Learning about Black Hole Accretion

What are Black Holes?
  • Black holes exist thanks to how gravity works in our universe. Mass gets pulled into a dense ball due to gravity, but in most cases gets pushed back out due to other forces, such as pressure that causes energy to radiate out. In some cases the force of gravity becomes much stronger than anything that would resist it. This happens during the deaths of very large stars. When a star dies it explodes and collapses into either a white dwarf, neutron star, or black hole, depending on mass. When the mass of the star is 20 times greater than the mass of our sun, mass gets condensed so tightly that it forms a singularity, which is a point of infinite density. Physical rules start to get confusing here.

  • Unfortunately, we can't see what's happening in the singularity. This is because in a black hole gravity is so strong that not even light can escape. This is where black holes get their distinct darkness from. The area around the singularity where light can't escape is called the event horizon.

  • There are also supermassive black holes. These are the black holes that are found in the centers of galaxies, and can be a million to a billion times larger than the mass of our sun. These must have gotten their immense size during early galaxy formation.

    The supermassive black hole pictured in Interstellar. Light bends around it due to gravity.

  • There are other interesting features of black holes as well. Around the event horizon of a black hole, the gravity is still very strong. It curves spacetime, which means that time and space are warped in a way that causes light to look like it's bending. It also causes clocks close to the black hole to appear to run more slowly than clocks further away (known as time dilation). If a person were to enter this intense gravity, they would experience "spaghettification," which is what happens when the difference between gravity on a person's upper and lower body is so different that the person gets stretched out.
How we observe Black Holes
  • Due to the black nature of black holes, we have to find creative ways to identify and observe them. This is primarily done by observing other objects interacting with black holes. One way to do this is called gravitational lensing. Gravitational lensing works by finding objects that are slightly behind black holes so that their light gets stretched and warped. Another way is by finding stars that are orbiting an invisible object. Finally, another way to observe black holes is by finding X-ray emissions from their accretion disks.
    Sag A*

    An infrared observation over several years of stars orbiting the supermassive black hole in the center of our galaxy!

How Accretion Works
  • As stated earlier, black holes are caused by the deaths of very large stars. Less directly, and just like the other big stuff in the universe, black holes are formed when material collects due to gravity. This material is usually from clouds of gases, such as hydrogen, getting cold and sticking together. Gases expand when they are hot, and contract when they are cold. When enough material groups into clumps, it begins to have its own gravitational pull, which brings even more material into it. This is called accretion.

  • Gravity would cause the material to collapse into a sphere if not for the fact that when it collapses, it moves around and starts to rotate. As the material gets closer together, the rotation gets faster. This rotation causes some material to orbit without falling into the center, and we say it has angular momentum. When an object orbits another object, such as the moon orbiting Earth, there’s nothing to slow its momentum down and make it fall inwards. It will keep orbiting forever unless some force alters it. This is basically a balance of gravity and centrifugal force, which is the outward force on a rotating object.
    Sag A*

    An artistic rendition of a black hole accreting material.

  • In accretion disks, gas is constantly orbiting around the object (in our case a black hole). For the most part it is still bound by angular momentum, but there are complex forces which pull the gas closer into the black hole. When the gases get closer, they lose potential energy. This energy, which is always conserved, gets converted into kinetic and then thermal energy when particles collide with each other. The high temperature of this emission causes the energy to be radiated as X-rays, which is how we can "see" black holes.

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