Stellar Evolution

 Each and every one definitely has had heard about the stars and wondering since their childhood. You woke up in the night and go to your terrace, see the night sky with thousands of shining stars in the sky. Many questions blow in your mind like why do stars shine? Why do stars round? How universe begin? What happened before the big bang? Thousands of questions are there which you want to know about stars and other things.

Astronomy is the oldest science ever known. The universe is full of mysteries and many things have been answered now, but still, we do not know nothing, because the universe is so vast, there are so many things which are still unrevealed. So, let’s begin with the basics of stars and comes up to the advance and measurable methods.

Stars are the basic building blocks of the galaxy, from which is it made up of. Stars are responsible for making galaxies and the formation of many elements like carbon, nitrogen and oxygen etc. Perhaps, to know about star birth, life, death is a central field to the astronomy.

Let me give you the basic idea about star formation, how stars made up of and why do stars shine?

Star formation- It is formed from interstellar clouds of dust and gases (hydrogen and helium). The first stage of the star’s life, gravity allows them to collapse under their own Gravity, it compresses the gas to the point where it has enough gravity to collapse more material itself. As the gas and dust, heat up and the temperature of the gas continues to rise, it starts to glow and emitting light. The cloud has now become a young star. So, when the core of the gas cloud becomes hot and dense that the hydrogen begins to collide and fuse into helium. Four atoms of hydrogen combine and form one atom of helium this process is called nuclear fusion. Since, the mass of he atom is 0.7% smaller than the mass of the four hydrogen atoms that formed it, remaining amount of mass of the atom has lost and converted into energy, Sun consumes 600 million tons of hydrogen every second. The total amount of energy produces is huge, this energy produced in the core and then reaches the surface. The plasma at the surface and begins to radiate hence, the star shines.

Core of the star

Image credit- Penn state Astronomy & Astrophysics

Image credit- astronomy.com

The energy liberated from the core of the star it creates a pressure. So, that the star collapses by the force of gravity. At this point, the star stabilizes called an Adult Star. The star’s temperature and color depend on its mass, a star’s lifetime also depends on the mass of the star. The greater the mass, the shorter the lifetime. The large mass stars can only sustain fusion for a few million years like Sun. The low mass stars cannot sustain fusion for a very long time and later become brown dwarfs.

A star the size of the sun requires about 50 million years to mature from the beginning to the adulthood. Main sequence star spans a wide range of luminosities and colors.

As we know, stars are made up of about 90% of hydrogen and about 10% helium, some other elements found in stars which is approximately 1% like- nitrogen, silicon, carbon, iron, copper, oxygen, silver, nickel, plutonium gold and uranium. These elements might have been present when the star is formed. Three helium atoms combine and form carbon then the carbon atom combine with helium to form an oxygen and this process is continues. In some stars, the hydrogen fusion began in the core and eventually becomes a shell of hydrogen fusion moving outward of the stars, while helium fusion continues to take place in the inward.

Why are stars round?

Stars form when the dust clouds collapse under the influence of gravity. It takes a spherical shape because any protuberance in the outer layer will exert pressure on the inner ones and sinks inwards. Stars are not perfectly circle in shape, in reality. Most of the stars rotate, and centrifugal force causes them to bulge out at the equator and be slightly flattened at the pole, as has happened with Earth. Our galaxy contains about 100 billion stars (10¹¹).

Let’s have a look at the star’s lifecycle

Image credit- scientioix.com

As we have discussed above about the star formation, here you can see the steps of evolving a star.

Stellar nebula- The interstellar dust and gas cloud which is responsible for the formation of stars.

Here the interstellar dust categorized in two different deaths of the star. If the mass of star is very low (less than 0.4 times one solar mass) the death is like the white dwarf. But when the mass of star is very high (8 times of the sun) the death will be like the neutron star or black hole.

Let’s study what happen to the low mass star?

The smallest stars, known as red dwarfs, may contain as little as 10% the mass of the Sun and emit only 0.01% as much energy, glowing feebly at temperatures between 3000-4000K. Despite their diminutive nature, red dwarfs are by far the most numerous stars in the Universe and have life spans of tens of billions of years.

On the other hand, the most massive stars, known as hypergiants, may be 100 or more times more massive than the Sun, and have surface temperatures of more than 30,000 K. Hypergiants emit hundreds of thousands of times more energy than the Sun, but have lifetimes of only a few million years. Although extreme stars such as these are believed to have been common in the early Universe, today they are extremely rare - the entire Milky Way galaxy contains only a handful of hypergiants- science. NASA. Gov

With stars of very low mass, the heat of the core diffuses and it's becoming dense, rising up to the surface where they cool down, then penetrate once again towards the core to repeat the cycle. Once all the hydrogen is consumed, the star has been transformed into helium gas. This process is extremely slow and it takes hundreds of billions of years.

The intermediate star like our sun, only the hydrogen in the core can be consumed, once that hydrogen is exhausted, the core contracts under high pressure. It raises the core temperature and allow hydrogen fusion to proceed in a spherical shell. Now the star has become the red giant. The process repeats itself, now helium fusion in the core to produce carbon and oxygen. When the helium is exhausted, it contracts. This heats it up and expels its outer atmosphere form a planetary nebula. It is all remains of the original is a small star with a hot, dense core of carbon and the oxygen, called a white dwarf.

So, now what happens to the large mass star?

Now, the stars of very large mass, it too enters a giant phase once its all hydrogen is consumed, but the event is comprehended and violent, leaves the star a supergiant. The temp in the core of a supergiant is higher than in stars of average mass, the nuclear fusion extends beyond the integration of carbon. The star self-annihilation in a supernova explosion. Then the core burst inwards and the residue is an extremely dense that can be either a neutron star or a black hole.

A neutron star is about 10-20km in diameter in which the density of diameter is so high that the protons and electron have merged to form neutrons.

Low mass stars can live for hundreds of billions of years. Stars of average mass, like the Sun, are stable and spend about 10 billion years on the main sequence stars and 100 billion years in the red giant phase. High mass stars spend about 70 million years on the main sequence stars, then 5 million years in their supergiant phase. Supernova explosions only last about 10s but the residual objects remain bright for months.

 So, till now we have studied the process of star formation and death of a star. Hope you understand it in a simple way and find it informative.

Star and their fates

Article by- Kirti Tomar (Amateur Astronomer)