The Mysteries of Black Holes

A black hole is a region in space where huge mass is concentrated in such a small space making the gravity so strong that nothing can escape it, not even light. Albert Einstein first predicted the existence of black holes in 1916, with his general theory of relativity. The term “black hole” was coined many years later in 1967 by American astronomer John Wheeler.

An artistic representation of a black hole

Singularity

Center of a black hole is called singularity. It is the point in spacetime which contains a huge mass in an infinitely small space, where density and gravity become infinite and space-time curves infinitely. Its purely a mathematical thing. Zero size is a consequence of general relativity which fails in the quantum world. We need a Quantum gravity theory to explain singularities.

Singularity and black hole

Event Horizon

A black hole’s “surface” is called its event horizon. It defines the boundary where the velocity needed to escape exceeds the speed of light which itself is the cosmic speed limit. Since the information cannot get past the Event horizon, it hides the singularity.

Different Elements of black holes

Schwarzschild radius

The distance from event horizon to singularity is called Schwarzschild radius. The Schwarzschild radius of a non-rotating black hole with mass of our sun would only be 2.95km and for earth it would be 8.87×10−3 m.

What are their types of black holes?

  • Rotating black holes
  • Non-rotating black holes(hypothetical)
  • Stellar black holes (3 to 100 solar masses)
  • Supermassive black holes (Millions to billions of SM)
  • Miniature black holes(hypothetical) .
  • Intermediate black holes (100 to 1000)

How do Black holes form?

Stellar black holes are made when the core of a massive star collapses after burning its fuel. It causes a supernova. A supernova of an exploding star blasts outer layers of the star into space. Only Massive stars can form stellar black holes. Intermediate black holes are formed by Either merging of several stellar black holes or by stellar black holes feeding on stars.

Scientists are not sure about the origin of Supermassive black holes. Either Hundreds or thousands of black holes can merge together to form supermassive black holes or They can arise from the stellar cluster collapsing. They are thought to be present in the center of every galaxy.

Star life cycle

How they feed on different stars?

Once a star gets close enough the enormous gravity of the black hole stretches the stars beyond its breaking point until it is partially devoured and torn apart. These events are often known as “stellar tidal disruptions”. Not all the mass of the star is consumed by the black holes. Most of it is lost to the galaxy. Mass of the star which is consumed forms an accretion disk around the black hole Which is then slowly consumed by the black hole. Once the black holes become massive enough they don’t tear the stars apart. They just swallow them whole.

An artistic representation of black hole swallowing a star

Can Black holes merge?

Yes, Black holes can merge. First stage is called Inspiral when the black holes start orbiting each other. This stage can last long. The orbit gradually shrinks and the rotation speed increases to the point that they collide. This is called a “merger”. This step completes in some milliseconds. A huge amount of energy is emitted in this stage in the form of gravitational waves. The first ever gravitational waves detected were caused by two black holes that merged and the total power output during the collision was 50 times greater than all the power of all the stars in the observable universe put together.

Two black holes of different sizes merging and giving birth to gravitational waves

How do we see black holes if light can’t escape them?

Black holes(without accretion disks) cannot be seen directly. Scientists observe these black holes through its interaction with other matter like weird movements of star in a region or gravitational lensing and electromagnetic radiations.

Stars orbiting a black hole which is invisible

We can see black holes with accretion disks. An accretion disk is the Material surrounding a black hole. It has been discussed above. Friction causes orbiting material in the disk to spiral inward towards the central body. It also makes the material hot and glow. In the first ever picture of black hole(M87), the glowing matter circling the black hole can be seen. The dark area surrounded by glowing matter is event horizon.

Using the Event Horizon Telescope, scientists obtained an image of the black hole at the center of galaxy M87.

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