A Sun's worth of matter in a small star
Sirius B packs roughly the Sun's mass into a body comparable in size to Earth. Such a star is called a white dwarf: a dense remnant that shines as it loses stored heat. Gravity is immensely strong at its surface, yet the star can remain stable while it cools. Subrahmanyan Chandrasekhar asked how that support works, and found that it has a limit. Above a certain mass, an ideal white dwarf cannot balance its own weight.
Start with a layer inside any spherical star. The material beneath it pushes outward; the material above pushes inward. Pressure must be greater on the lower face so that the difference can support the layer's weight. This condition is called hydrostatic equilibrium. It is a statement about force balance at each depth, whether the pressure comes from a hot gas, radiation or densely packed electrons.
Adding more mass strengthens gravity. Squeezing the star also changes the pressure, so the outcome depends on how rapidly pressure grows as matter becomes denser. Finding a stellar structure means fitting those two effects together throughout the whole sphere. The calculation has to follow pressure and enclosed mass together from the centre to the surface.
Electrons still resist compression when the star cools
For an ordinary gas, warming the particles strengthens their pressure. A white dwarf has another source of support that survives even when its temperature becomes low compared with the electrons' energy. Quantum mechanics restricts how electrons can share the available states of motion. When the low-energy states are occupied, further electrons must occupy states with larger momenta. Compressing the material makes this crowding more severe.
The resulting resistance is called electron degeneracy pressure. The name sounds exotic, but the consequence is concrete: cooling does not remove all the pressure that holds the remnant up. The heavy atomic nuclei provide most of the mass, while the much lighter electrons provide most of this pressure. Both belong to the same dense material.
Supporting a star and powering its light are different questions. A cooling white dwarf can remain in mechanical balance while its supply of stored heat gradually runs down. A model of that balance says how the layers fit together. Explaining the brightness and its change with time also requires the flow of energy through them.
Early white-dwarf calculations showed how greater mass could be supported by a smaller radius and higher density. Chandrasekhar improved the description by allowing the density to change from layer to layer. Those calculations naturally led to the next question: as compression forces electrons into faster motion, can their pressure keep increasing quickly enough forever?
Relativity changes the answer at high density
As the electrons become extremely energetic, their motion must obey special relativity. Increasing their momentum can no longer increase their speed beyond the speed of light. Pressure still grows when the material is compressed, but it grows less steeply with density than the simpler, slow-electron calculation predicts. Gravity can then catch up with the extra support supplied by squeezing the star.
In his 1931 paper, Chandrasekhar combined the relativistic electron-pressure law with a layered model of the star. In the limiting high-density form, the equations selected a particular mass, even as the central density changed. This was the crucial result: a more massive ideal white dwarf could not recover equilibrium merely by becoming smaller. The model had reached the end of its stable white-dwarf possibilities.
Readers opening that short original paper will find a value of about 0.91 solar masses. It used a different assumed composition from the carbon–oxygen white dwarfs usually discussed today. The familiar limit is around 1.4 solar masses for the usual idealized composition. The dependence on composition has a physical reason: different numbers of electrons per unit mass change how much pressure is available to support the same gravitational load.
A limit on the remnant changes the story of the star
The calculation gave stellar evolution a new fork. Once ordinary nuclear burning can no longer support a core, its remaining mass matters. A sufficiently light remnant can settle into a white dwarf; a core beyond the electron-supported limit needs a different outcome. Nuclear ignition, the production of a neutron star and gravitational collapse enter through further physics, depending on the core's composition and history. The limiting mass alone does not select one of them.
The number refers to the dense remnant, not simply to a star's mass at birth. Stars lose material, exchange it with companions and change their cores as they evolve. Observations can test the resulting structures: the orbit of Sirius B gives its mass independently of its faint light, and its small measured radius agrees with white-dwarf models. Such comparisons connect equations about unseen interiors to quantities a telescope can measure.
The 1983 Nobel Prize recognized Chandrasekhar's theoretical work on the physical processes governing stellar structure and evolution. William Fowler received the other half for work on the nuclear reactions that form the elements. One asks what reactions transform stellar matter; the other includes how that matter supports itself and whether the arrangement remains stable. Together they helped turn a sequence of stars in the sky into a physical account of changing interiors.