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.
The limit also explains one of astronomy’s most useful tools for measuring the universe. Because a white dwarf detonates only when it reaches almost exactly the same mass, the resulting type Ia supernova always releases roughly the same amount of energy and reaches nearly the same peak brightness. Astronomers call these standard candles, since comparing how bright one appears from Earth to how bright it actually is reveals its distance with unusual precision. That technique was central to the 1998 discovery that the universe’s expansion is accelerating, a finding that won the 2011 Nobel Prize in Physics and pointed directly toward the existence of dark energy.
When Chandrasekhar first presented his findings in 1935, the prominent astronomer Arthur Eddington publicly dismissed the idea in front of the Royal Astronomical Society, insisting nature would surely find a way to prevent such a catastrophic collapse. Chandrasekhar was proven right decades later, and he went on to win the 1983 Nobel Prize in Physics for the very work Eddington had once ridiculed.