The Chandrasekhar limit is the mass ceiling for a white dwarf, set at roughly one point four times the mass of the sun. Below that mass, a quantum effect called electron degeneracy pressure holds the star up against its own gravity and keeps it stable for billions of years.
It is named after Subrahmanyan Chandrasekhar, who worked out the calculation in 1930 as a nineteen year old physics student sailing from India to Cambridge, combining Einstein’s special relativity with the physics of degenerate electrons. The result showed that degeneracy pressure has a hard ceiling, past which it simply cannot hold gravity back any longer.
A white dwarf that pulls in extra mass from a companion star and crosses the limit does not just quietly grow bigger, it detonates as a type Ia supernova, an explosion so consistent in brightness that astronomers use it as a reliable ruler for measuring distances across the universe. More on the physics of white dwarfs is available from NASA’s Imagine the Universe.