Why can't a white dwarf exceed the 1.4-solar-mass limit?

A white dwarf cannot exceed the 1.4-solar-mass limit, known as the Chandrasekhar limit, primarily because it is supported against gravitational collapse by electron degeneracy pressure. This pressure arises from the principles of quantum mechanics, specifically the Pauli exclusion principle, which states that no two electrons can occupy the same quantum state simultaneously.

As the mass of a white dwarf increases and approaches this limit, the electrons within the star are forced into higher and higher energy states. When the mass surpasses approximately 1.4 solar masses, the velocities of these electrons approach a significant fraction of the speed of light, leading to a situation where electron degeneracy pressure can no longer counteract the gravitational force pulling inward. At this point, the white dwarf can no longer maintain its structure and may collapse further, potentially leading to the formation of a neutron star or triggering a type Ia supernova.

Understanding this concept clarifies that the behavior of electron degeneracy pressure is fundamentally tied to the relativistic speeds of electrons as the mass of the star increases. This interaction directly defines the limit at which a white dwarf can exist before undergoing catastrophic collapse.