STELLAR EVOLUTION-PART 2

Higher brightness of a star means it is using up more of its matter and is rapidly converting it into energy. Now why do stars shine with different brightness? That depends on the amount of matter they possess.

The rate of fusion at the stars core depends on its temperature and the temperature in turn depends on the mass of the star. Although large stars have more fuel they burn it much faster and hence their energy generating life is lesser than a star with lower mass.

A star with a mass of just 0.4 times the sun has a lifetime of 200 billion years, that of 0.8 times the sun 14 billion years, that of sun 9 billion years and that of 40 times the sun just 1 million years. See how rapidly the life span decreases as the mass of the star increases. Life evolves over billions of years, therefore only those stars which have a mass around that of the sun have ample time to evolve life.

Stars are classified as per their spectra and their temperature. The classifications are O B A F G K and M. to make one remember this is the sentence “Oh be a fine girl kiss me”. Sun belongs to the spectral class G and these are the stars that are most likely where life can evolve.   

For knowing the way the stars evolve an American called Russel and a Russian called Hertzsprung made a diagram known as the “Hertzsprung Russel Diagram”. or HR diagram.  In this diagram the X axis denotes absolute magnitude of the stars and the Y axis denotes temperature of the stars.

On the HR diagram a star that follows a specific pattern meaning the higher the temperature the higher the mass then those stars are known as main sequence stars. On the HR diagram they follow a slope from the top left corner of the diagram to the bottom right corner. If we take a main sequence star its central part can be considered as a large nuclear factory. On top of that is the convective zone where energy is transported. The temperature at which the fission reaction takes place is from about 1 crore centigrade onwards. Our sun has a core temperature of 1.40 crore degree Celsius.

Initially all the Hydrogen at the central part of the star gets used up and all of it is converted into Helium and the fusion reactions pass to the higher layers till all the hydrogen reserves of the star get exhausted. The fusion reactions in the star then stop and gravitational forces take over. As the star condenses on account of gravitation it reaches a temperature of 10 crore degrees Celsius where the fusion reaction start with Helium and Neon is produced. When the Neon is exhausted it passes onto the higher element of Magnesium and so on till Nickel. After that the fusion reactions stop.

Scientists assume that stars known as red giants with large surface areas and low temperatures are at this stage. Our Sun still has about 81.6% Hydrogen and for it to turn into a red giant it would take 500 crore years. As the size of the star expands when it reaches the red giant stage the temperature of the star cools down which is the reason why the star appears red. Compared to the surface temperature of the sun which si 6000 degrees, the red giants have a surface temperature of 3000 degrees. The star therefore leaves the main sequence and travels upwards and to the right of the HR diagram.

Some of the red giants are enormous in volume compared to the sun. For example the bright star Betelgeuse in the constellation of Orion has a diameter of 100 crore KM compared to just 14 lac KM for our Sun. However, the density of this star is almost negligible enough to be zero compared to 1.4 for our sun and this star weighs just about 16 times our sun.

As already said earlier, as the star is much bigger than the sun it loses its matter much faster and outshines the sun by far. Even in its red giant phase this star is 46,000 times more luminous than the sun. The result is while the sun is already 450 crore years old and would take another 500 crore years to reach the red giant stage, this star has splurged off its energy in just 1 crore years due to its heavier mass.

The stars have been categorized into different names depending on their size.

All those stars that are up to 20 times brighter than the sun are called Dwarf Stars. Our sun is a yellow dwarf. We consider the sun supreme but it just happens to be a dwarf star in the Universe.     

Super giant stars are the largest known stars in the Universe but they are rare. The star Betelgeuse mentioned above is a super giant. When these super giant stars die they become Super Novae and then form Black Holes.

When giant stars die they turn into Novae. In both Novae and Super Novae a single massive explosion would blow apart the matter in them. A star shines tens of thousands and sometimes 100,000 times brighter on turning into a Nova.

A Nova reaches maximum luminosity in a few hours after the explosion and return to its normal luminosity in days or a few weeks.

What happens after that? The star becomes a White Dwarf. Very hot and dense and full of carbon. With time it loses its heat and turns into a black dwarf and that can be called a dead star.  

A Super Nova explosion unlike that of the Nova is a massive event and is much more violent and it ends the life of a star. The Super Nova explosion would blast massive amounts of matter with a mass of several times the sun into space and at that time it outshines an entire galaxy consisting of 10,000 crore stars.

What happens after the Super Nova explosion? Bigger Super nova explosions turn into what we know as Black Holes. A black hole is so dense that it would not even let light escape form it. That is why a black hole cannot be seen.

Some black holes are very minute. These are just as big as an atom but contain the mass of a mountain. Another type of black holes known as Stellar black holes have a mass of up to 20 times the mass of the sun but the mass is concentrated in an extremely small area that gives it an immense gravitational attraction. Then there are the super massive black holes that have a mass of some 1 million times that of the sun. Scientists have postulated that all the galaxies have a super massive black hole at its center.   

However, not all super novae explosions give rise to black holes. Stars with masses up to 29 times the sun turn into Neutron stars. The Neutron stars have a diameter of about only 10 Km but have a mass that is up to 2 times that of the sun. Matter exists in them only in the form of Neutrons and that is why the name Neutron Star.