Image: The black hole is invisible, it is a point in the center of this image taken in the millimeter range (1.3 mm wavelength).
The dark central part that surrounds the black hole of the galaxy M87 is the shadow of the black hole, that is to say the sphere of the horizon of events.
The orange and yellow part is ionized gas spinning around the black hole. The more intense yellow light from ionized gas is due to the relativistic Doppler effect which causes light coming towards us to be brighter than light going away from us.
It was in April 2019 that astrophysicists unveiled this simple, blurry and asymmetrical orange-yellow halo of light, which evokes a black hole.
Image credit: EHT (Event Horizon Telescope) Interferometer.
A black hole is a fascinating and mysterious astrophysical object that defies our understanding. Its mass is extremely high and its volume very small, it's a hole in space-time, maybe a point! we are talking about singularity.
This is why we never see a black hole but only the phenomena which appear around it and which make it possible to define it.
In the region around the black hole, the escape velocity is equal to the speed of light (Vech = c).
From the escape velocity we can calculate the radius of this area, called the event horizon, V²ech = 2 GM/R.
The radius of the black hole, which corresponds to the event horizon, is proportional to its mass r = 2GM/c^2 (Schwarzschild radius) where G is the gravitational constant, M the mass of the black hole, and c the speed light.
For example, for a black hole of a solar mass we obtain a radius of 3 km, in other words, a sphere like the Sun which has a radius of 696340 km should be condensed on 3 km, to become a black hole.
The shadow of the black hole is a phenomenon predicted by the general theory of relativity of Albert Einstein (1879-1955).
Due to its intense gravitation, a black hole prevents light from escaping from the dark, spherical region that forms around it in spacetime. This region represents the shadow of the black hole. The light is so deflected by the gravity of the black hole that it remains trapped in this area which appears black to us.
The shadow of the black hole measures on this image a light week (about 18.6 billion km). The shadow of the black hole is not the black hole but its manifestation.
The event horizon and the shadow of a black hole are two different but closely related concepts because they represent the same region of space.
- The event horizon an imaginary spherical surface that marks the boundary of the region of spacetime where the gravitational field is strong enough to prevent any matter or radiation from escaping from it.
- The black hole shadow is the volume of space that appears dark in a black hole image. This region lies within the event horizon. In this zone, the light rays are so bent by the intense gravitation, that they are directed towards the center of the black hole and cannot be observed.
In other words, the formation of a black hole's shadow is caused by the bending effect of light around the black hole.
Light coming from behind a black hole is bent following the contours of distorted spacetime. It is so curved that it cannot reach a distant observer, thus creating a dark region in the observer's field of vision.
In summary, the shadow of the black hole is the projection on the cosmic background of the accretion disk of the black hole. It lies within the event horizon. This shadow can be observed indirectly using advanced imaging techniques, including very long baseline interferometry (VLBI) imaging or event horizon imaging (EHT).