The death of massive stars — Black holes

Whenever we think about black holes, two things are evidently clear in our minds. Black holes are black, and they are a kind of tunnel. Alright, a very bad joke. The way I described black holes is definitely incorrect, but their name does explain their nature. Welcome to another post of The Scientific Revelation in which we will cover black holes. in the previous posts, we have talked about inflation, fundamental forces of nature, matter formation and star formation among them last two are most related to this post. As explained by the title, The death of massive stars-Black holes are a very massive dead core of massive stars that had died in a supernova explosion. It has such strong gravity that even light can not escape it. Because we see everything around us due to visible and black hole don’t release light, we can’t observe it visibly.

A black hole is a very massive dead core of a massive star that had died in a supernova explosion. It has such strong gravity that even light can not escape it.

Black holes are where God divided by zero.

Albert Einstein

The emergence of the idea

Black holes made their first-ever appearance in Philosophical Transaction of the Royal Society of London in 1783 by John Michell. He wrote that black holes are a kind of star that had quite high mass and they are so compact which resulted in a strong gravitational field. The gravitational field is so high that the light leaving the star can not travel far enough before sucked back in. We can not observe these types of stars (that are in high number across the universe) but we can observe its gravitational attraction. Few years after French scientist Marquis de Laplace had also suggested the same phenomena in his book The system of the world.

The Chandrasekhar limit and light path

Another breakthrough for star death came from an Indian scientist Subrahmanyam Chandrasekhar when he had devised the Chandrasekhar limit. He had worked on how a star can support itself when its whole energy is exhausted. After the exhaustion of energy, the star gets smaller and its particle gets closer to each other. According to the Pauli exclusion principle they have different velocities so move away from each other makes star expands. So even after the death of a core, this happens. But this end is only granted to the stars that are lesser than 1.5 solar masses (Chandrasekhar limit) turning them into white dwarfs. The star heavier than this can not be stopped by the repulsion of Pauli exclusion. Their end will be very different.

The more detailed explanation about black holes comes around the 1960s when more scientists started working on large-scale structures. The simple explanation says that the heavier objects bend the spacetime around them. The path of light gets bend when it comes in the contact with a star’s gravitational field. The light bends inwards and as the star gets contracted its gravity increases which bends the light even more. The more star contracts the more it bends the light and to a distant observer, it seems fainter and redder. After some time light is bent so much that it can not escape out of the star. That is why we can not visibly see a black hole because can not escape from it.

Rotating and non-rotating black holes

In 1967 Werner Israel had revolutionized the study of black holes. He showed that no rotating black holes must be very simple and perfectly spherical. They can only be formed from a perfectly spherical star that does not exist. However, Robert Penrose(Nobel prize winner 2020 physics) and John wheeler suggested another explanation for Isreal’s work. When a star collapses the rapid movement at its collapse will release a lot of gravitational energy that will make the star spherical. It means that even if an imperfect star results in an in non-rotating black energy release will make it perfectly spherical. Its mass will depend on the assigned star.

In 1963 Roy Kerr explained rotating black holes. When the rotating star died rotating black holes formed that bulges in the equator the same way as earth. The faster a black hole will move the faster it has bulging on the equator. In 1970–1971 Brandon Carter and Stephan hawking had shown that a black hole has an axis of symmetry and its shape and size depend on its mass and rate of rotation. It shows that the size and shape of the black hole only depend on the mass and rate of rotation(rotating or non-rotating) of the star.

The first-ever discovered black hole system was Cygnus X-1. it was said that matter flown out of companion star became so hot that it had started emitting X-rays. There are many black hole systems like Cygnus X-1.

The detection of a black hole

We can not detect a black hole directly as they don’t emit light. The possible ways to detect them is through gravity and radiation (especially gamma rays). As the black hole glides across the cosmos it can swallow the star and interstellar matter when it comes in contact with them. Their gravitational pull is so strong that it pulls layers after layer of matter and when that matter sucks inside it makes an accretion disc around the black hole. As the infalling matter falls towards the black hole its temperature increase which in turn produce gamma radiations. At the same time, the star which had fallen prey to that star reacts differently than others. It losses its mass very fast and glows brightly.

When a black hole pulls the gas and stars inside it also creates Steller wind across interstellar gas clouds which is one of the reasons for the new star’s formation.

The most famous discovery of a black hole happened in 2015 after the gravitational waves from two merging black holes were detected. These two black holes were spinning at a random rate in opposite direction. This discovery had not only confirmed that gravitational waves exist but also shed light onto Stellar black holes.

The time effect of a black hole

The time is not absolute so the effect of the black hole will be different for the observer from a certain distance in comparison to the one who is traveling inside the black hole. Suppose there is a black hole and an astronaut is at the even horizon traveling towards the singularity. He is sending the signal every second to the spaceship which is at a safer distance from the star.

Let’s assume that he crosses the event horizon at 12 now there is no escape, nothing can go out of the black hole including the signal he was sending at every second. When the astronaut was sending signals before 12 it took little more than 1 second to reach his companions at the ship because time slows at the event horizon. But after going inside the even horizon his companion have to wait forever for the 12 o’clock signal that will never come.

At the center of the black hole, we have a singularity. Much like big bang singularity where all the matter and energy is contracted at one point. This singularity represents end here all the information will be lost.

The birth of a black hole

In the previous post on star formation, we have seen how stars form. The interstellar cloud of hydrogen and helium remains evenly spread across the cosmos. There are many ways the star formation can trigger. Firstly, interstellar molecular clouds are not very even. There are bumps here and there which acts as the center of gravitational pull. These bumps attract nearby matter towards them and this slow process takes momentum as the mass increase. The center of gravitational pull keeps on getting contracted and which increases the pressure. The increase in pressure rise the temperature. This process keeps on repeating itself until the core gets hotter enough to start nuclear fusion. After nuclear fusion, no nearby matter falls towards the core. The star also creates Steller winds that blow away surrounding gas and it creates other bumpy conditions for the birth of other stars. This new star is now visible.

Nucleosynthesis and die off

Star shines due to the process of nucleosynthesis in which hydrogen atom combines to form helium. This produces a tremendous amount of energy. This energy exerts pressure which balances the crushing force of gravity of the star. This balance remains for a long time until all the hydrogen gets exhausted and no more metal fusion can happen.

In nuclear fusion, lighter elements fuse into heavier elements. Even the biggest star can only synthesize the elements up to iron. The predecessors of iron during fusion releases energy, which balances the gravitational force but iron doesn’t. On top of that, stars don’t have enough energy to synthesize elements heavier than iron. So, firstly iron doesn’t release energy after fusion, and secondly, because heavier elements can not form, iron keeps getting pile up in the star core. The elements heavier than iron form during the high energy surge of a supernova

If the stars have medium mass they will die off as white dwarfs. But if this star is very massive with a very massive core it will die off in a supernova becoming either a neutron star or a black hole.

Structure of a black hole

Event horizon

The gateway to the hell of a black hole is the event horizon. It is membrane after crossing which nothing can come back. Stephan had defied the black hole in a simple way.

“event horizon is made by the space-time path of light of light that failed to get away from black hole forever struck there making an event horizon.” The path of light rays is always parallel or moving from each other. The surface area of the event horizon always increases as more matter and radiation fall into it. For instance, if two black holes collide and merged together, the new black hole’s event horizon will have more or equal surface areas in comparison to the sum of the surface area of both of the black holes. It is called the Non-decreasing behavior of a black hole.

Singularity

Singularity is the inner region of a black hole which is the grave of every matter/mass after it passes through the event horizon. Scientists don’t understand and accept singularity well Singularity as every physical law breaks down at it. A singularity is a place where all matter going inside the black hole concentrate and it has no volume or surface area.

Accretion disc

A black hole moves in the universe surviving on the matter. As it comes in the contact with surrounding gas or a star pulls the matter towards itself. The infalling matter creates an accretion disc around the black hole and it feeds from the accretion disc. Matter follows step to step guide before getting swallowed into the black hole. The definition of the accretion disc is “A disc-like flow of gas, particle, plasma, and dust around any high gravity astronomical objects including black holes, neutron stars, pulsars, quasars, or even some supernovas.

You can read more about accretion disc in this post of britannica.com.

The type of black holes

The first-ever black holes were discovered in 1971. The first person to propose anything black hole was Karl Schwarzschild who after solving Einstein’s equation came proposed this idea. People have this misconc3eption that black holes are massive vacuum cleaners means they will suck everything in their vicinity. They actually don’t suck anything. Whatever object comes under their gravitational influence fall towards them if you are out of its gravitational reach nothing will happen. If we replace our sun with a black hole of the same mass nothing will happen to planets and their orbits. They will not be sucked by the black hole, although earth will no longer be hospitable. The largest black known to date is S50014+81 with a mass of 40 billion times solar masses.

Lets us hop on to the types of black holes.

Mini black hole(primordial black hole)

There are many black holes having very less mass than our sun. These can not be formed by gravitational collapse because their mass is below the Chandrasekhar limit. They can only form when the matter is compressed heavily by very strong external pressure. Scientists think these primordial black holes were formed during the early universe. Because our universe was not uniform in matter distribution some places with overdensity had given rise to these black holes. If we can find them, we can discover so much more about the condition of the early universe.

These primordial black holes should have an age equivalent to the age of the universe because they formed in the very early universe. They have very little mass so their radiation must be very high. The more radiation means the black hole will evaporate very soon. We can detect them because they must be emitting high radiation in form of x and gamma rays.

The smallest one can be the size of grain with the mass of a mountain compressed inside them.

Stellar black hole

The Steller black holes are maybe the most common. They usually have 10–24 solar masses and they are formed during supernova events. When the very massive core of a very massive star collapse it burst in a supernova. The outer layers burst before releasing heavy elements along with radiation and the core shrinks into a black hole. As the massive star dies its core turns into a black hole or neutron star. There is a thumb rule for recognizing which core can turn into a black hole. If the core of the star is more than 3 times heavier than our sun it will turn into a black hole. According to the Harvard Smithsonian center for astrophysics, our galaxy might have one million of these Steller black holes.

Supermassive black hole

The largest kind of black holes are supermassive black holes that are found inside the center of galaxies like the milky way. The black hole at the center of the milky way is Sagittarius A* which is equivalent to 4 million solar masses. The formation of supermassive black holes is not clearly understood yet. There are many ways supermassive black holes can form like if interstellar cloud collapse under their gravity and make a black hole instead of stars. Due to heavy mass and Steller activity star clusters can also collapse to form a massive black hole. Some scientist thinks that a supermassive stars as Steller black hole which after getting access to the vast quantity of matter(interstellar gas clouds) turns into a supermassive black hole. They can also form if many Steller black holes collide together.

Intermediate black holes

The formation of an intermediate black hole is quite rare. They form when a chain reaction of the collision of many stars happens. It results in a star cluster which in the future forms a very very massive star. When these stars die off they form intermediate black holes. These black holes then move into the center merging into the supermassive black hole. Because these intermediate black holes are forms when many stars collide in star clusters there can many IBH in a star cluster. These IBH then can come together and form a supermassive black hole at the center of the galaxy.

Quasars

Quasars are a kind of black holes with masses larger than millions of solar mass. Their gravity is so strong that they engulf chunks of matter into them. As matter falls into the black hole it makes the black hole spin in the same direction which in return creates a heavy magnetic field. The infalling matter creates very high energy particles at the boundary of the black hole. The strong magnetic field directs these high energy particles towards poles and forms jets of matter. These jets can run across light-years and a very powerful source of radiation.

The death of the black hole

For a long time, it was thought that black holes are immortal bodies that will live through the end of the universe. But it was discovered that they too evaporate slowly leaving radiation behind. To understand this we have to understand entropy which is a measure of the disorder of a system. Let’s say we threw an object in the black hole we can say that the entropy outside went down but we can not find what happened to the entropy inside the black hole. Jacob Bekenstein had suggested that the area of the event horizon is a measure of the entropy of the black hole. As a matter which has entropy fall into black hole its entropy increase and area of event horizon also increase.

This definition had a flaw because if the black has entropy, it ought to have temperature and if it has temperature it must release radiation. But it was a common thought that black holes don’t emit or release anything.

The Hawking Radiation

Firstly two soviet experts Yakov Zeldovich and Alexander Starobinsky had put forward that according to quantum mechanics’ uncertainty principle rotating black hole should emit particles/radiations. During calculating the mathematics of how of emission a rotating black should emit Stephan hawking found out that non-rotating black holes should also create and emit radiation at a steady rate. It was proved that a black hole emits radiation just like a black body. This emission depends on the mass of the black hole; the higher the mass the less temperature. This radiation is called Hawking radiation after Stephen Hawking, one of the leading experts on the black hole.

The hawking radiation doesn’t come from within the black but from the empty space outside the event horizon. We know that empty space is not totally zero, it has quantum fluctuations. We can understand these fluctuations as light or gravity particles that sprout out sometimes, move apart and then come together annihilate each other. These particles are virtual particles, not real ones so we can’t detect them by particle colliders. One member of the pair is a particle, and another one is an antiparticle along with one being positive and another negative. The negative particle is short-lived as we see positive particles all around us. The one particle (-ve) from the pair of a virtual particle goes inside the black hole and the positive one escapes the black hole like a real particle.

The long demise

A person from far outside thinks that particles are coming from inside the black hole. Because negative particles are going inside the black hole it reduces its mass consequently the area of the event horizon also goes down. In this process of hawking radiation a black holes losses its mass very slowly but steadily. The more its mass goes down the more its temperature increases and more it emits the radiation resulting in more decrement of mass.

It is not clear as to how black holes die but some experts think that after getting very small when they no longer emit radiation they explode following a burst of gamma-ray.

Resources

1. Watch the documentary on Black Holes by Space and Astronomy.
2. Read this NASA post on black holes.
3. The post on Black Hole: Fact, formation, and discovery by Space.com.
4. The post where do black holes led by Space.com.
5. What is a Black Hole? by nationalgeographic.com.
6. A lecture on Physics of Black Holes with Prof. Chris Impey by The Royal Institution.

There are many more concepts to unravel so stay tuned and do the Revelation.

So Einstein was wrong when he said, “God does not play dice”. Consideration of black holes suggests, not only that God does play dice, but that he sometimes confuses us by throwing them where they can’t be seen

Stephen Hawking