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A star is a fiery luminous heavenly body, which emits light of its own. On a clear night we can see about 2500 stars at a time with naked eye through the total number of stars is much more. Sun is the nearest star to our earth. After the sun the next nearest star to our earth is Alpha Centauri.
It deals with the life history of a star. Like other things in this universe, a star is born, lives for a certain time and ultimately dies.
Birth of star: The vast space between the stars is filled with particles of dust and gas. Form the study of spectra of starlight after it passes through these gases shows that these gases contain 75% hydrogen, rest being helium and heavier elements. The birth of a star begins when the interstellar dust and gas particles come closer to form a cloud due to the gravitational force of attraction between them. As the particles come closer, the gravitational attraction increases.
Due to this, the original cloud becomes a rapidly shrinking, thicker mass. This is possible only if the original cloud has a mass of atleast on cloud continues, the pressure and the temperature inside the cloud rise. At a certain stage, the cloud breaks up into large number of fragments of steller size and each fragment continues to shrink nag its temperature continues to rise. When the temperature of a fragment reaches millions of degrees, nuclear fusion begins to take place. As a result of it, the fragment begins to radiate light energy from its surface. Then each fragment becomes a star and thus a cluster of stars is born.
Death of star: In each born star, the source of energy is the fusion process involving either proton-proton cycle or carbon – nitrogen cycle. In a fusion process, hydrogen is converted into helium and the energy produced keeps the star shining for millions of years. When the contents (i.e. hydrogen) of a star decrease to a certain proportion, the core begins to contract while the outer regions expand. Accordingly, the surface temperature of the star drops and it becomes giant or super giant star. At the end of this giant or super giant age (which lasts millions and millions of years) the star produces energy at a very high rate, to such an extent, that it explodes in the form of nova or supernova, throwing out the major portion of its mass into interstellar space. This is the death of a star. The core, which is left behind, may end in three ways depending on its mass as follows:
In case the original mass of a star is less than two times the mass of the sun, the gravitational compression leaves the core of star, composed of protons with electrons flying around forming the electron gas. When this electron gas can withstand the inward gravitational force then a stable equilibrium would be achieved. At this stage, the star is called a white dwarf. It gradually cools and finally ceases to be self-luminous. Then it becomes a black dwarf.
(2) In case the original mass of a star is between 2 to 5 times the mass of the sun, the recoil of supernova explosion compresses the core to such an extent that a high pressure is set up in the core. Now, the electrons are forced into the nuclei and the combination of electrons and protons will form neutrons. As a result of it a neutron star is produced.
(3) In case the original mass of a star is more than 5 times the solar mass, the recoil of supernova is extremely violent. Due to this, the core collapses to have an object, which is having very large density and gravity. As a result, any radiation (i.e. photon) or material particle that enters this object never comes out. This object is called a Black hole.
We will now proceed further and see galaxies.