In 1928 an Indian graduate student, Subrahmanyan Chandrasekhar, set sail for England to study at Cambridge with the British astronomer Sir Arthur Eddington, an expert on general relativity. (According to some accounts, a journalist told Eddington in the early 1920s that he had heard there were only three people in the world who understood general relativity. Eddington paused, then replied, “I am trying to think who the third person is.”) During his voyage from India, Chandrasekhar worked out how big a star could be and still support itself against its own gravity after it had used up all its fuel. The idea was this: when the star becomes small, the matter particles get very near each other, and so according to the Pauli exclusion principle, they must have very different velocities. This makes them move away from each other and so tends to make the star expand. A star can therefore maintain itself at a constant radius by a balance between the attraction of gravity and the repulsion that arises from the exclusion principle, just as earlier in its life gravity was balanced by the heat.

Subrahmanyan Chandrasekhar's journey to England and his work on the limits of stellar size is a significant part of the history of astrophysics. His insights during the voyage led to the formulation of what is now known as the Chandrasekhar limit, which is a critical concept in understanding the life cycle of stars, particularly white dwarfs.

The Chandrasekhar limit is the maximum mass that a white dwarf star can have and still be supported by electron degeneracy pressure. This pressure arises from the Pauli exclusion principle, which states that no two fermions (such as electrons) can occupy the same quantum state simultaneously. As a star exhausts its nuclear fuel, it can no longer generate the heat needed to counteract the force of gravity. If the star is massive enough, gravity will cause it to collapse, but if it is below the Chandrasekhar limit, the electron degeneracy pressure can prevent further collapse.

The value of the Chandrasekhar limit is approximately 1.4 solar masses (the mass of the Sun). White dwarfs with masses below this limit can remain stable indefinitely, while those above it will eventually undergo further gravitational collapse, potentially leading to the formation of a neutron star or a black hole, depending on their mass.

Chandrasekhar's work was initially met with resistance, particularly from Eddington, who believed that stars would not collapse beyond a certain point. However, Chandrasekhar's theory was later confirmed and is now a fundamental part of our understanding of stellar evolution and the behavior of matter under extreme conditions. His contributions to the field earned him the Nobel Prize in Physics in 1983.