STELLAR EVOLUTION

 

Just like people aging and then dying all the stars in the Universe too age and die. The only difference is that the death of a star does not mean the end of its matter but the end of its self luminosity and the thermonuclear reactions taking place in its core.

As I have written in my earlier note there are about 100,000 crore stars in our Galaxy. The average galaxy size in the Universe is 10,000 crore stars and there are 10,000 crore such galaxies in the Universe. Thus the Universe consists of 10¹¹*10¹¹ or 10²² stars.

Before going into the evolution of stars, other aspects of stars have to be considered.

Of the stars some are small, some are big and some are massive. Stars emit energy on account of the innumerable fusion reactions taking place in their core. The energy produced in a second in a star is enormous. For example, our Sun loses about 43 lac tons of mass each second and this mass is fully converted into energy as per the equation E=mc². If we can bring a pinhead of matter from the Sun to the earth it can turn some populated cities on the earth to ashes.

Depending on the mass and brightness of the star its evolution varies. Thus it is important to know the variety of stars in the Universe. On an average night, one can see just about 3000 stars with the naked eye. But instruments enhance our vision. With binoculars one can see 50,000 stars. If we view through a 2-inch telescope the figure rises to 3 lakh stars. The largest reflecting telescope in the world, the Gran Telescopio Canarias in Spain has a diameter of 410 inches and can virtually show us an enormous number of stars. Of course, there are also reflecting telescopes as well as radio telescopes that enable us to see further and further.  

For measuring the distance of the stars we have to use the unit Light Year which is the distance light travels in a period of 1 year. Light travels at a speed of approximately 300,000 Km/Sec. In a year this distance becomes approximately 9.5 lakh crore Km.

The nearest star to the sun is Proxima Centauri which is located at a distance of 4.2 light years away from us. It is in the star system Alpha Centauri which is a triple star. The 3 stars rotate against a common center of gravity. The next star is Barnards which is 6 light years away and after that Wolf 359 which is 7.7 light years away. The brightest star visible to us in the sky with the naked eye is Sirius and it is at a distance of 8.6 light years from us.  

Stars are classified into magnitudes depending on their luminosity. Stars that are visible to the naked eye are classified into 6 magnitudes. This classification was done by Greek astronomers. The brightest stars are classified as magnitude 1. Those with a little less luminosity are classified as magnitude 2, then 3 and so on till 6. A magnitude 6 star is so faint that it is barely visible to the naked eye. After the arrival of the telescopes, we have been able to see much higher magnitudes of stars. With the largest telescope stars up to magnitude 30 are visible to us.  

A 1^st magnitude star is 2.5 times brighter than the 2^nd magnitude star. A 3^rd magnitude star is 2.5 times brighter than the 2^nd magnitude star and so on. The brightest star visible to the naked eye is Sirius and it has a magnitude of -1.6. The minus sign indicates higher brightness. A 0-magnitude star is 2.5 times brighter than a star of magnitude 1. Similarly, a -1 magnitude star is 2.5 times brighter than a 0 magnitude star. As per this, a magnitude 1 star is 100 times brighter than the 6^th magnitude star.

On this magnitude scale, the brightness of our moon is -12.6 and that of the sun is -26.7.

What is visible to the naked eye is the apparent brightness of a star and not its real brightness as there is a tremendous variation in distances to different stars. To evaluate the actual luminosity of a star scientists have devised a mathematical model by which the stars are brought to a distance of 10 parsecs (1 parsec is 3.26 light years) or about 33 light years away from us and then evaluate their brightness. This is known as the absolute magnitude of that star.  

If our sun is taken to that distance it would shine as a star of magnitude 4.8.  The brightest star visible to the naked eye is Sirius and it shines with a magnitude of -1.6. Its absolute magnitude however is 1.3.

 There is one galaxy called the Larger Magellanic Cloud near to the Milky Way. In it is a star called S Doradus which has an absolute magnitude of -10. In the same galaxy, the star R136a1 has an absolute brightness of -12.5 which means if it is brought to a distance of 10 parsecs (33 light years) it would shine as bright as the moon itself while at the same distance our sun at a magnitude of 4.8 is barely visible to the naked eye.

This is the first part of a series I am writing on the evolution of stars and the next part will follow later.