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88 votes
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Why not take a picture of a closer black hole?

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 ...
Ingolifs's user avatar
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42 votes
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Why didn't the Event Horizon Telescope team mention Sagittarius A*?

There was a mention of Sagittarius A* during the Q+A portion of the press conference; the team indicated that they hope to produce an image sometime in the future (although they were careful to make ...
HDE 226868's user avatar
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37 votes
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Why are there uneven bright areas in this photo of black hole?

No, you aren't seeing the shape of the accretion disk. Although its plane is almost that of the picture it is far larger and fainter than the ring that is seen. The reason for this asymmetry is almost ...
ProfRob's user avatar
  • 153k
36 votes

Why not take a picture of a closer black hole?

There are a few criteria necessary to see a black hole with the Event Horizon Telescope. They are, in importance: Active Feeding: you need a thick accretion disk with lots of matter accreting onto ...
cms's user avatar
  • 606
21 votes

Why didn't the Event Horizon Telescope team mention Sagittarius A*?

I've found an explanation in Dutch here by Heino Falcke, one of the EHT founders. Translation: Hard to photograph It was easiest to take a picture of M87. "It is very difficult to photograph the ...
Glorfindel's user avatar
  • 4,790
21 votes

Why not take a picture of a closer black hole?

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: But it would take some more time because ...
user24582's user avatar
  • 311
19 votes
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Would Hubble Space Telescope improve black hole image observed by EHT if it joined array of telesopes?

No, it would not, because it operates in the visible spectrum and the EHT is an array of radio telescopes. For the "very long baseline interferometry" technique to work, all the telescopes ...
Mark Foskey's user avatar
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17 votes
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Doubt regarding size and shape of black hole images published by the Event Horizon Telescope (EHT)

The detail you seek is contained in Johansson (2014). The photon ring around a black hole is not the event horizon. It is the projection of unstable photon orbits that are able to loop around the ...
ProfRob's user avatar
  • 153k
16 votes

Why not take a picture of a closer black hole?

Another quick note - They are trying to get a photo of Sag. A*: From Space.com The project has been scrutinizing two black holes — the M87 behemoth, which harbors about 6.5 billion times the mass of ...
BruceWayne's user avatar
11 votes
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Why is the ring in this simulation of Sgr A* off center?

The photon ring around a non-spinning Schwarzschild black hole is perfectly circular and centered on the black hole. The photon ring around a spinning Kerr black hole is almost circular (except for ...
ProfRob's user avatar
  • 153k
10 votes

Why are there uneven bright areas in this photo of black hole?

There's some recent information which is worthy of an update to the answer (despite the difficulty of typing MathJax on my phone). I've quoted minimally as I wouldn't have improved upon what these ...
Rob's user avatar
  • 2,656
9 votes

Why are there uneven bright areas in this photo of black hole?

I believe we are seeing one of the effects of the accretion disk rotating at very high speeds. This is called relativistic beaming, and it occurs because particles (in this case matter in the ...
Jim421616's user avatar
  • 2,538
9 votes

How can light circle a black hole one or more times and then escape again?

As already explained in John Rennie's answer, the effective potential of the photon has a maximum at the photon sphere ($r = 1.5 r_s$). It can be shown that, in Newtonian mechanics, the effective ...
Prallax's user avatar
  • 4,431
9 votes
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Is this image of Sgr A* correctly labelled?

The event horizon of the Milky Way black hole (if it were non-spinning) has a diameter of 24 million km and is not directly related to the dark area. The black hole shadow should have an apparent ...
ProfRob's user avatar
  • 153k
8 votes

Do the "schwarzschild metric" or the "isotropic metric" fit the Event Horizon telescope data on Messier 87 better?

One of the things that can be confusing for beginners to GR is that there is no physical significance to the choice of coordinates we make. For example when studying static black holes we can use ...
John Rennie's user avatar
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7 votes
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Will the first Event Horizon Telescope image of the Milky Way's black hole Sgr A* just be another lumpy orange donut?

Yes. See today's press release from the Event Horizon Telescope. The image is remarkably similar to M87, but note that this a time-averaged composite over about 10 hours, because the emission from the ...
ProfRob's user avatar
  • 153k
7 votes
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Would the Event Horizon Telescope be able to produce a superior image of Betelgeuse?

Presumably, yes, EHT observations could improve on existing radio observations of Betelgeuse (e.g. recent ALMA images and comparatively ancient VLA images). Any observations would likely be targeted ...
HDE 226868's user avatar
  • 36.6k
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 ...
ProfRob's user avatar
  • 153k
6 votes

What else could the Event Horizon Telescope Observe?

How about Betelgeuse? Betelgeuse is about 640 light years away compared to 54 million light years for M87. It has an angular diameter of 0.042 to 0.056 arc seconds while the quoted resolution of the ...
Dave Gremlin's user avatar
  • 1,071
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*&...
Peter Erwin's user avatar
  • 17.1k
6 votes

Can the Event Horizon Telescope been used to find intergalactic distances?

For any type of imaging telescope: optical, radio, interferometric etc, the focal plane is at infinity. Consider a simple optical telescope, looking at a nearby planet (Jupiter). And suppose the ...
James K's user avatar
  • 123k
6 votes

Why is the ring in this simulation of Sgr A* off center?

The folder with the video this image comes from has a google docs document with captions and credits. The caption for this video reads "A video demonstrating the effect of inclination on the ...
planetmaker's user avatar
  • 19.5k
5 votes
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What else could the Event Horizon Telescope Observe?

If we could assume that most of the mm-wave emission from an ordinary star is photospheric, then the EHT could make a massive contribution to measuring the radii of stars. At the moment, this ...
ProfRob's user avatar
  • 153k
5 votes
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What is the orientation of the M87 black hole image relative to the jet?

The jet is projected onto the image roughly in the E-W direction with the (main) jet coming towards us to the right of the black hole. The brightening in the ring is perpendicular to this. It is ...
ProfRob's user avatar
  • 153k
5 votes

How, precisely, do radio astronomers detect (and record) the phases of waves for interferometry?

How, precisely, do radio astronomers detect (and record) the phases of waves for interferometry? The popular press always talks about directly 'interfering' two waves as they come in, but can they ...
uhoh's user avatar
  • 30.5k
4 votes
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Is there any update on the Availability of Event Horizon Telescope Images?

Today, April 10th 2019, there was a press conference where finally an image of M87 was released: Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations ...
Glorfindel's user avatar
  • 4,790
4 votes

What do the lines in this Event Horizon Telescope and Global mm-VLBI Array infographic represent?

I cant see a definitive statement, but I can see some infiormation and make some reasonable suggestions: The two colours represent the two different collaborations. Blue for the EHT, yellow for the ...
Steve Linton's user avatar
  • 10.3k
4 votes

Could images of (very) nearby targets be recovered from Event Horizon Telescope data?

The question asks about "targets" but doesn't specify black holes. However the other "targets" are related to infall of black holes. Nonetheless, I think the answer is a solid "yes" either way. Data ...
uhoh's user avatar
  • 30.5k
4 votes

Can the Event Horizon Telescope been used to find intergalactic distances?

The EHT is an array of radio telescopes, so it doesn't have a "focal plane" or "focal length" or "depth of field" like optical telescopes do. Instead, the EHT measures ...
Connor Garcia's user avatar
  • 16.3k
3 votes

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

Consider an extremely transparent lens. If you photograph the lens, what the camera really picks up where the lens covers is the distorted image of what is behind the lens. Would you say that is still ...
Mark Foskey's user avatar
  • 3,916

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