When the nuclear fusion stops in a star, then the stars start shrinking due to gravity, then the internal pressure i.e. the degenerative pressure of electrons cannot balance the gravity, then if the mass of the shrinking star is less than about 8 times that of the Sun, then these stars The upper surface is also destroyed. This fragmented debris travels around the star's atmosphere, forming a ring around the star. This type of star is called a planetary nebula.
When a star's core gradually contracts (due to gravity) until its density reaches a certain value, it is said to be in equilibrium, and such a star is known as a white dwarf. White dwarfs have the same mass as the Sun. Although its radius is around 5 to 10 thousand kilometers. Dwarf stars are blue to red in color, with temperatures ranging from high (more than 10,000 K) to low (a few thousand K). White dwarfs are dimmer than main sequence stars of the same surface temperature. Because of their small size, the difference in absolute magnitude of white dwarfs compared to main sequence stars is in the range of 5 to 10.
According to Chandrasekhar, if the mass of white dwarfs is about 1.4 times the mass of the Sun, beyond this limit, stars at the end of their lives either explode into supernovae or collapse into neutron stars or black holes. This limit is called Chandrasekhar limit.
According to Chandrasekhar, when the mass of white dwarfs increases, its radius decreases. If the mass of the white dwarf is greater than 1.4 of the solar mass, the gravitational contraction cannot be balanced by its internal pressure i.e. the degenerative pressure of electrons or A star whose mass is 1.44 times that of the Sun does not form a white dwarf, but continues to collapse, blowing off its gaseous envelope in a supernova explosion, and becoming a neutron star. An even more massive star continues to break apart and become a black hole.
In the absence of an energy-generating nuclear fusion reaction inside the core, a white dwarf has almost no source of energy left. So these stars continue to shine by radiating their thermal energy and in this process the temperature decreases forever and all the thermal energy is lost and the star becomes a cold object.
The thermal energy of a white dwarf can keep it shining for billions of years! Sirius B, the companion of the ‘Dog Star’ Sirius A, is the earliest white dwarf star to be observed.
Q.1 What happens if we keep adding mass to white dwarfs??
