Black holes

“Astronomy is older than physics. In fact, it got physics started, by showing the beautiful simplicity of the motion of stars and planets, the understanding of which was the beginning of physics. But the most remarkable discovery in all of astronomy is that the stars are made of atoms of the same kind as those on earth.*”

Richard Feynman 

How can a star turn into a Black Hole? From an object that shines due to nuclear fusion to something which does not allow light itself to escape from it. It becomes something so mysterious that cosmologists need to combine one of the greatest theories – General theory of relativity and Quantum mechanics to understand what is inside it. 

As Michael describes in his video "Travel INSIDE a black hole", theoretically anything, you, me or the Earth you are sitting right now can become a black hole if you shrink it under the Schwarzschild Radius. fortunately there is no means of doing it. But a star can do it at the end of its life.

But how? How can a star turn into a black hole.
To understand it we have to see what happens inside a star.

Stars emit electromagnetic radiation during nuclear fusion.

In our Sun during nuclear fusion Hydrogen nuclei fuse into Helium nuclei and produce electromagnetic radiation which we feel as heat. Photosynthesis starts as the radiation falls on leaves.

Thus everything we eat here on earth is just Bottled Sunshine!

Here are some of my favourite lines from Carl Sagan’s book “cosmos”,

It was more awesome to listen it in his beautiful voice in the video series Cosmos Episode 2 - One Voice in the Cosmic Fugue

Stars are very stable due to the thermal equilibrium that exists between the outward pressure and gravitational force. Outward pressure is maintained by heat generated in the nuclear reactions. But as more heavier elements form in the center of star, different layers start forming, star begins to swell and becomes a red giant. In the end it collapses into a dense core.

This is how we are told that the stars die, but its still described as one of the unsolved problems of physics, "What is the exact mechanism by which an implosion of a dying star becomes an explosion?"
 

So, every star becomes a black hole in the end?
No, two things are important for a star to become a black hole,

1.     Chandrashekhar limit
2.     Schwarzschild radius

If a star’s mass is less than 1.4 times the mass of sun (Chandrashekhar limit), it will settle down to a white dwarf, which will remain stable due to the balance between gravitational force and Pauli Exclusion Principle which is the repulsion between electrons. That is the the dense core left at the end will be so dense, it will have only electrons!

But if its mass is above Chandrashekhar limit, it will become a neutron star which is stable by the Pauli Exclusion Principle followed by neutrons. The dense core will be made of neutrons! But if the mass of a star is more than 10 or 20 times that of the sun it will collapse in on itself to a point! This is what is called a BLACK HOLE!

So what is inside a black hole?

We don't know!

One of the solutions of Einstein’s field equations is “Schwarzschild metric”. It describes spacetime surrounding a non-rotating massive object. Schwarzschild black holes which are simplest kind of black holes are described using this metric.

EINSTEIN'S FIELD EQUATIONS

Schwarzschild metric described by Matthew O'Dowd of PBS space time,

Metric is another word to describe the distance between two points in space. Metric can be different for different space. 

In spherical polar coordinate system the Schwarzschild metric is,

In the above equation, 2m=Rs (Schwarzschild Radius).
According to this metric, there is a Schwarzschild Radius surrounding a black hole where the escape velocity is greater than speed of light. This Schwarzschild Radius is actually whats written in many books as event horizon of a NON-ROTATING black hole.

As nothing can travel faster than light, this region surrounding a black hole looks black.

Usually when we see a picture of a black hole, we see the surroundings distorted and the black hole itself shown literally black. But as we know, all these images are not real. But they tell us what really is true. What we actually see when we look at a picture like this is the Schwarzschild radius or the sphere, from which nothing can escape, not even light.

jila.colorado.edu

And because of black hole's immense gravity, the galaxy behind it also seems to be distorted due to gravitational lensing.
In gravitational lensing, when a heavy object comes in front of a star, galaxy or quasar it bends the light coming from them and thus distorts is totally. Sometimes it forms a ring and sometimes multiple images. Like in yesterday’s APOD - https://apod.nasa.gov/apod/ap170227.html
And the following picture shows other gravitational lenses taken by Hubble Space Telescope.

hubblesite.org

The video shows how the bottom of a glass can be used to see a ring of light when the source of light and the bottom of glass are aligned.

As this experiment was inside my mind, I saw another way of looking at the effect using only water in a bowl.

You can see in the image below, when the light from LED falls in the center of bowl a ring is formed even though its not a full circle.

Schwarzschild black hole is the simplest black hole. Its assumed to be non-rotating and with no charge. So what would it be like to travel inside it?

There are some mindbending simulations on this page made by Andrew J. S. Hamilton.

http://jila.colorado.edu/~ajsh/insidebh/schw.html

But there are other black holes which are not formed after death of stars, they are called supermassive black holes.

And astronomers have found these supermassive black holes at the centre of every galaxy. So how these black holes are formed? And what came first galaxy or black hole?

As theoretical physicist Michio Kaku explain in this video, the latest theory tells us that supermassive black holes formed first and then the galaxy itself formed around it. But its still difficult to tell which formed first.

Theoretical physicist Carlo Rovelli in a course at World Science U describes that a quantum theory of gravity is needed to explain the interior of black holes. 

So we have still many mysteries to unravel as in Newton's time.

HAPPY SCIENCE DAY!