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As part of the materials supporting the press release for the
Event Horizon Telescope image of the supermassive black hole at the centre of our Galaxy,
ESO produced
this handy explainer of the environment around a supermassive black hole.
The text in the image reads:
Singularity:
At the very centre of a black hole, matter has collapsed into a region of infinite density called a singularity.
All the matter and energy that fall into the black hole ends up here.
The prediction of infinite density by general relativity is thought to
indicate the breakdown of the theory where quantum effects become important.
Event horizon: This is the radius around a singularity where matter and energy
cannot escape the black hole's gravity: the point of no return.
This is the "black" part of the black hole.
Photon sphere:
Although the black hole itself is dark, photons are emitted from nearby hot plasma in jets or an accretion disc (see below).
In the absence of gravity, these photons would travel in straight lines, but just outside the event horizon
of a black hole, gravity is strong enough to bend their paths so that we see a bright ring surrounding a roughly circular
dark "shadow".
Relativistic jets:
When a black hole feeds on stars, gas or dust, the meal produces jets of particles and radiation blasting out
from the black hole's poles at near light speed. They can extend for thousands of light-years into space.
Innermost stable orbit:
The inner edge of an accretion disc is the last place that material can orbit safely without the risk
of falling past the point of no return.
Accretion disc:
A disc of superheated gas and dust whirls around a black hole at immense speeds, producing electromagnetic radiation
(X-rays, optical, infrared and radio) that reveal the black hole's location. Some of this material is doomed to cross the event
horizon, while other parts may be forced out to create jets.
(Image credit: ESO)
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