The star story- from Big Bang to black holes


Want to know about stars and how they formed? The Universe began with a big bang, giving birth to countless galaxies and trillions of stars. Now not all the stars are same. Stars are of different types and they live and die in different ways. This article is about the life cycles of stars.

In the beginning, there was nothing. Then the universe rapidly expanded within a split second, a phenomenon called Big Bang. Big Bang occurred 14 billion years ago. Giant gaseous clouds called Nebulae present during the birth of universe formed stars and galaxies. So stars are one of the first things that formed.

Birth of a star

At a temperature right above absolute temperature, gas becomes molecular, with carbon monoxide and hydrogen being the most common gases. The denser molecules collapse into core due to their own gravity and the clouds of lighter molecule keep forming gasesous layers over and over for millions of years birthing a star.

Cores of stars very massive. They try to pull the surface towards them. But the fusion reaction in the core, where hydrogen fuses to helium, releases an impressive amount of energy which counteracts the gravitational pull of core. Thus a star balances itself.

Types of stars

Brown dwarf

Our Sun isn't even considered a medium sized star. Sun is very young, energetic and active yet. A star should have 1/8th of the mass of the Sun in order to sustain and perform fusion reactions in core. A Brown dwarf is any star that lacks such mass. So, it has a gaseous surface with an inactive core. They still emit energy due to potential energy of collapse. They are probably the most common type of star.

Death of a Brown dwarf

After 15 million years of burning relentlessly the Brown dwarf finally fades away to a black dwarf, meaning, it becomes intolerably cold and undetectable by light. Such is the death of a pathetic Brown dwarf.

G- type main sequence star

Our Sun is a fine example for it, with a surface temperature up to 6000 C. They are also called yellow dwarfs, though our Sun is actually white. These stars are also very common.

Now here's a sad news. Sun was formed 4 billion years ago. It has lived more than half of its life. We only have 5 million years left to cherish our lovely and only star.

Death of our Sun and similar stars

During the final stages of their life, star core, where the fusion reaction occurs, is infested with heavy helium because hydrogen is now exhausted. Sun will turn into a red giant, which has a several times huge diameter than present. In the process of this expansion, Sun will devour the first three or four planets of our solar system.

The source of life will then snatch away the life. The outer layers of the star then, due to a great decrease in the pressure from now degenerate core, will fly away as a nebula. What will be left is called a White dwarf.

But before looking into a white dwarf, let us explore the remaining categories.

Blue giants

Blue super giants to be precise are the hottest stars, with 10-100 times of mass greater than sun, with surface temperature ranging from 9000-10000 degree Celsius. They have a very short life span because of rapid fusion in core. These enormous and extremely hot stars die in an extreme way too.

Death of a blue supergiant

Ever spotted Orion constellation? Try to find the star Rigel this time around. That is a blue giant. Blue giant, like any other star when depleted away of all of its hydrogen in its core, turns into a blue supergiant for a few hundred years, until the mass of heavy elements accumulate on the surface of the star. Then within seconds an explosion like none other occurs in the core of the star ejecting vast volumes of material and energy into empty space. These radiations travel through space for millions and billions of kilometres. Devouring everything in its path. This phenomenon is called a supernova.

What happens to the fate of the star after supernova is equally interesting. Remember the white dwarf we mentioned during the death of main sequence stars?

Let's take a brief look about them -

White dwarfs are the stars in their final stage who's mass isn't big enough to die a splendid way like supernova. White dwarfs, retains the star's mass, while being about our Earth' s size. To explain it, imagine a school bus. Shrink it to a size of an eraser. Now the eraser still weighs the same as the school bus.

This amazing stage of a star life is where there is no fusion reaction in the core. In fact a white dwarf is the core of a dead star. Hence it is still hot, for a few more million years. Then it cools down forming a black dwarf, as we previously saw, a fully dead star not emitting energy and undetectable.

A blue giant with lesser mass might end up a white dwarf.
But if a blue giant is 30 times larger than Sun, then it becomes a neutron star.

A Neutron star is similar to a white dwarf, but when white dwarfs are formed from G-sequence stars, Neutron stars are remains of a supernova. Because they were once a core of massive star, their gravitational pull is a billion times stronger than Earth.

A neutron star would later collapse in itself to form an empty, undetectable, void inside the void, a black hole.

But Black holes must wait until we unravel our last star type.

Red Dwarf

These are the oldest stars in universe and are the among the smallest too. About 1/3rd of the stars in Milky way are Red dwarfs. Their surface temperature is just around 3000 degree centigrade. Because of lower mass, the gravitational pull of these stars is very low.

Death of red dwarfs

Proxima Centauri, the nearest star next to Sun, is a red dwarf. Red dwarfs' unusual and overly large lifespan helped us estimate the age and the birth of the universe to a great extent. These stars don't die a spectacular way though. Unlike Sun and other stars, a red dwarf keeps altering it's brightness every second. At present no red dwarf is in it's final stage. But just like any other star, after trillions of years, a red dwarf would turn into a black dwarf and get lost in space.

With the discussion about black holes we can conclude our interstellar journey. Huge neutron stars collapse under their own weight to give rise to an absolute nothingness. A void in space that even light cannot escape. The imaginary border that separates a black hole from remaining space is called Event Horizon. Come too close to event horizon and you'll be sucked into nothingness and no one knows what happens inside a black hole.

Black holes are mysterious. Every galaxy has a massive Black hole in their centre which cause the stars in a spiral galaxy to revolve around it.

Understood now? Moons revolve around planets. Planets around stars. Stars revolve around a massive Black hole.

Wow! A star's life and death are both amazing and confusing.


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