Black hole birth report[신아형의 코스모스]

The birth of all life is a wonder. When humans are born, they cry out loud and face the outside world for the first time. Recently, there was a being that roared louder than anything else and announced its birth. It is an unknown celestial body, a black hole.

The brightest gamma ray in history

The afterglow of the gamma-ray burst ‘GRB221009A’ observed by NASA’s Swift X-ray Space Telescope on the 9th. screw

On the morning of the 9th (local time), the National Aeronautics and Space Administration (NASA) observed a very bright beam of light from space. The identification of the light was a ‘Gamma Ray Burst (GRB)’ flash that occurred approximately 2.4 billion light years from Earth (1 light year is the distance light travels in a year approximately 9.46 trillion km). It is estimated that this light flew for 1.9 billion years before we could see it.

In fact, gamma ray bursts are common in space. In 2015, NASA observed more than 900 gamma-ray bursts. And these gamma ray bursts release more energy in just one second than our Sun emits in about 10 billion years of its life. In addition, the light of gamma ray bursts is not light that is scattered and dimmed in all directions, but is concentrated in one focus like laser light, so it can reach further and its destructive power is huge.

GRB221009A light taken at the Gemini Observatory in Chile on the 14th. US National Infrared and Optical Astronomy Laboratory (NOIRLab)

Dubbed ‘GRB221009A’, the gamma-ray burst flash is much brighter than those seen previously, giving it the nickname ‘BOAT (Brightest Of All Time)’, meaning ‘ the brightest in history’ among scientists. The energy of light is a staggering 18 TeV (tera electron volt, 10 to 12 electron volts), and it is said to have an effect on the long wave radio communication signal of the ionosphere of the Earth’s atmosphere. Brandon O’Connor, professor of astrophysics at the University of Maryland and George Washington University, admired, “There is a gamma-ray burst that is so bright and has been seen so close to us[Y Ddaear]is something that can only happen once in a hundred years, maybe a thousand years.”

The gamma ray burst ‘GRB221009A’ emits light. screw

The end of a star defeated by gravity

So, why do gamma ray bursts happen? It is associated with the birth of a black hole in exchange for the death of a star.

The process of evolution by mass of a star. still youtube

Stars have slightly different lives depending on their mass. Based on the mass of the Sun, we will classify stars that are lighter than 8 times the mass of the Sun and stars that are greater than 8 times the mass of the Sun. In this episode, we’re going to look at a star with a mass more than 8 times that of the Sun that turns into a black hole at the end of its life.

An equilibrium diagram of the forces of gravity and fusion. The force of gravity that tries to shrink the star (black arrow) against the force produced by nuclear fusion against it (yellow arrow). Snapshot of the Science on Your Doorstep homepage

The fate of a star depends on the ‘balance of forces’. As long as the balance between the ‘gravity’ that tries to contract the star and the force that resists it is maintained, the star can survive without dying. The force against gravity comes from the nuclear fusion of hydrogen, the substance that forms stars.

nuclear fusion. International Atomic Energy Agency (IAEA)

Last month, we looked at ‘nuclear fission’, where neutrons and uranium-235 atoms collide and split into smaller particles (refer to ‘Big Story of Small Things – Nuclear Fuel Uranium’ on 25 September). This time it’s the other way around. In the center of the star, hydrogen atoms combine to form a heavier atom, helium, and helium atoms collide to form a chain of fusion reactions that transform them into heavier carbon, oxygen, silicon, and even iron. The mass lost in this process is converted into energy according to our classic formula, Albert Einstein’s ‘Mass and Energy Equivalence Formula (E=mc^2)’.

But everyone, the heavier the weight, the greater the force of gravity that must be endured, right? Because of this, a heavier star will run out of fuel much faster than a lighter one. In other words, it has to fuse faster than a lighter star to produce energy.

No matter how hard nuclear fusion is, the fuel a star can use is limited. In the end, gravity has no choice but to win. As the fusion fuel is depleted, the core of a star continues to contract due to gravity and burn more vigorously, causing the shell and surface around the core to expand. The core of a star that used to be filled with hydrogen is now replaced by a heavier atom, such as iron. Now this star means it is time to die.

Supernova and neutron star mergers

expandSupernova drawing. oxford university england

pop! Simultaneously with the death of the star, the most powerful and violent explosion occurs. It’s a ‘supernova’. The material on the surface of the expanding star is blown away with the explosion, leaving only the core. Sometimes elements heavier than iron, such as nickel, zinc, silver and gold, are formed under special circumstances, such as supernova explosions. By tracing the tracks of supernovae, scientists study the origin of heavy elements such as gold in the universe.

At this point, the remaining core will face two fates. First, if the mass of the core is between about 1.4 and 3 times the mass of the Sun, the protons and electrons in the atom are gathered together by the force of gravity to form a neutron star made only of neutrons. Secondly, if the mass of the core is more than three times the mass of the sun, it will collapse completely due to gravity, creating a ‘black hole’ with a volume of ‘0’ but an infinitely large density.

Two nearby neutron stars. screw

Neutron stars can also turn into black holes. Two nearby neutron stars spin slowly in a circle, and when they come close enough to touch, they spin at insane speeds. Eventually, the two collide and merge into one, eventually collapsing into a black hole. When two neutron stars orbit together in one orbit, they emit gravitational waves. In August 2017, NASA directly detected gravitational waves produced when two neutron stars merged.

Binary neutron star collision, merger. screw

Now let’s go back to the gamma ray burst we introduced earlier. I’m back from afar. Gamma-ray bursts occur when supernovae occur, or when binary neutron stars merge. At the same time as the birth of a black hole, a gas of magnetized matter rotates around the black hole, and the strong magnetic field pressure formed at this time ejects matter and energy in the form of a jet at a very high speed close to speed of light. This phenomenon is a gamma-ray burst. Gamma-ray bursts from neutron star mergers are very short, less than a second, while gamma-ray bursts from supernovae are relatively longer, about a minute.

A gamma ray burst. screw

The British astrophysicist Stephen Hawking (1942-2018), famous for his study of black holes, borrowed Einstein’s saying “God does not play dice” and said, “God not only plays dice, he n throw dice where we can’t see.” They complained of difficulties in understanding the phenomenon of mass ejection.

British physicist Stephen Hawking. Wikipedia

The gamma-ray burst GRB221009A, seen on the 9th, lasted for hundreds of seconds and is said to have been caused by the death of a star more than 30 times the mass of the Sun. Scientists will use observational data over the coming months to better understand the background of gamma ray bursts. The scientific community is excited as we have never seen such a bright and vivid burst of gamma rays before. This is the most vivid witness of the birth of a black hole which is still full of mystery. It is hoped that this observation will provide another clue to solving the mystery of black holes.

Reporter Shin Ah-hyung abro@donga.com

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