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Comment by nvahalik | Hacker News Reader

FTA: "An object in the centre of our galaxy (the Milky Way) called Sagittarius A* is thought to be such a “supermassive” black hole."

If this black hole is 55M LY away and the center of our galaxy should have one. So, why is it that we can see this one, but not the one at the center of our galaxy? Is it because there are things in our line of site with it?

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1) This one is actively ingesting material (active galaxy) so there are emissions from the accretion disk. Our galaxy is quiet, so there isn't the same emission from the black hole.

2) The one in our galaxy is 1600x closer and roughly that much smaller, so they're approximately the same angular size.

3) They have tracked the orbit of a star that orbits our galaxy's central black hole through an orbit, and it goes ~2% (iirc) of the speed of light at closest approach, but it's nowhere near the event horizon. There are other things that have been tracked near A*, but nothing at the event horizon like this one.

From what I've read, they actually are a similar size to image when size/distance was taken into account, and they did initially consider imaging SagA*, but M87 proved easier for them to image.

The black hole that they revealed is about 1,000 times larger than the one at the center of our milky way. So even though it's also 1,000 times further away, it's about the same angular diameter in the sky. Same size but less stuff in the way/more active makes it an easier target.

Our local SMBH isn't presently eating anything substantial, so it isnt very bright. M87's black hole is eating something enormous, so it's easier to get a picture.

I heard it can be hard to see our center black hole because we are on the Galaxy disk so we have to look through a lot of light pollution and dust things to see it?

Yeah, I was researching further and found this[0] SE question. Seems to also say the same thing that there is a lot of "space dust" between here and there.

[0]: https://astronomy.stackexchange.com/questions/30339/why-not-...

Sure, but at these long wavelengths the dust is effectively transparent.

My understanding is that the black hole imaged is both vastly bigger and much more active (i.e. actively consuming matter from its accretion disc) than Sgr A*. That makes it easier to get a compelling image out of it.

I don't really know if line of sight is a factor here though.

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