Life of a Star

They are truly the celebrities of the cosmos. On a dark moonless and cloudless night, we look up into the sky and see countless number of stars. They look like small points which shine brightly. It provides beauty to the dark and gloomy sky in the night. But what are they really? What is a star made of? How do stars shine so brightly?


Milky Way Galaxy

Stars in the Milky Way Galaxy (Image: Wikipedia)

What is a Star?

A star is hot, glowing and burning ball of gases, which record some of the highest temperatures in the cosmos. Although they look so small from earth, stars are quite enormous in size and some are even hundreds of thousands of times larger than the earth. The reason they appear so small to us is the huge distance between the earth and the stars. This distance is often measured in “Light years”, but the closest star to the earth is the Sun which is at the centre of the solar system. A star being so massive will have a huge mass and therefore a strong gravitational field. This gravitational field will attract other planets and form a “Stellar-system” much similar to the Solar System. Stars can exist either independently or partnered with other stars, a two star system is called a “binary system”. Stars existing in large groups of hundreds to millions of stars are called a “Cluster”. They come in different sizes, colours and temperatures.


• A common misconception about the term light years is that it sounds like a unit of time, but it is actually an astronomical unit of distance. One light year is approximately equal to 10 trillion kilometres. Another unit of astronomical distance is “parsec” which is equal to 3.3 light years. It distinguishes the stars based on proximity of the stars from the earth.


Characteristics of a Star

Similar to the genetic traits of a person, a star has its own properties. Some are extremely hot, some are cooler, and some are five times the size of the sun making the sun a relatively small star. Based on temperatures, colours of the stars vary. Since the stars are extremely far away to conduct visual research, the astronomers have to deduct important properties of the star based on the light that it emits. The light emitted by a star has to travel lengthy distances.


• An interesting fact is that the light that we see from the distant stars takes a long time (years to decades and even centuries) to reach the earth. So, the light that we are seeing was produced a long time ago. We are actually looking at the past. We are seeing through time. A mind boggling concept!


The stars are usually present in a fixed state, in groups called “constellations” (all Constellations are given ancient Greek names similar to our zodiac signs such as Aries, Sagittarius, Pisces and other famous Greek Gods and Creatures like Orion, Pegasus etc.). If we look out into a clear dark sky that is devoid of any light pollution, we will notice under cautious observation that the relative positions of the stars never change. Therefore the stars have a specific geography to them. Like the earth, where the positions of landmasses are fixed, the positions of constellations are also fixed. Therefore, the entire viewing sphere of the stars is termed by astronomers as the “Celestial Sphere”. Here continents are analogous to the Constellations and celestial sphere analogous to the spherical earth. Under telescopic observations it is seen that stars are present as either combined with nebulas, clusters, galaxies etc. These look like beautiful sparkling, colourful and bright Christmas lights.


Formation of a Star

Every single star in the universe makes a huge difference. Take for instance our very own local star, the “Sun”. It holds an entire system of planets and one (Earth) in which (as far as we know, only one, leaving out the existence of aliens) life exists. The primary reason for life on earth is because of the Sun. Let us see how such mega events occur.


Protostar (The Birth)

As everything else on the universe starts out, stars are initially formed by mere gases and dust clouds in the universe which is called a “Nebula”. But these nebulae remain dormant for quite a while until they are stirred up by an external force. This can be injected by a passing comet or the shockwave of a Supernova. This causes the particles to collide and form clumps. Finally the clump that has a greater mass attracts the other clumps and particles and forms a larger mass (due to the gravitational pull of a mass oriented object). This makes the centre hotter and denser. In the course of a million years, the clumps form into a small body called the “Protostar”. This can be compared to a “foetal stage” of the star formation.



The Carina Nebula containing clouds of gas and dust which can potentially form stars

Teenage and Middle Age

As the Protostar reaches a critical temperature of 7,000,000 kelvins, a nuclear fusion reaction takes place causing hydrogen particles to fuse to form Helium atoms which release enormous amounts of energy. But this energy is still lesser than the gravitational pull which pulls in more and more particles into it until it finally tips and the Protostar becomes unstable causing an energy flow between its poles. It collapses to a stable star in which the energy from fusion overcomes the gravitational pull. After this the fusion continues to take place until there is fuel left to burn. This process is called the main sequence (the stars spend 90% of their lives in the main sequence). The life of the stars depends on the size and hotness of the star.


Old Age and Death (Red Giants, White Dwarfs and Black Dwarfs)

Finally when the fuel runs out, there is no fusion reaction between the hydrogen and helium atoms. This relieves the energy outflow from the core of the sun which keeps the helium on the outer layers. Therefore the helium collapses into the inner core and the star starts collapsing onto itself. When the helium enters the core the high temperatures cause the helium to undergo fusion and cause a layer of hydrogen to burn around it. The fusion of the helium results in the formation of Carbon and Oxygen. This causes the star to swell up to either “Red Giants” or “Red Super Giants” according to the size of the star. The Red Giant swell up to a maximum extent covering a diameter of up to 60,000,000 miles and brighten to 1,000 to 10,000 times. The red giant cools down over a few hundred million years and forms a white dwarf after shrinking. On further cooling it forms a black dwarf.


For larger stars, which are more than 8 times the mass of the sun, the fusion takes place further than carbon and oxygen to gradually form Iron. In the periodic table, Iron is the most stable element. Therefore an iron core settles down at the centre of the star. To collapse the iron tremendous amount of energy is required. This energy is obtained from the explosion called a “Supernova”. When the Iron Core reaches a value of 1.4 times the mass of the sun, the gravitational force again returns to shrink the Black dwarf from a diameter of 5000 miles to about 12 miles. This crushing force causes electrons and protons of to combine forming neutrons an expelling out high energy subatomic particles called neutrinos.


• The energy released is equivalent to the energy produced by 100 stars for a period of 10,000,000,000 years!


During this time the burning core burns brighter than a billion suns. The iron core finally fuses to form heavier metals. The stars therefore truly exit by giving out a spectacular performance, a true “Grand Finale”.