4.1.3. Final Stages of a Star̛ s Life

In the first part of the final stage of its life, a star enters the red-giant phase where it becomes a red-giant star. After that, depending on its mass, the red-giant star can die out by becoming a white dwarf star, or by exploding as a supernova star, which ultimately ends in the formation of neutron star and black holes.

(1) Red- Giant Phase. Initially, the stars contain mainly hydrogen. With the passage of time, hydrogen gets converted into helium from the centre outwards. Now, when all the hydrogen present in the core of the star gets converted into helium, then the fusion reactions in the core would stop. Therefore, ultimately, the matter in the core of the star would consist only of helium. Due to the stoppage of fusion reactions, the pressure inside the core of the star would diminish, and the core would begin to shrink under its own gravity. In the outer shell or envelope of the star, however, some hydrogen still remains, the fusion reactions would continue to liberate energy but with much reduced intensity. Due to all these changes, the overall equilibrium in the star is upset and in order to readjust it, the star has to expand considerably in its exterior region(outer region). Thus the star becomes very big ( it becomes a giant), and its colour changes to red. At this stage, the star enters the red-giant phase and it is said to become a red- giant star. Our own star, the sun, will ultimately turn into a red-giant star after about 5000 million year from now. The expanding outer shell of the sun will then become so big that it will engulf the inner planets like mercury and Venus, and even the earth. When a star reaches the red-giant phase, then its future depends on its initial mass. Two cases arise:

(a) If the initial mass of the star is comparable to that of the sun, then the red-giant star loses its expanding outer shell and its core shrinks to form a white dwarf star which ultimately dies out as a dense lump of matter into the space.

(b) If the initial mass of the star is much more than of the sun, then the red-giant star formed from its explodes in the form of a supernova star, and the core of this exploding supernova star can shrink to form a neutron star or black hole.

(2) Formation of White Dwarf Star: If the mass of red-giant star is similar to that of the sun, the red-giant star would lose its expanding outer shell or envelope because then the comparatively smaller amount of hydrogen fuel present in it will be used up rapidly, and only the core of the red-giant star will gradually shrink into an extremely dense ball of matter due to gravitation. Because of this enormous shrinking of helium core, the

temperature of core would rise greatly and start another set of nuclear fusion reactions in which helium is converted into heavier elements like carbon, and an extremely large amount of energy will be released. When the mass of a star is similar to the mass of the sun (which is comparatively a small mass), then all the helium is converted into carbon in a short time and then further fusion reactions stop completely. Now, as the energy being produced inside the star stops, the core of star contracts (shrinks) under its own weight. And it becomes a white dwarf star.

A great Indian scientist Chandrasekhar made a detailed study of the stars which end their lives by becoming white dwarf stars. Chandrasekhar concluded that the start having a mass less than

1.44 times the solar mass (or sun’s mass) would end up as white dwarf stars. The maximum limit of 1.44 times the solar mass (for a star to end its life as a white dwarf) is known as ‘Chandrasekhar Limit’. If, however, a star has a mass more than 1.44 times the solar mass or sun’s mass, then it will not die out by becoming a white dwarf star. This is because due to greater mass, it will have more nuclear fuel in it, which will not get exhausted in a short time. The stars having mass much more than solar mass (or sun’s mass) led to supernova explosions and end their lives by becoming neutron stars or black holes. This point will become clearer from the following discussion: