Mass of Sirius B

Sirius B carries a full solar mass in a body the size of Earth, and that single juxtaposition founded the physics of compact objects. When its size became clear in the 1910s from its temperature and faintness, Eddington called the implied density — about a tonne per cubic centimetre — "impossible"; quantum mechanics soon said otherwise, with Fowler showing in 1926 that electron degeneracy pressure, the Pauli exclusion principle acting in bulk, holds the star up with no fusion at all. Sirius B is the ash of a star originally five times the Sun's mass that evolved, swelled and shed its envelope while its brilliant companion was still young.

The mass, from the same Hubble-tracked orbit that weighs Sirius A, is the most precise for any white dwarf — and at 1.018 M⊙ it sits usefully close to the 1.44 M⊙ Chandrasekhar limit, the ceiling this library records separately, above which degeneracy loses and a supernova follows.