Meghnad Saha: Master of stellar spectra

MEGHNAND SAHA was born on October 6, 1893 in Seoratali (now in Bangladesh). His father, a small shopkeeper, could not afford to send him to a primary school. He studied at the Government Collegiate School where he received both a free studentship and a stipend. Saha lost this educational support, when he participated in the protest against partition of Bengal. He then attended a private school and came to Calcutta in 1911 to join the Presidency College. Saha earned by giving tutions to children. A major turning point occurred when he called on Sir Ashutosh Mukherjee along with Satyen Bose. They were given a monthly scholarship of Rs.125.

In 1916, Saha started his career in the mathematics department — subsequently transferred to the physics department, in the newly-founded University College of Science.
He published his first paper (1917) in the Philosophical Magazine; several more on light, solar spectrum and quantum theory followed in the next two years. For these contributions he was awarded by the Calcutta University the D.Sc. degree in 1918.

In 1814 Fraunhofer had discovered a large number of dark lines in the spectrum of sunlight, which was proved by Kirchoff in 1859 to represent definite chemical elements. Later it was found that there were bright as well as dark lines, which exceeded the number of known elements. This created confusion, that was swept away by Saha's discovery. He developed the theory in his epoch- making paper: On Ionisation in the Solar Chromosphere, (Philosophical Magazine, 1920).
He came across a paper by J.Eggert who applied the heat theorem of his teacher Nernst (Nobel Laureate 1923) to explain high ionisation in stars due to high temperatures.''
Saha at once saw the error: Eggert's formula missed the significance of the ionisation potential of the atoms. He showed that the degree of ionisation depends on the pressure (P), temperature (T) and the ionisation potential of a gas. So the same gas would produce different spectral lines.
Saha took the idea from chemistry and enunciated his famous equation
Log10(x2/1-x2)P = U/4.517T+2.5log10T-6.5

where x is the fraction of ionised atoms and U is the ionisation energy for the gram atom. (G.Venkataraman: Saha and his formula : Universities Press, Hyderabad, 1995).

Saha set sail for Europe (September 1919) to A.Fowler's laboratory. The prize-winning essay of Saha rewritten under Fowler's guidance was published in Proceedings of the Royal Society, 1921 and spurred developments in astrophysics.
Alongside, Saha wanted to verify experimentally his theory of thermal ionisation, for which high temperature facilities were not available in Imperial College. On Fowler's advice, he wrote to Nernst . Saha spent a year (1920) in Nernst's laboratory, which proved fruitful. In a long paper sent in 1919 to the prestigious Astrophysical Journal ., Saha showed that what encountered solar gravity was selective radiation pressure. This met with a sad fate. He communicated the contents of this paper as a note to Nature, which E.A.Milne exploited and developed a theory which has since come to be known as ``Milne's theory''.

Saha had in fact many disappointments in respect of recognition of his work. But his ionisation formula cleared up many mysteries about the Sun and stars as well. His theory may indeed be considered to be the beginning of modern astrophysics.
Saha's career was spent in various capacities in scientific research and public life . He was active in research until his death on February 14, 1956. .