HERTZSPRUNG RUSSEL DIAGRAM -EVOLUTION OF STARS

 

Now 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.

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 the absolute magnitude of the stars and the Y axis denotes the temperature of the stars.

On the HR diagram, a star 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. The temperature at which the fission reaction takes place is from about 1 crore 40 lakh degrees centigrade onwards.

Initially, all the Hydrogen at the central part gets used up and all of it is converted into Helium and no further Hydrogen is available there to continue the fusion reaction. The fusion in the convective zone then stops. The core of the star starts condensing and becomes more and more denser. By that time the temperature over the convective zone increases to about 2 crore degrees Celsius and the fusion reaction is initiated there. After the Hydrogen reserves there are exhausted the fusion reaction moves to the higher layers. As this happens the temperature of the star increases and the volume of the star also expands. Due to this expansion, the temperature of the star decreases but as the star expands there is more surface area for radiation of light. At this stage, the star is known as a Red Giant. The surface temperature of the star cools down from about 6000 degrees to around 3000 degrees and it looks red.  

As the outer layer of the star starts expanding the central core full of Helium starts condensing more and more till such time it reaches a temperature of 10 crore degrees Celsius. At this point, the Helium nuclear reactions take over. Because of this, the fusion reaction comes back to the center but with a difference. Now instead of Hydrogen Helium uses up and starts getting into higher elements on account of the fusion reaction. These reactions produce Neon, then Magnesium, and gradually higher elements up to nickel. After that, the fusion reaction stops. The scientists assume that red giants are in this condition. Our sun still has 81.6% Hydrogen in it and for it to turn into a Red Giant it would take another 500 crore years.   

Finally, in the red giant stage does the star die because the thermonuclear reaction stops?