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Comment by pjungwir | Hacker News Reader

The video JumpCrisscross shared above says this is the black hole at the center of our galaxy, so why isn't its accretion disk oriented the same as the rest of the galaxy? Isn't that weird?

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It's actually in a galaxy called Messier 87 which is 55 million lightyears away.

Oh, interesting! Wouldn't it be easier to photograph our own? :-)

Nope, for the same reason we had photos of the Moon before we had photos of Earth.

Can you expand on this?

What they're saying is that we had to send a camera away from the Earth (in a rocket) in order to photograph it properly, and similarly you'd need to send a camera away from our galaxy (in a biiiig rocket) in order to be able to photograph it properly.

During the press conference, they said photographing the M87 black hole was like shooting a hibernating bear, and photographing the Sag A* black hole of our galaxy like photographing a quickly moving toddler. Something about the speed making it much harder. It is also much smaller, but that's less of a problem because it's much closer. All in all apparently making it about the same angular size. But it sounds like they'll get to it, it's just harder.

That's a... weird analogy.

I think the actual analogy was lost in translation: the point is that, unlike its older brother, Sgr A* is not going to "pose" when you point your camera at it.

Ah, I thought it was about actually shooting bears and toddlers.

Actually not. M87 is a 1000 times farther away than Sgr A* but also a 1000 heavier and thus a 1000 times bigger in diameter. (Diameter/radius scale proportionally with the black hole's mass.) Therefore, the actual angular size on our night sky is the same for both black holes and, from this point of view, both would be equally difficult to observe.

However, as they mention in the press conference, Sgr A* moves a lot faster relative to us than M87, so it's much harder to take a still image. (In the press conference they used the example of trying to take a photo of a toddler with an exposure time of 8 hours.)

M87's black hole is currently eating something big, which makes it brighter. The black hole at the center of the milky way doesn't seem to have eaten anything lately, so it's accretion disk may be small or nonexistent.

> is currently eating something big

You mean: was eating something big 55 million years ago ;)

Nope. The view of our own galaxy's supermassive black hole is completely obstructed by matter within our own galaxy. You can't see to the core of our galaxy; it's too dense. You'd have to send a rocket quite some distance outside of the galactic plane to get a good view of it.

The images are made with radio telescopes, which cuts through the dust quite easily. We have many other radio observations of Sagittarius A* [1], albeit at much lower resolutions. There are also numerous observations of Sagittarius A* in X-ray wavelengths, which is also fine because they are so energetic they simply punch through. All the dust and gas in the galaxy is transparent at most wavelengths except the visible one.

The Event Horizon Telescope is interesting because it is, in essence, a radio telescope that uses a "sensor" that is the size of the entire Earth. As such, it is able to make much higher resolution observations.

[1] https://en.wikipedia.org/wiki/Sagittarius_A*#/media/File:Clo...

[2] https://en.wikipedia.org/wiki/Sagittarius_A*#/media/File:X-R...

The observed both supermassive black holes at the center of M87 and our Milky Way galaxy Sagittarius A

Not according to Veritasium's comment on his own video as well as the RelAstro group who produced the material[1]: "As there seems to be some general confusion, please note that the image shown here is a simulated one and not an actual image. So far we only have an image of M87. Kind regards, the RelAstro group. "
[1] https://www.youtube.com/watch?v=VnsZj9RvhFU

Ah you're right. Looks like they all should've skipped Sagittarius A instead of adding confusion.

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