From: veritasium
An accretion disk is a structure formed by diffuse material orbiting a massive central body, such as a black hole, under the influence of gravity [00:01:32]. This material gradually spirals inward towards the central body.
The Event Horizon Telescope released what is likely to be the first-ever image of a black hole on April 10th, 2019 [00:00:00]. The black hole specifically observed is Sagittarius A*, located at the center of our Milky Way galaxy [00:01:28]. This black hole is surrounded by an accretion disk [00:01:32].
Characteristics of the Accretion Disk
The accretion disk is composed of dust and gas swirling chaotically [00:01:39]. Key characteristics include:
- Temperature The material in the disk is incredibly hot, reaching millions of degrees [00:01:45].
- Speed The matter moves at a significant fraction of the speed of light [00:01:48].
- Growth The black hole feeds off this matter, growing larger over time [00:01:50].
Innermost Stable Circular Orbit (ISCO)
The accretion disk does not extend all the way to the event horizon of the black hole [00:01:59]. This is due to the presence of an innermost stable circular orbit (ISCO) [00:02:04]. For a non-spinning black hole, this orbit is located at three Schwarzschild radii [00:02:10]. If matter goes inside this orbit, it quickly spirals into the center of the black hole [00:02:35].
While the black hole at the center of the Milky Way is likely spinning, models often simplify by considering the non-spinning case for explanation [00:02:21].
Appearance of the Accretion Disk
If there were no matter around a black hole, it would be impossible to image because it absorbs all electromagnetic radiation that falls on it [00:01:18]. The light from the accretion disk is what allows us to “see” the black hole’s shadow [00:01:32].
The way light rays are warped by the black hole’s gravity significantly affects the appearance of the accretion disk [00:03:51].
Viewing Angle Effects
The image of the accretion disk depends on the observer’s viewing angle [00:06:42].
- Perpendicular View If observed perpendicular to the disk, one would see the black hole’s shadow with the disk material around it [00:06:36].
- Edge-on View When viewed edge-on, the black hole’s warping of space-time causes light rays from the back of the accretion disk to bend over the top, appearing as if they are coming from the front [00:06:57]. Similarly, light from the bottom of the disk can bend underneath the black hole and appear to come from the top [00:07:21]. This can create an image resembling the black hole from the movie Interstellar [00:07:31].
- Multiple Images Light from the top of the disk can go around the back of the black hole, graze the photon sphere, and appear as a very thin ring underneath the shadow [00:07:39]. Similarly, light from underneath the disk can bend around the back and appear over the top, forming another ring of light [00:07:53].
Relativistic Beaming
Due to the high speeds of matter in the accretion disk, relativistic beaming (also known as Doppler beaming) occurs [00:08:14]. This effect causes the side of the accretion disk that is moving towards the observer to appear much brighter than the side moving away [00:08:18]. This phenomenon creates a distinct bright spot in the black hole image [00:08:29].
Observing how “blobs” of matter move in the accretion disk over time could provide further insights into black hole dynamics [00:09:08].