White Dwarfs, Red Giants, Neutron Stars, Black Holes and Chandrashekhar's Limit

Quick Take

Nothing is forever; even stars are born and eventually die. What happens to stars like our Sun, though, after they die? Ultimately, they become white dwarfs.

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What are White Dwarfs?

White dwarfs are remnants of dead stars with a size comparable to that of Earth and a mass comparable to that of the Sun . Due to their low volume and high mass, these stars are extremely dense. The gravity balancing the outward force in such stars is the “electron degeneracy pressure.” This is a consequence of Pauli’s exclusion principle, which states that no two electrons can exist in the same quantum state. These stars are called “white” due to their extreme temperatures at the time of formation. The surface temperature of a newly formed white dwarf is about 100,000 K; however, it is believed they slowly cool down to form black dwarfs in an estimated period of a few billion years. No black dwarfs have yet been detected.

How are They formed?

A star is said to be dead if it has exhausted all its nuclear fuel and, hence, no further energy production through fusion takes place. In such cases, the force of gravity dominates and pulls the entire mass of the star toward the center, and the star starts to shrink and collapse under its own weight. While in the phase of contraction, the temperature of the star increases and it fuses the hydrogen that remains in the outer shell. Due to this, the shell expands and the star enters the red giant phase. The Sun in its red giant phase would be so huge that it would engulf Mercury . After about a billion years, the outer layers will dissipate and form a cloud of gas around the central core that contains heavier elements, like carbon. This central region continues contracting until the “electron degeneracy pressure” is enough to balance gravity. When the star reaches equilibrium, it is said to be in the white dwarf phase. For stars with a core mass of more than 1.4 solar masses, the electron degeneracy pressure is not enough to balance gravity. They go on to form neutron stars or black holes. This mass limit is known as “Chandrasekhar’s Limit ”.

Discovery and Further Observation

Friedrich Wilhelm Herschel, on January 31, 1783, discovered the white dwarf, 40 Eridani B. The next star to be discovered was Sirius B, by astronomer Friedrich Bessel in 1844. He predicted that Sirius has a companion star, by observing the periodic change in its position. In 1863, Alvan Clark observed this previously unseen object, which was later determined to be a white dwarf. Recently, a lot of white dwarfs have been observed by Hubble and other large telescopes across the globe. The X-ray satellite, ROSAT, also observed a white dwarf: HZ 43. The X-rays in white dwarfs come from the inner layers, unlike most other X-ray sources in the sky. As of now, around 10,000 white dwarfs have been observed.