> is 200 M light years away with a central black hole estimated to be 17 billion solar masses heavy. This would put it at about 40% the apparent diameter of Sgr A*. </p>

<p><a href="https://i.sstatic.net/cdsKB.png" rel="noreferrer"><img src="https://i.sstatic.net/cdsKB.png" alt="black hole size comparison, self made"></a>
<strong>Comparison of the apparent size of the largest nearby black holes</strong></p>

<p>And of course <a href="https://xkcd.com/1276/" rel="noreferrer">obligatory XKCD</a> to remind us how small these objects really appear.</p>

CC BY-SA 4.0
2019-04-11T04:20:10.033Z
Don't forget to say space is kind of dusty in the direction of Sgr A*. It's quite a bit clearer in the direction of M87.
Florin Andrei
CC BY-SA 4.0
2019-04-11T09:16:16.943Z
@FlorinAndrei Actually that doesn't really matter much, since the observations are made in radio where there's barely any extinction.
pela
CC BY-SA 4.0
2019-04-11T11:16:04.797Z
@FlorinAndrei It may be dusty in the direction of Sgr A* but it’s messier in the direction of M87.
Konrad Rudolph
CC BY-SA 4.0
2019-04-11T12:18:17.997Z
@KonradRudolph I see what you did there :D
pela
CC BY-SA 4.0
2019-04-11T18:11:30.040Z
Given that we are on the edge of the milky way, half the dust and contents of the milky way are between us and Sgr A*. I'm not sure the exact orientation of milky way relative to M87, but I believe they mentioned Sgr A* being obscured in the press conference. They also said Sgr A*, being much smaller, was also much more highly variable over time. I REALLY hope they make a time lapse video or something. That'd rule.
S. Imp
CC BY-SA 4.0
2019-04-12T07:44:54.723Z
Don't forget. Sgr A* would be seen rather from the side then from zenith/nadir
yunzen
CC BY-SA 4.0
2019-04-12T09:25:58.540Z
@pela on the other hand, we never saw what he did in the direction of Sgr A*.
John Dvorak
CC BY-SA 4.0
2019-04-12T10:47:38.337Z
I'm not sure this answer really fully answers the question. In the "short answer" paragraph, it states that M87's apparent size is 70% that of Sgr A*. That, by itself, would appear to make it a *worse* candidate than Sgr A*. The question wants to know why a closer black hole wasn't chosen. Sgr A* is closer - why not choose it (and indeed the question even wants to know why not "even ours")? The answer would be improved by elaborating on the other factors that make M87 a better candidate than Sgr A*
JBentley
CC BY-SA 4.0
2019-04-12T19:58:49.960Z
@JBentley You're right, I skipped over Sgr A* because I assumed people knew they were also trying to image it. I'll edit my answer to make that explicit.
Ingolifs
CC BY-SA 4.0
2019-04-13T13:15:16.780Z
I am wondering though, how many of those candidates are actually visible to the whole array. From what I understand, they need the south pole telescope to achieve their incredible baseline, so that should restrict observation targets to the southern hemisphere.
mlk
CC BY-SA 4.0
2022-04-30T16:21:57.930Z
Keep an eye on EHT news May 12 :)
pela
CC BY-SA 4.0
2019-04-11T00:23:47.707Z
2019-04-12T20:22:33.543Z
2019-04-12T20:22:33.543Z
Ingolifs
<p>There are closer galaxies than Messier 87 for sure, even ours! It sparked my curiosity that they went with one 53 million light years away. Is there a reason for this?</p>

CC BY-SA 4.0
2019-04-11T17:18:49.853Z
Because black holes are dangerous and the IRB wouldn't let them get too close.
David Richerby
CC BY-SA 4.0
2019-04-12T03:15:40.047Z
53 million ly is just a short trip down the road, galactically speaking.
PM 2Ring
CC BY-SA 4.0
2019-04-12T22:52:41.053Z
I'd like to ask, so as better understand your question: Why take a picture of a closer black hole?
Williham Totland
CC BY-SA 4.0
2019-04-14T14:36:53.810Z
Elliptical galaxy with a nice jet may have made M87 a very tempting target: https://duckduckgo.com/?q=Messier+87&t=ffsb&ia=images&iax=images&iai=https%3A%2F%2Fwww.universetoday.com%2Fwp-content%2Fuploads%2F2009%2F12%2FM87AGN-239x250.jpg Keep scrolling. The BH image looks to be taking over.
Wayfaring Stranger
CC BY-SA 4.0
2020-10-13T15:31:33.093Z
Interesting question! I've just asked [Will the first Event Horizon Telescope image of the Milky Way's black hole be just another orange donut?](https://astronomy.stackexchange.com/q/39352/7982)
uhoh
<black-hole><supermassive-black-hole><event-horizon-telescope>
Why not take a picture of a closer black hole?
Morgan
# Why not take a picture of a closer black hole?

(Asked by: username_0 on 2019-04-10)

There are closer galaxies than Messier 87 for sure, even ours! It sparked my curiosity that they went with one 53 million light years away. Is there a reason for this?

**Tags:** black-hole, supermassive-black-hole, event-horizon-telescope

> **Comments:**
> **username_1**: Because black holes are dangerous and the IRB wouldn't let them get too close.  
> **username_2**: 53 million ly is just a short trip down the road, galactically speaking.  
> **username_3**: I'd like to ask, so as better understand your question: Why take a picture of a closer black hole?  
> **username_4**: Elliptical galaxy with a nice jet may have made M87 a very tempting target: https://duckduckgo.com/?q=Messier+87&t=ffsb&ia=images&iax=images&iai=https%3A%2F%2Fwww.universetoday.com%2Fwp-content%2Fuploads%2F2009%2F12%2FM87AGN-239x250.jpg Keep scrolling. The BH image looks to be taking over.  
> **username_5**: Interesting question! I've just asked [Will the first Event Horizon Telescope image of the Milky Way's black hole be just another orange donut?](https://astronomy.stackexchange.com/q/39352/7982)  

---

## 5 answers

### Answer by username_15 on 2019-04-11. Score: 88 | Accepted answer

I was surprised too when I first heard they were trying to image M87's black hole.

The short answer is because it's really, really big. It is 1500 times bigger (diameter) than our Sagittarius A\*, and 2100 times farther away. This makes its apparent size about 70% of that of Sgr A\*, which they are also attempting to image.

A cursory search of wikipedia's [List of Largest black holes](https://en.wikipedia.org/wiki/List_of_most_massive_black_holes) shows that there's no other black holes with a combination of size and closeness greater than these two.

A couple of other candidates are not too far off. Andromeda's black hole is 50x the size of ours, and at 100x the distance, it would appear half the size of Sgr A\*. The Sombrero galaxy is 380 times farther way than Sgr A\*, and has a black hole estimated to be 1 billion solar masses, which is 232 times Sr A\*, resulting in an angular diameter about 60% of Sgr A\*.

There appear to be many other considerations to which black holes were chosen, as explained in [this similar question.](https://astronomy.stackexchange.com/questions/30313/why-didnt-the-event-horizon-telescope-team-mention-sagittarius-a) At a guess these would include how obscured each black hole is with foreground dust/stars etc, how active (and therefore bright) the nuclei are, and their inclination w.r.t earth affecting which observatories could observe them at which times.

Edit: I've found another plausible candidate. [NGC\_1600](https://en.wikipedia.org/wiki/NGC_1600) is 200 M light years away with a central black hole estimated to be 17 billion solar masses heavy. This would put it at about 40% the apparent diameter of Sgr A\*.

[![black hole size comparison, self made](https://i.sstatic.net/cdsKB.png)](https://i.sstatic.net/cdsKB.png)
**Comparison of the apparent size of the largest nearby black holes**

And of course [obligatory XKCD](https://xkcd.com/1276/) to remind us how small these objects really appear.

> **Comments:**
> **username_16**: Don't forget to say space is kind of dusty in the direction of Sgr A*. It's quite a bit clearer in the direction of M87.  
> **username_17**: @FlorinAndrei Actually that doesn't really matter much, since the observations are made in radio where there's barely any extinction.  
> **username_18**: @FlorinAndrei It may be dusty in the direction of Sgr A* but it’s messier in the direction of M87.  
> **username_17**: @KonradRudolph I see what you did there :D  
> **username_19**: Given that we are on the edge of the milky way, half the dust and contents of the milky way are between us and Sgr A*. I'm not sure the exact orientation of milky way relative to M87, but I believe they mentioned Sgr A* being obscured in the press conference. They also said Sgr A*, being much smaller, was also much more highly variable over time. I REALLY hope they make a time lapse video or something. That'd rule.  
> **username_20**: Don't forget. Sgr A* would be seen rather from the side then from zenith/nadir  
> **username_21**: @username_17 on the other hand, we never saw what he did in the direction of Sgr A*.  
> **username_22**: I'm not sure this answer really fully answers the question. In the "short answer" paragraph, it states that M87's apparent size is 70% that of Sgr A*. That, by itself, would appear to make it a *worse* candidate than Sgr A*. The question wants to know why a closer black hole wasn't chosen. Sgr A* is closer - why not choose it (and indeed the question even wants to know why not "even ours")? The answer would be improved by elaborating on the other factors that make M87 a better candidate than Sgr A*  
> **username_15**: @username_22 You're right, I skipped over Sgr A* because I assumed people knew they were also trying to image it. I'll edit my answer to make that explicit.  
> **username_23**: I am wondering though, how many of those candidates are actually visible to the whole array. From what I understand, they need the south pole telescope to achieve their incredible baseline, so that should restrict observation targets to the southern hemisphere.  
> **username_17**: Keep an eye on EHT news May 12 :)  

---

### Answer by username_9 on 2019-04-11. Score: 36

There are a few criteria necessary to see a black hole with the [Event Horizon Telescope](https://en.wikipedia.org/wiki/Event_Horizon_Telescope). They are, in importance:

* Active Feeding: you need a thick accretion disk with lots of matter accreting onto the black hole. M87 fits this criteria, and is a glut, [consuming about 90 Earth masses a day](https://arxiv.org/abs/astro-ph/0202238).
* Apparent size. Even though it is 53 million light-years away, M87 is 6.5 billion solar masses. Since the radius of the event horizon scales linearly with mass, its distance is made up for by sheer scale.

> **Comments:**
> **username_10**: Calculates quickly... the M87 BH consumes one Earth mass every 16 MINUTES!  
> **username_11**: Mmmmm, Earth-masses. :)  

---

### Answer by username_6 on 2019-04-11. Score: 21

As username_15 says, Sgr A\* and M87\* are the obvious candidates. At the press conference, Heino Falcke explained why they got a picture of M87\* first:

> But it would take some more time because Sagittarius A Star is 1000 times faster and smaller. Its like a toddler who is moving constantly. In comparison, M87 is much slower, like a big bear.

— [The Deccan Herald](https://www.deccanherald.com/science-and-environment/astronomers-release-first-photo-of-black-hole-727987.html)

> **Comments:**
> **username_7**: That analogy doesn't really work. I can reliably confirm that a toddler runs slower than a bear.  
> **username_8**: @username_7 I'm so sorry for your loss  

---

### Answer by username_12 on 2019-04-11. Score: 16

Another quick note - They are trying to get a photo of Sag. A\*:

From [Space.com](https://www.space.com/first-black-hole-photo-by-event-horizon-telescope.html)

> The project has been scrutinizing two black holes — the M87 behemoth, which harbors about 6.5 billion times the mass of Earth's sun, and our own Milky Way galaxy's central black hole, known as Sagittarius A\*. This latter object, while still a supermassive black hole, is a runt compared to M87's beast, containing a mere 4.3 million solar masses.
>
> Both of these objects are tough targets because of their immense distance from Earth. Sagittarius A\* lies about 26,000 light-years from us, and M87's black hole is a whopping 53.5 million light-years away.
>
> From our perspective, Sagittarius A\*'s event horizon "is so small that it's the equivalent of seeing an orange on the moon or being able to read the newspaper in Los Angeles while you're sitting in New York City," Doeleman said during the SXSW event last month.
>
> ...
>
> And in case you're wondering about Sagittarius A\*: The EHT team hopes to get imagery of that supermassive black hole soon, Doeleman said today. The researchers looked at M87 first, and it's a bit easier to resolve than Sagittarius A\* because it's less variable over short timescales, he explained.


---

### Answer by username_13 on 2019-04-13. Score: 0

The difficulty in imaging anything on the visible-light spectrum is dust. Sagitarrius A is clouded by dust clouds which can be penetrated with infrared. The M87 fulfilled the criteria of being big and relatively close while at the same time enabling light to reflect off the event horizon and not being blocked by dust clouds.

> **Comments:**
> **username_14**: They weren't imaging in visible light, or even anything we'd normally call IR. Very short radio waves, really. Also nothing reflects of an event horizon.  
> **username_13**: Well, if nothing did reflect off an event horizon there would be no photo to take.  QED  
> **username_14**: The photo is essentially a silhouette. A **very** crude approximation is that there is light coming from hot stuff *behind* the black hole, and we see it with a dark area in the middle corresponding to the black hole.  This is an approximation because a black hole is a distorted are of space and time, and light takes strange paths near to one. However what you see does look like a bright ring around a dark center from the right angle  

---

