Meghnad Saha

Meghnad Saha was an Indian astrophysicist who in 1920 formulated the Saha ionization equation, a result that became foundational to modern stellar astrophysics. The equation relates the degree to which atoms in a hot gas are ionized, meaning stripped of one or more electrons, to the temperature and pressure of the gas, combining ideas from thermodynamics, quantum mechanics and statistical mechanics into a single quantitative relationship. Applied to the light emitted by stars, the Saha equation allowed astronomers for the first time to work out the actual surface temperature of a star directly from the pattern of absorption lines in its spectrum, explaining why stars of different spectral classes show very different patterns of spectral lines even when their underlying chemical composition is similar, and it became a central tool used to interpret stellar spectra and classify stars by temperature. The same underlying physics also describes how an ordinary gas becomes a plasma as it is heated, giving the equation broader importance across plasma physics. Beyond this signature contribution, Saha founded and led several Indian scientific institutions and journals, including the influential publication Science and Culture, and later in his career was elected to the Parliament of India, where he continued to advocate for scientific research and education.