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I'm a newbie in this field so forgive me if I'm considering things wrong. As much as I can see, the newly published image of the Sgr A* looks pretty much the same as M87 released years ago. I'm wondering why the appearance of both nuclei are the same while M87 has a massive, active galactic nucleus with strong radio emissions and long relativistic jets, but the Sgr A* is not an AGN, is not as massive as M87 and doesn't have radio emissions as strong as M87 and yet the image of the accretion disks look quite the same?

From Wikipedia:

The comparatively small mass of this supermassive black hole, along with the low luminosity of the radio and infrared emission lines, imply that the Milky Way is not a Seyfert galaxy.

Actually I was expecting a simple dark circle with a narrow accretion disk with somehow lower luminosity(temperature?) in the image.

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The rings are not direct images of accretion disks. They are blurred images of the gravitationally lensed light from all around the black holes, with a central "shadow" due to photon orbits that fall into the black hole.

These are consequences of extreme bending of light close to the black hole, which means that light coming from multiple directions around the black hole is bent into the line of sight and appears as a ring around the black hole, while light that gets any closer falls into the black hole. When observed from a great distance, the central shadow appears to have a diameter of about 5 times the Schwarzschild radius of the black hole.

The central shadows of M87 and Sgr A* are a similar apparent diameter because their linear size depends on the Schwarzschild radius and hence the mass of the black hole, whilst the apparent angular size then depends how far away the black hole is. Since the M87 black hole is 1500 times more massive and 2000 times further away than Sgr A*, then their rings have a similar angular diameter.

In terms of brightness and temperature, you can't tell anything from false-colour, monochromatic images. The Sgr A* source is much less luminous, because the accretion rate is much lower than in M87. But it is 2000 times closer. In fact there is about 4 times as much flux at 1.3 mm wavelengths coming from the compact source in Sgr A*.

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    $\begingroup$ +n! for an "illuminating" overview $\endgroup$
    – uhoh
    May 14 at 0:30

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