Amazing world of stars

Every one of us had sung the rhyme – twinkle twinkle little star, how I wonder what you are ... numerous times during our childhood, isn’t it? Apart from featuring in our nursery rhymes, stars have been important to civilizations throughout the world. They have been part of various religious practices and used for celestial navigation and orientation. Many ancient astronomers believed stars were permanently affixed to a heavenly sphere, and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and inferred the position of the Sun. The motion of the Sun against the background stars (and the horizon) was used to create calendars that in turn regulated agricultural practices. The currently used Gregorian calendar is a solar calendar, based on the angle of the Earth's rotational axis relative to its local star, the Sun.

Let us explore the amazing world or stars that had mesmerized us with its scintillating presence. Ask a layman, he would say stars are those heavenly bodies that twinkle in the night sky. Those with little scientific know how would describe Stars as heavenly bodies which has its own light. Some may add Sun is the nearest star. Perhaps all the statements are true.

A star is a massive, luminous ball of plasma held together by gravity. There are countless number of stars spread un-uniformly across the universe. Stars, along with interstellar gas and dust are normally grouped into galaxies. A typical galaxy contains hundreds of billions of stars, and there are more than 100 billion (10¹¹) galaxies in the observable universe. The sun is only one among the billions of billions of stars that light up our night sky.  Apart from the Sun, Proxima Centauri, which is 39.9 trillion kilometres or 4.2 light-years away is the next nearest star. Light from the Sun takes about 8 minutes to reach earth whereas it takes about 4.2 years from Proxima Centauri.

But terming Stars as mere heavenly bodies is just one side of story. Sun, the star nearest to the Earth is the ultimate source of energy and in fact is the driver of all life on our planet Earth. You are well aware, green plants convert the energy from sunlight into useful organic compounds, especially sugars through photosynthesis. Another important aspect of photosynthesis is the process uses carbon dioxide and releases oxygen essential for our survival. Green plants draw energy directly from the sun, whereas all other life forms depend on green plants for their energy requirements. Therefore it could be said nearly all life depends directly or indirectly on the sun for its survival. The most interesting aspect however is that a Star has a life cycle similar to that of human beings except that the life cycle of a star lasts for several thousand million years.

Stars are formed by accretion of clouds of gases containing Hydrogen, Helium and dust, known as nebulae. These clouds under its own gravity contract setting up a dramatic process which converts this dense gas cloud into a Protostar. As the protostar contract, the atoms in the gas collide frequently raising the inner temperature to about 10⁷ degree Celsius. At this high temperature the Hydrogen nuclei fuse to form Helium, liberating enormous amount of energy in the form of light radiations of different wavelengths. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes. The star's internal pressure prevents it from collapsing further under its own gravity.

The star at this stage is in a state of dynamic equilibrium under two opposing forces. The gravitational force trying to compress and ignite the fusion reaction and the internal pressure generated by the energy released by fusion reaction. This balance can continue for several thousand million years. Our Sun, which was formed some 5000 million years ago is presently in this balanced stage of its life. It is estimated our Sun will continue in this stage for another 5000 million years driving different life processes on earth.

As fusion reaction continues in the interior of the star, more and more Hydrogen is converted to Helium and after some time, the core will consist Helium in large quantities. The fusion reaction would then stop in the core, the internal pressure would drop and it would begin to contract once again under its gravity. However in the outer shell, fusion reaction will continue. The outer shell will expand resulting in increase in surface area and decrease in the intensity of the light radiation. The star at this stage would change colour and appear red hence this stage is called Red giant.

Once a star reaches the Red giant phase, its future depends on its initial mass. A star with an initial mass similar to that of our Sun would loose its expanding shell. The core left behind would gradually condense into a dense ball causing the helium to fuse and form higher elements like Carbon. At this stage the star is called a White dwarf.

On the other hand, if the initial mass of the star is much more than that of the Sun, its end would be quite spectacular. The core of the helium formed during the Red giant stage would continue to contract building up higher and higher temperature. The outer envelop then would explode with a brilliant flash called a Supernova explosion. Such explosion could release in one second the same amount of energy that the sun would radiate in about a hundred years time.

After the supernova explosion, only the core would survive. The vast gravitational forces would crush the core to tremendous densities forming what is called a Neutron star. The fate of the neutron star again depends on its mass. It has been estimated that heavy neutron star could continue to contract infinitely. This implies the vast amount of matter in the neutron star would eventually be packed into a small ball of highly dense matter called a Black hole. The gravitational pull in a black hole is so great even light cannot escape from it. Black holes have been known to distort the space around them, and can suck neighbouring matter into them including stars. This is how stars pass through different stages of their life as we do. They take birth, grow younger and die after a certain age, lighting up our life during the intervening period of birth and death.

So next time when you hear a child singing twinkle twinkle little star, you know there is more to it than the rhyme. Stars are infact the driver of all life on our planet earth.

Based on my talk aired by Akashwani, Port Blair on 21/05/2011 and sourced from my article published in The Daily Telegrams dated 30/08/1998.