We all know that without the sun there would nothing be there on our earth. Sun is the source of energy for our earth which always glows its pale yellowish light. Without sunlight, there will be no plants, no living beings or ever no earth! But does this powerful god always stay the same? How it was formed and if it has a death, then how will it die?
We all know very well that the sun is a star and there are also a lot of other stars there in the vast universe other than the sun. It is found to be that there are about 100-400 stars there in our galaxy milky was itself with varying size and energy.
How the stars are formed?
On saying simply, stars are formed from the molecular clouds of the interstellar medium.
And what is called interstellar medium and molecular cloud?
Interstellar medium, as the name denotes, is the space inside the galaxy other than the stars. But it may be composed of dust and gaseous particles. Well, this states about the molecular cloud as well. Molecular clouds are giant clouds of dust and gaseous particles (containing atoms and molecules(typically hydrogen and helium and can also have methanol, water, etc.)) found inside the interstellar medium.
The molecular cloud is so cold that it is just a few degrees above zero kelvin(also called absolute zero).
You can compare the size of a typical molecular cloud with that of the sun and can realise the difference. Molecular clouds are so big that they seem larger in size than our solar system and can be hundreds of thousands of times heavier than the sun.
Molecular clouds are dark areas in the night sky. It is so hard to see a molecular cloud with human eyes or even with telescopes as those molecular clouds are composed of scattered dust particles which will absorb the light from the surrounding stars. This prevents the journey of the starlight to reach us. But we have solutions for that too.
Though the indigenous light can’t go through these giant dark masses, it permits the passage of radio waves. Therefore our astronomers make use of radio waves to see and find the constituents of the molecular cloud.
Also read: Did Earth really form like the other planets?
How molecular clouds came useful?
This molecular cloud undergoes turbulent motion and as a consequence, some regions in the cloud acquire more matter than the other regions. When the gas and other particles in the molecular cloud pile up highly at a certain region, that region starts to collapse under the influence of its own gravity.
When the collapse happens, the particles slowly heat up. Some part which has a greater concentration of matter forms the core of the star. And we call it a pre-stellar core when the size of the collapsing region nearly reaches about 10,000 astronomical units. Over the next 50000 years or so, the pre-stellar core starts contracting.
The core contracts until it change to 1000 astronomical units. Within this time density increases massively as the size shrank to a great degree without any change in the amount of matter inside.
According to the physics laws, the random motion of the gas and dust combined with the system’s contraction as the prestellar core causes the whole system to rotate.
The matter outside this core will form a flat disc around the core as a result of this rotation. It is notable that the core ejects materials out from its poles as the core spins around. This structure from which particles are ejected out is called by the name jets. These jets are the main factors which keep the system in balance.
The system at this phase can be called the protostar. and it must be said that the disc plays a major role in the development of our protostar. It enables the protostar to grow into a properly sized star.
Due to gravity, the core gains particles from the disc by the way of its rotation and grows. This process of growing is what we generally refer to as the accretion process.
After 1000 years or so, the star accumulates most of the matter from the disc. The rest makes used to create other celestial objects like planets and asteroids which are meant to rotate around this parent star(the materials are eventually clumped together to form these bodies). This is why the planets around our solar system and other star systems be in a plane (the plane of the early protoplanetary disc).
Now our star has been born and then it's the time to turn to its duty. (the star would become able to initiate nuclear fission on it. This causes the star to shine. So we call the star at this phase the T-Tauri star).
Apparently, this theory of the formation of stars can explain all the major pieces of information related to the stars and their structure. So this theory is what presently all believe. But still, there exists a main dilemma for astronomers which tempts all to say that all the stars aren’t formed the exact same way early mentioned in this article.
The formation of larger stars, the dilemma of scientists.
Astronomers were able to explain the formation of stars from those of smaller sizes to eight times bigger ones than our sun. The rest, bigger ones, weren’t good to fit in this theory.
Stars of greater masses required another process because the pressure from such stars' radiation (this would be discussed below) would push away the disc circled around. And if there is no disc nearer to the star, how can it grow further by accretion? This would prevent the growth of stars beyond approximately eight times the size of the sun.
So if all the stars have formed by the accretion theory, then there may not be any stars in the universe which are greater in size than roughly eight times the size of the sun. But as a matter of fact, such stars do exist in our universe!
So it is clear that the enormous outward pressure of the thermal radiation that is created as the stars heat up should definitely blow away the dusty cloud of gas trying to accrete onto them. Then the scientists' endeavour turned to find another alternative to explain this.
A lot of alternatives have been proposed by many scientists to explain the cause for the existence of these massive masses. But each has its own problems.
One of the prevailing theories among those new steams of theories was the merging of low mass protostars. Only a few percentages of the total stars are these giant ones (approximately about 1% of the total stellar population). So the reason for this small population may be because of the fact that a few per cent low massed stars merged together. Anyway, this theory too has limitations.
Also read: Did Moon form from Earth? - Moon formation
Duty and life of a star
After the formation onwards, stars come to do their job of supplying energy mainly in the form of heat and radiation. It’s because of the sunlight our earth remains verdant and comfortable. No one can even imagine a world without solar energy. But you may have doubts about the evolution of these kinds of energy inside the sun. How does it work?
What happens inside a star?
We describe a star as a bright sphere of hot, glowing gas held together by its own gravity. Our sun stays with the other billions of stars in our Milkyway galaxy. They may vary in size, mass and other characteristics, but they do work on the same basic principle.
What happens inside a star is Nuclear fusion. Which liberates an enormous amount of energy outside.
First, let's get acquainted with what’s nuclear fusion. Nuclear fusion is the process of merging two light nuclei to form a single heavier nucleus. The total mass of the resulting heavier nucleus is lower than that of the sum of the two lighter nuclei that merged. The difference in this mass is converted to resulting energy (according to Einstein’s equation, energy is equivalent to the product of mass and the square of the speed of light in vacuum (E=MC^2). Which means energy and mass can be converted into each other).
How nuclear fusion happens inside a star?
Okay, we know by now that the molecular cloud supplies the materials for star formation. The molecular cloud comprises a huge amount of hydrogen as I early specified. Collision of the random motioned particles will cause the temperature to rise. Not a little, it changes the temperature to approximately 15000000 degrees in Celcius!
As this temperature is reached, the hydrogen atom slowly starts to involve in the fusion process causing the formation of Helium at the core of the cloud (the lighter hydrogen nucleus merges to form the heavier helium nucleus).
Along with the formation of helium atoms, a huge amount of energy is released in the form of heat and light radiations(you can calculate how much energy will liberate according to Einstein's equation E=MC^2. Even, the corresponding energy of one-kilogram mass is E = 1 X (3.08 X 10^8)^2 = approximately 89,875,517,873,681,764 joules!). This process is called by means of another name: Stellar nucleosynthesis.
This nuclear fusion is the source of many elements in our universe heavier than hydrogen. Like carbon, gold, iron, etc. With time, the hydrogen completely converts to helium and when the temperature increases more and more, helium starts fusion to result in carbon. And the process goes on until iron(according to the size of the star. Massive stars produce gold and iron in the old times of their life).
Over 10 billion years, an average star will burn nuclear fuel in its centre to emit energy as the radiations that we call sunlight(according to the sun).
Every celestial objects have gravity towards its centre. Star, being a massive body compared to the other planets and asteroids, have high gravity which is continuously pulling the star inwards. If there is no equal and opposing fore there, the star will, for sure collapse due to gravity. Actually, that opposing force which enables the star to be balanced is the outward pressure of heat and light formed as the by-product of nuclear fusion taking place inside the core. This pressure is also called radiation pressure.
This equilibrium which makes the star exist is known as Hydrostatic equilibrium.
The time period in the life of a star during which the star consumes hydrogen to form helium is called the Main-sequence. Our sun is in going through this phase. Our sun is estimated to have a 10 billion-year main sequence and it has covered almost half of it (almost 4.5 billion years).
The mass of the star has a dominant role in its lifetime. More massive stars use more fuel rapidly and die faster than smaller ones. If the smaller one lives for billions of years, the larger ones will live only a few hundred thousand years.
It is important to mention here that stars spend almost 90% of their life in the main-sequence stage.
How do stars destroy?
The villain of all stories - Death is not a thing confined to living beings only. Stars too die! Retire from their duty of supplying energy, changes to what they were formed with - to the dust cloud from which the new generation (new stars) sprout out.
When the star at the main-sequence stage loses all of its hydrogen from its centre(core), it begins to use hydrogen outside the core. This causes the outer layer of the star to expand. The star grows almost 400 times bigger than its main-sequence size and gradually cools. Due to this, it came to appear red and giant and thus we call it the red giant.
The red giant may explode and acquires the power to swallow planets. If the sun explodes the same way approximately after five billion years, it would, for sure, engulf all the planets in our solar system.
As the outer layer of the stars expanded and the fuel (hydrogen) almost is in a state of completion, gravity overrides the radiation pressure and thus starts to shrink in. This contraction will increase the temperature more and more causing the helium to start fusing.
The core in this stage fuses helium rather than hydrogen. The star again shrinks and gets hotter and blue. This stage only remains for a few periods (almost one million year) as helium quickly runs out.
Then, the same process that occurred when hydrogen ran out happens again with helium. As helium ran out from the core, the star begins to use the helium from outside of the core. The outer layer of the star expands, cools and turns red again. This is called the second red giant phase as the same process that happened at the red giant phase using hydrogen happened here too but with the help of helium.
The next phase of the star is not the same for all kinds of stars. It depends on the size.
What happens to smaller stars after the red giant phase?
A smaller star like our sun will cool down eventually and stops growing. The stars begin shedding their outer layers. This results in a glowing and expanding shell of hot gases. Researchers gave this the name planetary nebula as it looks a bit like planets if we gaze through a small telescope. We call this phase the planetary nebula phase.
Stars are highly unstable now. So it pulsates producing strong stellar winds which throw off the outer layer of the star. It leaves only a small hot and bright core behind called the white dwarf. It emits ultraviolet radiation and this radiation lights up the layers of gas around the star.
Planetary nebulae only last a few tens of thousands of years. By this time, the materials from it scatter into space forming clouds of dust and gases which will eventually mix with other clouds forming giant molecular clouds from which other stars take birth!
This is the same situation for our star - the sun, too!
Also read: https://www.fluratech.com/2022/10/18-gk-questions-and-detailed-answers-on.html
What happens to larger stars after the red giant phase?
Massive stars have much more violent endings. They explode into a supernova. It is said to be that the supernova can shine brighter than a whole galaxy.
There are different kinds of supernovae. The prominent among them are the core-collapse supernovae that form when a star at least eight times bigger than our sun runs out of fuel.
In massive stars, fusion continues until the iron has formed by fusion. So those stars would be made mostly of iron at that old stage. Stars cannot fuse any element heavier than iron and thus the fusion stops. At this point, the hydrostatic equilibrium does not hold good(the radiation pressure would be overridden by the high gravity of the star). So a sudden collapse happens here as the high gravity pulls all materials towards its centre.
The outer layer collapses and bounces back at about 30000 kilometres per second. This sends shock waves and causes the star to explode as a supernova. A huge amount of energy is liberated as well as new elements are formed in this process.
The star brightens fast and then gradually fades away leaving only the core. During the explosion, the core may be has collapsed down to create either a neutron star or a black hole.
So this all implies that the whole life of a star happens in a cyclic manner. one forms from the remnants of the dead one.
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