What is a white dwarf?
To understand what a white dwarf is, we need to know how is a star formed.
The process begins when a large amount of gas like Hydrogen starts collapsing in itself due to gravitational force. As it compresses, the atoms of the gas start colliding with each ither at greater speeds. This increases the kinetic energy of the system leading to increase in the temperature. Thus the system gets heated. Now the gas will rise up to an extreme temperature so instead of the hydrogen particles colliding with each other and bouncing off, they will combine to form helium. The heat released is what provides the shine to a star. This heat also increases the pressure of the gas until it is sufficient to balance the gravitational attraction, and the gas stops contracting and makes a star more stable. Eventually, the star runs out of its nuclear fuels & hydrogen. So the more big the star the more heat will be used in balancing the gravitational force resulting the more use of the nuclear fuel. When a star looses its fuel its temperature starts decreasing, it becomes cooler and begins to contract.
However, this is not exactly the case. Chandrasekhar (a renowned Indian-American astrophysicist) suggested that when the star got sufficiently dense, the repulsion caused by the moving particles won’t be able to overcome the gravitational attraction.
He calculated that a cold star of more than about one and a half times the mass of
the sun would not be able to support itself against its own gravity. (This mass is now known as the Chandrasekhar
limit.)
If a star’s mass is less than the Chandrasekhar limit, it will eventually stop contracting and will be converted into a “white dwarf”. The radius of white dwarf is few thousand miles and it has a density of hundreds of tons per cubic inch. A lot of white dwarfs have now been found. Sirius, being the first one to be discovered.
