Black Holes in Space
The Milky Way Galaxy's Central Black Hole called "Sagittarius A," has finally been imaged !
Eventhough it is in excess of 4 million solar masses, it is nothing compared to the colossal 6.5 billion solar masses of that supergiant elliptical external galaxy designated M87, that was imaged in 2019, in the constellation Virgo.
Black Holes were predicted to exist by the theory of general relativity propounded by the great theoretical physicist of all time, Albert Einstein, in 1915.
When the end product of a massive star breathes its last nuclear breath, it either ends up as a White Dwarf star (upto 1.4 solar masses, as derived by the nobel laureate Subramaniyan Chandrasekhar) or a Neutron star (upto 3 solar masses) or a Black Hole (> 3 solar masses), only depending on its final end mass.
A Black Hole is the end product of a massive star which got exhausted of its core nuclear fuel. What happens is, when a massive star dies, it collapses due to its inwardly acting gravitational field.
Stars are burning balls of emanating electromagnetic radiation, generated in their cores by nuclear fusion reactions. In its prime life time called the "main sequence time," a star is balanced by the inwardly acting gravitational force countering its enormous radiation pressure, escaping outward from its millions of degrees hot centre.
With the passage of time (i mean, cosmic time scales - of millions and billions), stars get exhausted of its hydrogen nuclear fuel, and succumb to the inwardly acting gravitational force as the outwardly acting counter-balancing radiation pressure gets depleted. Now, a star collapses on itself and gets reduced in radius.
The collapse is of such magnitude that the stars atoms get closely packed to one another. It gets into a state of electron degeneracy where the atomic electrons resist further collapse. A star like the Sun, shrinks to a radius of a planet like Earth. It becomes a high density star called a "White Dwarf star." This as the name suggests is a dwarf in terms of its size and incandescent white in terms of its surface temperature. It burns by a process called "triple-alpha process," in its core, and is supposed to live for a few million years, till it reduces itself further into many stages towards a final state of becoming a blown-out brown dead stuff.
Suppose, if the end product of a post main sequence star, is more than 1.4 solar masses, as defined by the chandrasekhar limit, it turns out into a Neutron star. How ?
As I said, the white dwarf star is in a state of electron degeneracy. In a massive body of > 1.4 solar masses, the orbital electrons are forced to break down. The atom ceases to exist and becomes only nuclear matter. The electrons fuse with the protons and become neutrons. It is called a state of neutron degeneracy.
The entire star is composed of only neutrons. Its radius is enormously reduced. It's just like, if the planet Earth, all of a sudden, became a small island ! A spoon-full of its matter now, weighs billions of tons !
These neutron stars, that rotate super-fast are also called "pulsars," and these radio-pulsars, are picked-up by radio telescopes on Earth. The famous Crab pulsar, is located within the crab nebula supernova remnant.
Super-Novae, are gigantic explosions of the end products of massive stars, before their core collapses into a neutron star.
And don't forget, our Sun is a star, but, its not likely to explode into a supernova. It will slowly swell up to gigantic proportions to the extent of gobbling up the inner planets, becoming into a red giant star and even becoming a red-supergiant star with a luminosity of ten thousand suns, only to finally expel it's extended atmospheres, to leave a remnant nebulae. After these stages, it will end up finally, into a white dwarf star, in another five billion years. These are theoretical astrophysical predictions, from well-established models of observational studies of
"stellar evolution."
Well then, what is a Black Hole ?
When the end product of a massive star is more than three solar masses, a star remnant collapses into a Black Hole. Even light with its fabulous velocity of 300,000 kms/s, can't escape from the gravitational field of a Black Hole. How can we see it ? Simply, we can't see it exist in any part of the EM spectrum, other than by its attractive gravitational field. They were predicted to exist indirectly by the behaviour of its nearby stars. But, now with advanced imaging methods and technology, scientists have been able to image its location negatively, by imaging the infalling surrounding gas that emits EM rays before its final disappearance into the Black Hole, all around it circumferentially.
The Black Hole is simply a space-time region of a finite mass of the end product of a massive star that turned into a point of singularity a mathematical proposition, which has an infinite density, infinite temperature and infinite pressure.
It's gravitational boundary is called event horizon. Any thing including light that enters it is doomed to be ever within it. Its gobbled up for good. Actually, there is only empty space between the event horizon and singularity.
Black Holes are regions of collapsed space-time itself due to the presence of not only mass, but singularities. They are supposed to be of many kinds. Stellar black holes, relic primordial black holes of the big bang and supermassive black holes at the galactic centres.
M87 and Sagittarius-A are SMBHs.
- to be continued....
...this writeup is not a forward or cut copy paste. It was written spontaneously.
