One of the most mysterious objects in space are Black Holes, dark voids in space formed by the collapse of massive stars. I’ll be covering most of the topics of interest in this story.
The idea of the existence of Black Holes was first given by Indian-American Astrophysicist Subrahmanyan Chandrasekhar. On his way to England, to study in Cambridge, he calculated the maximum mass a white dwarf(dead star) could have. Everyone else thought there was no maximum, but Chandra showed that there was a limit to the gravitational force normal matter could withstand. If a star was too massive it would collapse to almost nothing.
Scientists like Eddington and Einstein refused to believe Chandra’s conclusions. Chandra abandoned this line of study after being ridiculed for proposing something so unnatural.
Eventually, scientists realized he had been right. When he was awarded the 1983 Nobel Prize, it was is part of his early research in “limiting the masses of stars”. The maximum limit came to be known as the “Chandrasekhar Limit” which’s value is 1.4 msun or 1.4 times the mass of the sun.
Stars are balls of gas, pulled together by gravity. The gas is Hydrogen, and a star has enormous amounts of it. As gravity tries to pull the gas to the centre, the temperature starts to rise and the Hydrogen molecules fuse to form Helium, the next element, by the process of nuclear fission. This releases a tremendous amount of energy, which causes the star to expand countering the force of gravity.
This balance between pressure and gravity keeps the star alive and kicking for millions of years. But nothing lasts forever.
After millions of years, the star starts to run out of fuel. The pressure against gravity starts to die out and gravity starts to compress the star. The helium starts fusing into lithium, lithium into beryllium. But when all the stars fuel turns into iron, whose fusion releases no energy, the star’s death has arrived.
The gravity of the star compresses it even more and the star dies with a final burst of light, releasing more energy than it ever did in its whole life, a supernova.
Decision of Fate:
Remember from the introduction, the Chandrasekhar Limit? Well, the time has come to put it to use. We will call it M.
After the supernovae, if the star weighs less than M(1.4 msun), it turns into a white dwarf, a dead star. A white dwarf emits a very faint light for some years before it fades away. Examples are Sirius B and Procyon B.
Stars weighing equal to c are compressed by their gravity into a diameter of about 12 miles while weighing 1.4 msun. Just a tablespoon of a neutron star can weigh more than a billion tons.
On the other hand, stars with masses more than M have a problem. Their gravity is so powerful that it compresses it till it cannot be compressed any more, till the smallest possible unit of length, the Planck length (1.616255(18) × 10^-35 m). The star’s entire mass is compressed in that single point in space. Thus, a Black Hole is formed.
Misconceptions about Black Holes:
Let's discuss some of the most common misconceptions about black holes and what really the truth is:
- Black Holes are very big : One of the most common misconceptions about Black Holes is that they are very large. As discussed above, Black Holes are one of the smallest things in the universe, measuring a diameter of 1 Planck length.
- Black Holes have infinite gravity : I have heard many people say that Black holes have infinite gravity. This is completely wrong. Black Holes have the same gravity as a star of the same mass outside the surface. If our sun was replaced by a black hole of the same mass, our orbits would stay the same. The difference is felt only inside the surface. The star’s gravity starts decreasing and we go closer to the centre, while the black hole’s gravity keeps increasing.
- Black Holes devour everything close to them : As discussed above, black holes don’t have infinite gravity. So they don’t devour everything close to them. Only after you cross the Event Horizon(will be discussed), can you never come back.
Structure of a Black Hole:
As we have discussed the origin of and the common misconceptions about Black Holes, we can go into discussing its structure.
A black hole has mainly two parts, an event horizon, and a singularity. Among these the event horizon is imaginary. There is also an ergosphere, but it is related to a complex topic of spinning black holes, so I am not mentioning it here.
The Event Horizon:
When you get close to a black hole, its gravity start’s pulling you. Slowly, your speed starts increasing. This speed is called escape velocity, or the velocity required to escape from the black hole’s gravity. The event horizon is the imaginary circle around the black hole where escape velocity v equals the speed of light, c. Since nothing can travel faster than light, nothing can escape once they reach the event horizon, not even light. The diameter of the event horizon is directly proportional to the mass of the black hole. It is the limit of what we can see from the outside.
The singularity is the Planck length sized point which contains all the mass of the black hole. It is at the centre of the event horizon and pulls everything in the event horizon.
Journey to a Black Hole:
Let's discuss what you’ll see if you travel to a black hole. The circumstances are:
The mother ship is in a safe orbit around the Black hole outside the event horizon. You have a radio transmitter attached to your watch which sends a signal every second. You jump out of the ship towards the Black hole. You’ll reach the Event Horizon at 09:00:00 AM.
As you get closer and closer to the event horizon, your crewmates at the mother ship will notice that the difference between intervals of individual signals is increasing. The difference will be very slight till 08:59:59, but they will have to wait forever for the 09:00:00 signal as you have crossed the Event Horizon. From your jump, they will see you travel slower and slower till you come to a stop at 09:00:00. Your image will slowly turn redder and redder(being red-shifted) till it becomes too dim to see and you are trapped in the black hole forever with your 09:00:00 signal and everything else that ever ‘fell in’.
On the other hand, for you, the mother ship appears to travel slower and slower while you accelerate very fast. You won't even notice when you travel past the event horizon. Soon, you start feeling the tidal forces of the black hole. Centimetres of difference will cause an enormous difference in gravity and you will be pulled till you become very long and very thin. This process is called spaghettification. Soon the gravity will be so powerful that every atom of your body will be ripped apart and you’ll die a quick and painful death.
But let’s go beyond death. Soon you’ll reach the singularity and the black hole’s gravity will compress you into the singularity.
From the above-stated characteristics of Black Holes, you may conclude that Black Holes never die. But late American Physicist Stephen W. Hawking said that black holes can evaporate by a process called Hawking Radiation.
The theory states that a pair of virtual particles(resultants of quantum fluctuations) may form just outside the event horizon of a Black Hole. While one escapes to safety and becomes a real particle the other enters the black hole and thus lowering its mass by a tiny bit.
But Black holes can take enormous amounts of time to evaporate completely. It takes a Black Hole of 1 msun 10⁶⁷ years to evaporate completely. If the universe lasted 10¹⁰ more years for each of its 10⁸⁰ electrons, that would still be 10 million times less than the lifespan of the most massive black hole.
Black Holes are mysterious things with a strange birth, a long life and an even stranger death. I have just scraped the surface of what is possible in the world of black holes. But hope you got the basic idea of them.