Journey to the formation of Black Hole

Life cycle of stars

Massive clouds of gases (Hydrogen, Helium and other organic complexes) starts contracting due to gravitational pull. Because of this, molecules will move firstly towards one another or towards the centre or core.

A stage comes, when their Kinetic Energy is so high (implies temperature is also high) such that the molecules breaks into atom on further contraction. Then, there is an increase in ionisation of atoms which results in the formation of bare nuclei with electron cloud. And the fusion of Hydrogen begins due to less positive energy.

Sequence phase (comprises of 90% of life)

Due to fusion of Hydrogen and Helium, expansion of gas happens. An outward thermal pressure acts. So the star becomes stable as the Inward Gravitational Contraction equates to the Outward Thermal Expansion. Generally, it continues for 10 billion years. 

Hence size of star is stable. For example the sequence phase for Sun is 5 billion years (approximately). More the massive star is more quickly it will die, as the amount of mass of the star is proportional to its Gravitational pull.

Then a stage comes when all the Hydrogen exhaust from the core of the star. The process of Nuclear Fusion stops and there is no thermal pressure to balance the Gravitational contraction. The star gets cooler and appears red. As :-

Violet – Indigo – Blue- Green- Yellow- Orange- Red (VIBGYOR – the famous Rainbow acronym)

Moving from left to right the wavelength of the light spectrum increases which results in decrease in frequency and hence decrease in Energy. So now the star is less energetic and hence appears red.

But there is still some Hydrogen left in outer shell of the core of the star. Though the depletion took place in its inner core. This Hydrogen now fuses to give Helium and the outer layer of the star expands due to heat of the Nuclear-fusion. Here at this stage the star appears as a “Red Giant”, results in “Red Giant Phase”.

 

Case 1: The star having mass within the range of 1 to 8 solar masses (solar mass is the mass of the Sun).

Red Giant Phase

Now the core still contracts due to Gravitational pull. Its molecules keep speeding up until the Kinetic Energy becomes so high (due to the intense temperature), that fusion of Helium to Helium begin to occur, results in the formation of Carbon (Here the star have stable size). 

This process continues for approximately one billion years, until all the Helium got depleted.

Then again the core contracts without any thermal pressure to balance. But these stars do not have enough mass or gravitational pull to fuse the Carbon. The Kinetic Energy never so high in such stars that it can cause the fusion of Carbon nuclei.

Death Phase

When all the Helium depletes, only Carbon is left in the reacted core. The unreacted subsequent outer core that have Helium is some amount, contracts without any thermal pressure. Which causes the fusion of left over Helium to Carbon and heat again. This heat expands the outer layer of the star. The star loses mass from the outer layer and the core contracts until electron-degeneracy stops it. And hence by the electron degeneracy principle it gets stabilised. Results in the formation of “White-dwarfs” like Nebula (Neptune + Uranus), having very high intensity. After a large amount of time (10 billion years) it cools to become “Black-dwarf”, that is the black lumps of Carbon.

Note: Our sun could not form “Black-dwarf”, as it does not have enough mass.

Case 2: The star having mass greater than or equal to 10 solar masses (solar mass is the mass of the Sun).

Note: the probability of formation of black hole of the star is directly proportional to its size.

Red Giant Phase

Now the star reach to Carbon after the depletion Hydrogen and the subsequent fusion of Helium. As it is a massive star, so its Kinetic Energy is still high implies high temperature. Hence it is ready for the further nuclear-fusion process to achieve a stable nuclei. This process goes on from Carbon to Nitrogen, from Nitrogen to Oxygen (Popularly known as C-N-O cycle), until Iron (Fe). As the nucleus of Iron is the most-stale nucleus among the other elements.

Death Phase

After Iron (Fe), fusion stops, but the contraction continues due to the absence of thermal pressure. This contraction reaches a phase where the star (having the core of Fe) could not stands on its own weight. Here the implosion (an inward explosion) took place to form a “Supernova” (Death of massive star).

Now after supernova, the star become stable by forming “Neutron Star” or very unstable by forming “Black Hole”.

Note: Sun could not reach up-to this stage (As it lies under Case 1)

Neutron Star: Stars whose mass of the core (of Iron) is within the range of 1.4 solar mass to 5 solar mass (The Chandrashekhara-limit).

Then the contraction of core (Implosion) is stopped by neutrons, forming a very highly dense Neutron Star. Whose mass of the core is in radius of 30 kilo-meter. It is very hot and emits X-ray radiation and is invisible even to Hubble Space Telescope.

Note: A sugar cube of neutron star will weigh as much as all existing humans.   

Note: Stars having core mass less than 1.4 solar mass form “White-dwarf”.

Black Hole: Stars whose core mass (of Iron) greater than 5 solar masses.

The core is so massive such that the gravitational pull is so strong that it cannot be stopped by the electron-degeneracy or neutron. The core falls onto itself. It becomes so dense that its gravitational pull doesn’t allow even light to escape.

 

 

Special thanks to Alakh sir (physics wallah).