Meghnad Saha

Meghnad Saha was an Indian astrophysicist whose ionization equation connected stellar spectra to the temperature and pressure of stars. He also helped build scientific institutions and public policy in India.

Thermal equilibrium: A state in which interacting systems exchange energy without net change in their macroscopic properties. The equation assumes matter in a star is close to thermal equilibrium.
Saha ionization equation: An equation relating the ionization balance of a gas to its temperature, pressure, and atomic properties. Saha’s defining result predicts the relative populations of successive ionization stages.
University of Calcutta: A public research university in Kolkata, India, founded in 1857. Saha studied and later taught there during the early stages of his scientific career.
Harvard spectral classification: A system classifying stars by the patterns of absorption lines in their spectra. Saha explained why the spectral sequence largely tracks stellar temperature and ionization.
Ionization: The process by which an atom or molecule gains or loses electrons, forming charged particles. Saha quantified how stellar conditions shift atoms between neutral and ionized states.
Partition function: A quantity that sums the statistical weights of a system’s accessible energy states. Atomic partition functions account for excited states in calculations of ionization balance.
Allahabad University: A public university in Prayagraj, India, established in 1887. Saha joined its physics department and developed a major center for theoretical research.
Cecilia Payne-Gaposchkin: An astronomer whose 1925 dissertation established that hydrogen is the most abundant element in stars. She applied Saha’s ionization theory to infer stellar temperatures and chemical composition.
Boltzmann distribution: A probability distribution describing how particles occupy energy states at thermal equilibrium. It supplies the temperature-dependent populations of atomic energy levels in the equation.
Electron pressure: The pressure contributed by free electrons in a gas or plasma. Electron pressure influences how readily atoms remain ionized under stellar conditions.

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