APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Comments and questions about the APOD on the main view screen.
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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Martin » Thu Apr 11, 2019 11:29 pm

Nitpicker wrote: Thu Apr 11, 2019 10:16 pm
Martin wrote: Thu Apr 11, 2019 7:28 pm
Nitpicker, not to bust bubbles but 'fake news' is an exploitation of the lowest common denominator. Try harder to rise above it, my friend.
Martin, you have incorrectly attributed the words of "Confused" to me, by mangling the quote tags.

It might be best to ease off on the condescension, too.
I see your point. I'm going back to just reading the comments.

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Ann » Fri Apr 12, 2019 2:04 am

rstevenson wrote: Thu Apr 11, 2019 7:32 pm
joe25 wrote: Thu Apr 11, 2019 1:35 pm This is actually a photo of angels dancing on the head of a pin. Can you count the number of them ?
42 -- obviously.

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Ann » Fri Apr 12, 2019 2:18 am

Chris Peterson wrote: Thu Apr 11, 2019 8:19 pm
JohnD wrote: Thu Apr 11, 2019 8:14 pm Again, hang on! All this talk of light that is 1.5, 2 or 2.5 (various figures quoted) the radius of the Event Horizon, "will be sucked in!!!"
The Event Horizon is the Event Horizon - that is the distance from the BH itself at which, thanks to the gravity well, escape velocity equals light speed. Anything further out CAN get away, if it goes at light speed, in the right direction. Hawking himself said so, in theorising Hawking radiation. Stuff further out MAY go in, but it's not on that handcart to … whatever there is inside the EH!
That's true. But in this case, absent some kind of external influence that doesn't appear to exist, the light in that region is too close to escape (or at the least, so little escapes that the region appears black to our instruments). The important point is to explain why the actual event horizon is deep within the donut hole here.
APOD Robot wrote:

The first image was released yesterday and resolved the area around the black hole at the center of galaxy M87 on a scale below that expected for its event horizon.
So the image resolved the area around the black hole on a scale below that expected for its event horizon. But that means that the event horizon is smaller than expected? Otherwise we wouldn't be able to see anything there?

I'm also thinking of the extreme gravity near a supergiant black hole, and the gravity-induced reddening that must occur there. Is that why the picture was taken in radio waves? Okay, no. I can imagine several reasons why no attempt was made to photograph the black hole in visible light, including the fact that M87 is not a quasar or a blazar, its black hole accretion disk is not blindingly bright, and the light from it might so easily be overwhelmed by all the light generated by trillions of stars in this huge galaxy.
NASA wrote:
The elliptical galaxy M87 is the home of several trillion stars, a supermassive black hole and a family of roughly 15,000 globular star clusters.
In principle though, would the inner edge of the accretion disk even be visible in optical light, assuming there was a way to photograph it? Or would the gravity-induced reddening change the visible light (and even the far ultraviolet light) from that area into invisible long wavelengths?

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by neufer » Fri Apr 12, 2019 3:23 am

Ann wrote: Fri Apr 12, 2019 2:18 am
So the image resolved the area around the black hole on a scale below that expected for its event horizon. But that means that the event horizon is smaller than expected? Otherwise we wouldn't be able to see anything there?

I'm also thinking of the extreme gravity near a supergiant black hole, and the gravity-induced reddening that must occur there. Is that why the picture was taken in radio waves? Okay, no. I can imagine several reasons why no attempt was made to photograph the black hole in visible light, including the fact that M87 is not a quasar or a blazar, its black hole accretion disk is not blindingly bright, and the light from it might so easily be overwhelmed by all the light generated by trillions of stars in this huge galaxy.

In principle though, would the inner edge of the accretion disk even be visible in optical light, assuming there was a way to photograph it? Or would the gravity-induced reddening change the visible light (and even the far ultraviolet light) from that area into invisible long wavelengths?
The APOD black hole shadow (approximately 0.01 light year wide) lies within a broad hot disk that spans a maximum diameter of 0.39 light-years. Only the warmest central portion of that disk is visible in today's APOD. None of the observed hot disk is close enough to the event horizon that the thermal emission is significantly reddened to anywhere near the extent you are thinking about. Millimeter wavelengths are used because:
  • 1) they can both penetrate to the center of the activity
    2) as well as resolve the shadow using global interferometry.
https://en.wikipedia.org/wiki/Messier_87#Supermassive_black_hole wrote:
<<In April 2019, the Event Horizon Telescope released measurements of M87's supermassive black hole mass as 6.5 × 109 M. A rotating disk of ionized gas surrounds the black hole, and is roughly perpendicular to the relativistic jet. The disk rotates at velocities of up to roughly 1,000 km/s, and spans a maximum diameter of 0.12 parsecs (0.39 light-years). Gas accretes onto the black hole at an estimated rate of one solar mass every ten years (about 90 earth masses per day). Observation suggests that the black hole may be displaced from the galactic center by about seven parsecs (23 light-years). The displacement is in the opposite direction of the one-sided jet, which may indicate that the black hole was accelerated away by the jet. Another possibility is that the change in location occurred during the merger of two supermassive black holes.>>

:arrow: [top left] - This radio image of the galaxy M87, taken with the Very Large Array (VLA) radio telescope in February 1989, shows giant bubble-like structures where radio emission is thought to be powered by the jets of subatomic particles coming from the galaxy's central black hole. The false color corresponds to the intensity of the radio energy being emitted by the jet.

[top right] - A visible light image of the giant elliptical galaxy M87, taken with NASA Hubble Space Telescope's Wide Field Planetary Camera 2 in February 1998, reveals a brilliant jet of high-speed electrons emitted from the nucleus (diagonal line across image).

[bottom] - A Very Long Baseline Array (VLBA) radio image of the region close to the black hole, where an extragalactic jet is formed into a narrow beam by magnetic fields. The false color corresponds to the intensity of the radio energy being emitted by the jet. The red region is about 1/10 light-year across. The image was taken in March 1999.
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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Confused » Fri Apr 12, 2019 3:31 am

Chris Peterson wrote: Thu Apr 11, 2019 4:29 pm In astronomical and physical terms, "light" is usually synonymous with "electromagnetic radiation". That includes radio waves.
You are obviously a very knowledgeable astronomer but I was talking about people watching the TV news. You seem relatively inexperienced at communicating with common people. Common people do not consider radio waves to be light. Someone talking in TV news should make it clear that this is not visible light, not even ultraviolet or infrared. We should not require non-scientists to think like scientists to be understood. Mike Massimino in Fox News should not have said that the image that is shown as visible light is light. In previous posts here, there are descriptions of the methodology used to generate the image from the data. The image is something representing data, it is very different from what non-scientists would call visible light.
Last edited by Confused on Fri Apr 12, 2019 3:43 am, edited 1 time in total.

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by geckzilla » Fri Apr 12, 2019 3:41 am

Confused wrote: Fri Apr 12, 2019 3:31 am
Chris Peterson wrote: Thu Apr 11, 2019 4:29 pm In astronomical and physical terms, "light" is usually synonymous with "electromagnetic radiation". That includes radio waves.
You are obviously a very knowledgeable astronomer but I was talking about people watching the TV news. You seem relatively inexperienced at communicating with common people. Common people do not consider radio waves to be light. Someone talking in TV news should make it clear that this is not visible light, not even ultraviolet or infrared. We should not require non-scientists to think like scientists to be understood. The astronaut on TV that I described should not have said that the image that is shown as visible light is light.
On the contrary, I felt a greater understanding of the EMS as soon as I started thinking of it all as light, just that most of it my eyes do not receive. The public is afraid of things like microwaves and even sometimes radio waves, because someone, somewhere decided to make the distinction that it's not light, and that's a mistake. Everything from the way cell towers work to your microwave oven makes a bit more sense when you think of it in terms of invisible light that reflects, transmits, absorbs, and scatters all these different wavelengths of light.

In my opinion, calling it radiation has been far more confusing and less intuitive. In some regards, it's even harmful and prevents understanding as the fear outweighs the explanation.
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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Nitpicker » Fri Apr 12, 2019 3:47 am

Ann wrote: Fri Apr 12, 2019 2:18 am So the image resolved the area around the black hole on a scale below that expected for its event horizon. But that means that the event horizon is smaller than expected? Otherwise we wouldn't be able to see anything there?

I'm also thinking of the extreme gravity near a supergiant black hole, and the gravity-induced reddening that must occur there. Is that why the picture was taken in radio waves? Okay, no. I can imagine several reasons why no attempt was made to photograph the black hole in visible light, including the fact that M87 is not a quasar or a blazar, its black hole accretion disk is not blindingly bright, and the light from it might so easily be overwhelmed by all the light generated by trillions of stars in this huge galaxy.
The resolution achieved by the EHT on M87, was 20 μas, which is about the size of the shadow, which is ~2.5 times larger than the event horizon. But the image scale is smaller, in the sense that the pixels in the image are much smaller than the event horizon. The shadow is pretty much exactly the size it was expected to be.

It was recorded in (almost infra-red, Extremely High Frequency) radio waves, because the best interferometers yet built by humans for infrared and visible light can "only" achieve resolutions a little smaller than a milli arc second. They needed close to the shortest possible radio wavelength and the longest possible baseline on Earth, to achieve a 20 μas resolution.

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Confused » Fri Apr 12, 2019 3:52 am

geckzilla wrote: Fri Apr 12, 2019 3:41 am The public is afraid of things like microwaves and even sometimes radio waves, because someone, somewhere decided to make the distinction that it's not light, and that's a mistake.
I don't think so but that is off-topic for here so I won't comment further.
geckzilla wrote: Fri Apr 12, 2019 3:41 amEverything from the way cell towers work to your microwave oven makes a bit more sense when you think of it in terms of invisible light that reflects, transmits, absorbs, and scatters all these different wavelengths of light.

In my opinion, calling it radiation has been far more confusing and less intuitive. In some regards, it's even harmful and prevents understanding as the fear outweighs the explanation.
Many people can benefit from what you can teach us but we need common terminology to communicate. I am saying that using the terminology that scientists use with people unaccustomed to scientific discussion will confuse people.

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Confused » Fri Apr 12, 2019 4:05 am

Martin wrote: Thu Apr 11, 2019 7:28 pm
Confused wrote: Thu Apr 11, 2019 4:19 pm Well let me put it another way. Is there any chance that this is fake news? I assume that among scientists it is understood that this is a theory that could be flawed in some way, but the TV news people are reporting is fact.
Confused, is it your assertion that everything in the observable universe is identical? Not to bust bubbles but 'fake news' is an exploitation of the lowest common denominator. Try harder to rise above it, my friend.
I made a mistake in my previous post so I will assume that caused a miscommunication. I have changed the previous post saying TV news people are reporting is fact to instead say TV news people are reporting it as fact.

If the current findings persist as being fact then it is a fantastic development. If however further data leads to a different discovery then the current news is fake news. I am not a scientist but I think that astronomers are too quick to assume things.

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Ann » Fri Apr 12, 2019 4:07 am

Thanks for your explanations, Art and Nitpicker.
APOD Robot wrote:
Pictured, the dark central region is not the event horizon, but rather the black hole's shadow -- the central region of emitting gas darkened by the central black hole's gravity.
Is it correct to say that the black hole's shadow is caused by gravity-induced reddening?

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Chris Peterson » Fri Apr 12, 2019 4:10 am

Confused wrote: Fri Apr 12, 2019 3:52 am Many people can benefit from what you can teach us but we need common terminology to communicate. I am saying that using the terminology that scientists use with people unaccustomed to scientific discussion will confuse people.
The proper terminology is what the scientists are using. Dumbing it down does nobody any good. There's nothing very confusing about this, and every single story I've seen, aimed at all different audiences, has made it very clear (complete with pictures and diagrams) that this image was obtained with radio telescopes.
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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Nitpicker » Fri Apr 12, 2019 4:28 am

Confused wrote: Fri Apr 12, 2019 4:05 am I made a mistake in my previous post so I will assume that caused a miscommunication. I have changed the previous post saying TV news people are reporting is fact to instead say TV news people are reporting it as fact.

If the current findings persist as being fact then it is a fantastic development. If however further data leads to a different discovery then the current news is fake news. I am not a scientist but I think that astronomers are too quick to assume things.
I think your beef is with the people reporting the news on TV, not with science, or scientists, or astronomers in particular. Science is far less sensationalist than the TV news. And if evidence is one day found that the thing in this image is not the surrounds of a black hole, and the consensus of the scientific community is to agree with the new evidence, then science will simply be working in the way it is intended to work. It is a very big if, however. And I would not call it "fake news" unless I suspected some form of deliberate misinformation, for which I see no evidence at all.

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Astronymus » Fri Apr 12, 2019 4:35 am

Confused wrote: Fri Apr 12, 2019 4:05 am If however further data leads to a different discovery then the current news is fake news. I am not a scientist but I think that astronomers are too quick to assume things.
Then it would be "false news".
But you can be quite sure scientists today learned from mistakes in history not to make quick assumptions. You also have to bear in mind that the information from the science community is filtered by journalists to be comprehensible for the public. So each report is a mere interpretation by someone (in most cases) not a scientist to help you understand what the scientists discovered. Because most of us lack the qualification for astrophysics. As journalists are humans also or they lack the insight in the area of studies their reports sometimes end up a false interpretation of scientific information too.

"Fake news" is the deliberate misinformation of the public by political intent.


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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by geckzilla » Fri Apr 12, 2019 4:37 am

don't take it from me... my authority is all but nonexistent, anyway... the people who study how the public perceives astronomical imagery call it light. and those people who have to deal with some of the most challenging astronomical imagery to convey to the public are at the chandra x-ray observatory. http://chandra.harvard.edu/learn_light.html

this is how it is. they didn't just come up with this idea and say eh, let's go with it. they have run surveys over multiple years and used that data to make these decisions. they've posted papers about it... look up aesthetics and astronomy if you want to read some of the literature.
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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by RocketRon » Fri Apr 12, 2019 6:57 am

Referring to anything here as 'light' suggests that this image may have been taken with a camera ?
Which is terribly misleading - radio telescopes operate as anything but visual images.

Again, this 'image' has been assembled by an algorithm.
Which has had no discussion yet ?

Depending on the intracies of that algorithm will determine what the final image consists of.
Different algorithms could (conceivably) produce pyramids, squares or any shape, or random non-shape you can imagine ??
OR, conceivably, a random mish-mash of data, which has no structure !!

So, how did that algorithm assemble that image ??
In some detail, generalities aren't going to be enough.
ALL that data is coming from a pinpoint source 55 million light years away,
so the methodology is going to have to be thoroughly examined and questioned.

??

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by RocketRon » Fri Apr 12, 2019 7:06 am

Ann wrote: Fri Apr 12, 2019 4:07 am Is it correct to say that the black hole's shadow is caused by gravity-induced reddening?
I'm not even sure what gravity induced reddening is,
but I would question how a radio astronomy-produced image even had any red component in it.
Or any color really.

The data is all just radio noise/signals, so any color has to have come from post observation processing. ?
Although we have become used here to color introduced into astro images, even if there is no standardisation of how this is applied.

??

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by JohnD » Fri Apr 12, 2019 7:38 am

Nitpicker wrote: Thu Apr 11, 2019 10:48 pm With M87 being 12 degrees north of the celestial equator, I am not sure how the South Pole Telescope might have helped in this image. Certainly useful for Sgr A*, however, the data from which, I assume, is currently being analysed. I am now wondering how a resolution of 20 micro arc seconds was achieved on M87 without the SPT.
Since the Tropics are 23 degrees either side of the Equator, M87 will be 11 degrees above the South Pole's horizon on their mid-summer day. That's how.
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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by moontrail » Fri Apr 12, 2019 7:50 am

Are there any schemes depicting this black hole, its spinning axis, the disk accretion and our relative position to them? Is the black hole spinnig causing the disk accretion configuration instead of a spherical one. Thanks in advance.

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Ann » Fri Apr 12, 2019 8:47 am

RocketRon wrote: Fri Apr 12, 2019 7:06 am
Ann wrote: Fri Apr 12, 2019 4:07 am Is it correct to say that the black hole's shadow is caused by gravity-induced reddening?
I'm not even sure what gravity induced reddening is,
but I would question how a radio astronomy-produced image even had any red component in it.
Or any color really.

The data is all just radio noise/signals, so any color has to have come from post observation processing. ?
Although we have become used here to color introduced into astro images, even if there is no standardisation of how this is applied.

??
"Reddening" means that the wavelengths are getting longer, for example by being stretched by strong gravity. So even radio waves can get reddened, even though they don't have any visible color. They, too, can get stretched in a field of strong gravity.

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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Nitpicker » Fri Apr 12, 2019 8:48 am

JohnD wrote: Fri Apr 12, 2019 7:38 am
Nitpicker wrote: Thu Apr 11, 2019 10:48 pm With M87 being 12 degrees north of the celestial equator, I am not sure how the South Pole Telescope might have helped in this image. Certainly useful for Sgr A*, however, the data from which, I assume, is currently being analysed. I am now wondering how a resolution of 20 micro arc seconds was achieved on M87 without the SPT.
Since the Tropics are 23 degrees either side of the Equator, M87 will be 11 degrees above the South Pole's horizon on their mid-summer day. That's how.
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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by JohnD » Fri Apr 12, 2019 10:56 am

No? Please explain?
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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by neufer » Fri Apr 12, 2019 11:19 am

JohnD wrote: Fri Apr 12, 2019 10:56 am
No? Please explain?
Until the Earth's 'gyroscope' precesses in a few thousand years the bottom of the Earth cannot observe fixed celestial objects at positive Declination. Nevertheless, the South Pole Telescope (SPT) is still a member of the Event Horizon Telescope (EHT) telescope array and one that should prove invaluable in mapping the Sagittarius A* black hole (Declination −29° 0′ 28″).
https://en.wikipedia.org/wiki/Event_Horizon_Telescope wrote:
<<The Event Horizon Telescope (EHT) is a large telescope array consisting of a global network of radio telescopes and combining data from several very-long-baseline interferometry (VLBI) stations around the Earth. The aim of the EHT project is to observe the immediate environment of supermassive black holes, e.g., Sagittarius A* at the center of the Milky Way as well as the even larger black hole in the center of the supergiant elliptical galaxy Messier 87, with an angular resolution of about 25 micro-arcseconds, comparable to the angle subtended by the black hole's event horizon.

The first image of a black hole, the supermassive one at the center of galaxy Messier 87, was published by the EHT Collaboration on April 10, 2019. The black hole was given the nickname Pōwehi, meaning "embellished dark source of unending creation" in Hawaiian.>>
https://en.wikipedia.org/wiki/South_Pole_Telescope wrote:

<<The South Pole Telescope (SPT) is a 10-meter diameter telescope located at the Amundsen–Scott South Pole Station, Antarctica. The telescope is designed for observations in the microwave, millimeter-wave, and submillimeter-wave regions of the electromagnetic spectrum, with the particular design goal of measuring the faint, diffuse emission from the cosmic microwave background (CMB). The first major survey with the SPT–designed to find distant, massive, clusters of galaxies through their interaction with the CMB, with the goal of constraining the dark energy equation of state–was completed in October 2011. In early 2012, a new camera (SPTpol) was installed on the SPT with even greater sensitivity and the capability to measure the polarization of incoming light. This camera operated from 2012–2016 and was used to make unprecedentedly deep high-resolution maps of hundreds of square degrees of the Southern sky. In 2017, the third-generation camera SPT-3G was installed on the telescope, providing nearly an order-of-magnitude increase in mapping speed over SPTpol.

The SPT collaboration is made up of over a dozen (mostly North American) institutions, including the University of Chicago, the University of California, Berkeley, Case Western Reserve University, Harvard/Smithsonian Astrophysical Observatory, the University of Colorado Boulder, McGill University, The University of Illinois at Urbana-Champaign, University of California, Davis, Ludwig Maximilian University of Munich, Argonne National Laboratory, and the Fermi National Accelerator Laboratory. It is funded by the National Science Foundation and the Department of Energy.

The South Pole is the premier observing site in the world for millimeter-wavelength observations. The Pole's high altitude (2.8 km above sea level) means the atmosphere is thin, and the extreme cold keeps the amount of water vapor in the air low. This is particularly important for observing at millimeter wavelengths, where incoming signals can be absorbed by water vapor. Because the sun does not rise and set daily, the atmosphere at the pole is particularly stable. Further, there is no interference from the sun in the millimeter range during the months of polar night.
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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by neufer » Fri Apr 12, 2019 11:43 am

Ann wrote: Fri Apr 12, 2019 4:07 am
Is it correct to say that the black hole's shadow is caused by gravity-induced reddening?
It is correct to say that the black hole's shadow is an extreme example of the Sun's extended 'shadow' observed by Arthur Eddington caused by background stars nearest the Sun appearing to be pushed outward by GR light bending. Replace the Sun with its BH and there would be a region (Einstein ring) of avoidance at the center larger than the event horizon.

However, instead of pertaining to a large Einstein ring (10,000 times the Solar black hole size!) for a distant radiation source behind a BH it pertains to a much smaller Einstein ring for a close (and slightly reddened) radiation source behind a BH.
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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Ann » Fri Apr 12, 2019 12:37 pm

RocketRon wrote: Fri Apr 12, 2019 7:06 am
Ann wrote: Fri Apr 12, 2019 4:07 am Is it correct to say that the black hole's shadow is caused by gravity-induced reddening?
I'm not even sure what gravity induced reddening is,
but I would question how a radio astronomy-produced image even had any red component in it.
Or any color really.

The data is all just radio noise/signals, so any color has to have come from post observation processing. ?
Although we have become used here to color introduced into astro images, even if there is no standardisation of how this is applied.

??
You deserve a better answer than the one I just gave you.

Near the tremendous gravity of a black hole, all wavelengths will be stretched and become longer than they were originally.

The Tadpole Galaxy.
NASA, H. Ford (JHU), G. Illingworth (USCS/LO), M. Clampin (STScI),
G. Hartig (STScI), the ACS Science Team, and ESA.
There is another cause that will stretch wavelengths and make them become longer, and that is the expansion of the Universe.

At right you can see a picture of the Tadpole Galaxy, located (according to Wikipedia) some 420 million light-years away.

The picture at right is too small for me to demonstrate my point to you. You need to look at this 600 KB picture of the Tadpole Galaxy. Enlarge the picture as much as you can, and look at the background galaxies. As you can see, the background galaxies are quite small, because almost all of them are much farther away than the Tadpole Galaxy. (A few might be dwarf galaxies at the same distance as the Tadpole Galaxy.)

Please note that almost all the background galaxies are quite reddish in color. Note in particular the group of galaxies at far left, seemingly just below the Tadpole Galaxy. They are very orange-red. Why is that?

The answer is that all the orange background galaxies in this picture have been redshift-reddened. That means that the light that they once emitted has been stretched, so that it became longer and redder, because of the expansion of the Universe. (I'm sure you know of the expansion of the Universe.) This means that when the light waves travelled through space in the direction of the Earth, space itself expanded, and the wavelengths of the light stretched and became longer because the Universe "stretched".
This means, too, that the farther away a galaxy is, the redder its light will be. That is because their light has been travelling and "stretching" for the longest time. Some galaxies will be so "red" that they can't be detected in visible light at all, and astronomers have to look for them using longer wavelengths.

But there is another form of redshift that doesn't involve stretching. That is the Doppler shift redshift. Have you ever heard an ambulance or a police car approaching you, sirens blaring? You may have noticed that when they pass you, the sound of the sirens changes, becoming more low-pitched.

It is the same thing with stars and galaxies in the nearby universe. In our own galaxy, some objects are redshifted because they are moving away from us, and other are blueshifted because they approach.

The light from every star contains spectral lines, and the position, number and strength of these spectral lines tell us what kind of star emitted them.

When a nearby object becomes Doppler redshifted because it moves away from us, the spectral lines move towards the red part of the spectrum. When a nearby object becomes Doppler blueshifted because it moves towards us, the spectral lines move towards the blue part of the spectrum.

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Chris Peterson
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Re: APOD: First Horizon-Scale Image of a Black... (2019 Apr 11)

Post by Chris Peterson » Fri Apr 12, 2019 1:16 pm

RocketRon wrote: Fri Apr 12, 2019 6:57 am Referring to anything here as 'light' suggests that this image may have been taken with a camera ?
Which is terribly misleading - radio telescopes operate as anything but visual images.

Again, this 'image' has been assembled by an algorithm.
Which has had no discussion yet ?

Depending on the intracies of that algorithm will determine what the final image consists of.
Different algorithms could (conceivably) produce pyramids, squares or any shape, or random non-shape you can imagine ??
OR, conceivably, a random mish-mash of data, which has no structure !!

So, how did that algorithm assemble that image ??
In some detail, generalities aren't going to be enough.
ALL that data is coming from a pinpoint source 55 million light years away,
so the methodology is going to have to be thoroughly examined and questioned.
There are a dozen links in the caption and subsequent pages of discussion here to sites that explain, at all different levels of detail, how this image was made, what wavelength was used, how interferometry works, what the different parts of the image are showing us and why they look the way they do as a consequence of general relativity. There are also superb and easily used resources like Wikipedia where you can quickly look up concepts like "reddening" and see what they mean.

BTW, this is hardly the first image produced this way, or the first one to be featured on APOD. A number of images have been shown which were created by radio telescope arrays, and they all use similar processing, because the method is straightforward and based on well understood math and optics. The exceptional thing about this image was the technology that increased the spatial resolution, and the consequences of this particular observation in terms of strengthening support for GR.
Chris

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