So, we all heard what black holes are.
Let's not get into that "even light cannot escape, blla blla"...
They are regions of space where gravity is really strong.
Because matter has been crammed into such a small space, gravity is very powerful.
This can happen when a star is dying.
People cannot observe black holes because no light can escape outside; hence they are
invisible.
When matter falls toward a black hole, it does not fall straight into it.
Instead, it forms a large, thin disk as it spirals around the black hole.
This disk is known as an accretion disk.
An accretion disk, a disk-like flow of gas, plasma, dust, or particles around a black
hole in which material orbiting in the object's gravitational field steadily spirals inward,
losing energy and angular momentum.
The term accretion is used in astronomy to describe the increase in mass of any celestial
object due to gravitational attraction.
The astronomical object whose mass is increasing is known as the accretor.
Although no one has ever seen a black hole or even its event horizon, we can observe
the accretion disk because the spinning particles are pushed to incredible speeds by the black
hole's massive gravity, generating heat and intense x-rays and gamma rays out into the
universe as they collide.
When the matter being accreted has enough rotational or angular momentum, it cannot
simply fall inward into the black hole in a straight line, an accretion disk forms.
The flow tends to flatten into a disk in directions perpendicular to the black hole's rotation
axis because the rotation prevents the inflow of matter.
Matter contracts toward a plane in directions parallel to the rotation axis until the heat
pressure inside the accretion disk approximately equals the gravitational force.
The accretion disk will be geometrically thin, and its thickness will be much smaller than
its radial extent, if the heat pressure, which resists compression, is minimal compared to
gravity and rotation, which cause the accretion disk to contract vertically.
If pressure forces are comparable to rotation and gravity, the accretion disk will be geometrically
thick, looking more like a torus than a disk.
Due to turbulence and viscosity, the matter within the accretion disk spirals inward,
losing energy and angular momentum.
The material orbiting the black hole would swiftly sink toward the equatorial plane of
the accretion disk in the absence of any viscosity, turbulence, or other types of friction, and
then continue to travel in circular orbits.
For matter to flow inward and for accretion to occur, some type of friction that dissipates
angular momentum is required.
Viscosity in the black hole is too low to generate inward drift at the densities and
temperatures found in accretion disks.
The friction is considered to be caused by turbulence due to the rotation of the accretion
disk increasing any magnetic fields that already exist.
The effective viscosity that pulls matter inward while conveying extra angular momentum
outward is provided by turbulence.
As the accreted material falls or spirals inward, accretion disks generate enormous
amounts of energy from converting gravitational potential energy into radiation.
In the black hole, accretion can release energy more efficiently than thermonuclear reactions,
and is thus assumed to fuel the universe's most energetic objects, such as quasars, radio
galaxies, and X-ray binaries.
The primary sources of the mass supply for the accretion disk material can be from a
companion star (in the case of X-ray binaries and cataclysmic variables), the interstellar
medium (in the case of active galactic nuclei), or dust and gas in a protoplanetary disk (in
the case of planet formation).
The accretion disk's physical state and chemical composition are determined by the physical
conditions inside the disk and the source of the accretion disk material.
The temperatures in the accretion disks range from a few thousand to several million kelvins.
As a result, the accretion disk emits light with wavelengths ranging from infrared to
low-energy (soft) X-rays.
Parts of the accretion disk may evaporate, forming an even hotter low-density corona,
comparable to that of the Sun, releasing high-energy (hard) X-ray radiation.
Other accreting objects, like quasars, radio galaxies, X-ray binaries, and newborn stars
produce supersonic jets that eject a portion of the accreted material from their poles.
It is generally assumed that these jets are probably propelled by magnetic forces emerging
in the magnetic field lines that are helically twisted by the rotation of the accretion disk
and directed perpendicular to it.
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