Final Stages in the Life Cycle of a Star
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When all the hydrogen gets exhausted, fusion stops and as a result the internal pressure, which keeps the star from collapsing under its high gravitational force decreases and the core contracts under its own gravity. However, the hydrogen in the outer shell fuses to helium and releases energy. But this energy does not create enough internal pressure and the shell begins to expand thus increasing the surface area. The intensity of energy released is less and the color becomes red. At this stage the star enters the Red Giant Phase. Our Sun is going to enter this phase only after 5000 millions years from now and its expanding shell will engulf the first three planets of the solar system.
Red-giant Phase of a Star
Once the star reaches the red giant phase, its future depends on its initial mass. A star, whose mass is equal or less than that of the Sun loses its expanding shell and the core condenses into an extremely dense ball of matter and the resulting high temperature favors the fusion of helium nuclei to carbon.
The energy liberated by the fusion of helium makes this small core glow like a White Dwarf star, as long as the helium lasts before fading into a dense lump.
Death of a Small Star
Note : The density of white dwarf is 10,000 kg/cc and one white dwarf has been observed near the star Sirius.
In the case of a star, whose mass is greater than that of the sun, the core of helium formed during red giant phase contracts building up higher temperature and thus inducing the fusion of helium to carbon and carbon to higher elements. The energy released during fusion is so high that it causes the explosion of the shell and this exploding star is called a Supernova. In a supernova explosion, clouds of hydrogen gas in the outer shell are liberated into space, thus providing raw material for the formation of new stars.
Supernova
Nova
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In some of the binary stars, it is possible that one of stars reaches the white dwarf stage first and it sucks in the matter of the other star, which is comparatively young. This results in a mild explosion called Nova.
After the supernova explosion, the core continues to contract and forms a dense lump called
Neutron Star.
[Density of neutron star is about million tones per cubic centimeter].
Some of the neutron stars spin on their axis and emit radio waves and these neutron stars are called
Pulsars.
[Quasars are quasi-stellar objects or galaxies, which emit radio waves].
If the star is massive, then the neutron star further contracts indefinitely and the mass gets concentrated at a single point called
Black Hole. A black hole traps even light and therefore it is invisible. However, their presence can be guessed if we see a star moving in a circle with no other visible star at the center. One such black hole has been located in the Cygnus constellation.
Note:
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Dr. S. Chandrasekhar, the great Indian Astrophysicist, after several years of hard work, proved that star smaller than 1.44 times the solar mass end up as "white dwarfs". Bigger ones explode as supernova and developing on their size, end up as Neutron stars or Black holes. The limit 1.44 solar mass has come to be known as "Chandrasekhar limit".
Thus we can conclude that the life cycle of a star is highly spectacular. The left over gases after the formation of the star condense to form other heavenly bodies like planets.
Occurrence of Supernova Explosions
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The five supernova explosions, which have been recorded so far, are,
3. In the year 1572 Tycho Brahe observed a supernova explosion.
4. Johannes Kepler observed it in the year 1604.
5. Ian Shelton from Chile observed a supernova explosion on February 24, 1987.
