Meghnad Saha Sent His Ionisation Paper From Calcutta on 4 March 1920, Gave Astronomers a Temperature Scale for the Stars, and Was Nominated for the Nobel Prize Seven Times Without Winning

The Saha ionisation equation gives the share of atoms in a hot gas that have lost an electron, for a given temperature and pressure. Astronomers use it to turn the dark lines in a star’s light into a surface temperature. Meghnad Saha, then a lecturer at Calcutta University, sent the first of his papers on it to the Philosophical Magazine on 4 March 1920, according to the physicist Arnab Rai Choudhuri of the Indian Institute of Science, who puts Saha’s age at 26.

The Nobel Prize archive lists Saha as a nominee seven times, between 1930 and 1955, and he never won. Two points in his story are disputed: who wrote the equation down first, and why the Nobel Committee passed him over. Each is marked below, with the name of the person making the claim.

A lecturer who never had a research supervisor

Saha was born on 6 October 1893 in the village of Seoratali, near Dacca, the son of a small grocery shop owner, Choudhuri writes. He joined Presidency College in Calcutta in 1911 and took a BSc in 1913 and an MSc in 1915. His classmate Satyendra Nath Bose came first in both examinations, with Saha second.

Banglapedia says Ashutosh Mookerjee appointed him a lecturer at the new College of Science in Calcutta in 1916. In a later letter to the astronomer HH Plaskett, quoted by Choudhuri, Saha said he was given an odd assortment to teach in 1917: thermodynamics, spectroscopy and the figure of the Earth. He had been reading whatever astronomy and physics he could find, including two books by Agnes Clerke on the Sun and the stars. Choudhuri argues that this scattered reading let him join three fields that specialists kept apart: chemical equilibrium theory, atomic physics experiments and stellar spectroscopy.

The puzzle at the edge of the Sun

During a total solar eclipse the Moon covers the Sun’s disk for a few seconds, and the thin gas just above the surface shows its own light. That layer is the chromosphere, and its light is called the flash spectrum. Choudhuri says its bright lines often sat at quite different positions from the dark lines of ordinary sunlight, and nobody could explain why.

Calcium was the clearest case. Ordinary sunlight has a dark line of neutral calcium at 422.7 nanometres and two lines of calcium with one electron missing, the H and K lines, at 396.9 and 393.4 nanometres. In the upper chromosphere the neutral line was absent and the H and K lines were much stronger. Astronomers then thought that layer was cooler than the Sun’s surface, which should mean less ionisation, not more.

Saha treated the stripping of an electron from an atom like a chemical reaction that settles into a balance. He showed that the balance depends on pressure as well as temperature. Pressure falls with height, so the share of ionised calcium rises with height, and in the high chromosphere almost no neutral calcium is left. To put numbers on it he needed the energy that strips an electron from each element, which he took from ionisation potentials measured by Franck and Hertz, MacLennan and others. The paper, “Ionisation in the solar chromosphere”, is in volume 40 of the Philosophical Magazine, from page 472.

From calcium lines to a temperature scale

Harvard astronomers had sorted stars by their spectra into classes O, B, A, F, G, K and M. The open question, in Choudhuri’s account, was whether stars of different colours were made of different things. In “On a physical theory of stellar spectra”, Saha showed that one composition at different temperatures gives different spectra.

Assuming a pressure of one atmosphere, he found calcium completely ionised once, with one electron gone, near 13,000 K. The neutral line is barely seen in the hottest B stars, so he put those stars at about 13,000 K. A second electron comes off near 20,000 K, which fixes where the H and K lines vanish. Working through many lines, Choudhuri writes, Saha mapped the whole spectral sequence onto a temperature scale. He ended the paper by saying the spectra unfold physical processes as temperature varies from 3000 K to 40,000 K.

Saha had gone to England on a scholarship, and between November 1920 and February 1921 he worked in the laboratory of Alfred Fowler at Imperial College, London. The fourth paper was withdrawn from the Philosophical Magazine, rewritten over about four months with Fowler’s criticism, and printed in the Proceedings of the Royal Society in 1921. Choudhuri notes that Saha used 7,500 K for the Sun’s surface, where the true figure is close to 5,800 K.

Others took the theory up quickly. Henry Norris Russell of Princeton extended it to mixtures of gases, and S Rosseland wrote in his 1936 textbook that “The impetus given to astrophysics by Saha’s work can scarcely be over-estimated”.

A prediction was tested within weeks. Saha expected lines of potassium, rubidium and caesium in sunspot spectra, where the gas is cooler, and on 9 July 1921 he asked George Ellery Hale of the Mount Wilson Observatory in California to have someone look. Russell, working at Mount Wilson, answered on 3 August that “your predictions about the lines of alkali metals have been completely verified”. Rubidium was present, he added.

Cecilia Payne’s thesis

Cecilia Payne, a young Englishwoman at Harvard, knew Saha’s work. APS News gives 1 January 1925 as the date she finished her thesis, Stellar Atmospheres, which used his equations on the Harvard spectra. Physics World’s Sidney Perkowitz says she combined newly measured energy levels with Saha’s theory and arrived at temperatures of about 20,000 K for B stars and 3,000 K for M stars.

Her hydrogen result was the surprise. APS News says that in the Sun’s atmosphere at 5,700 K only about one hydrogen atom in 200 million is in the state that produces its Balmer absorption lines, so those lines understate how much hydrogen there is. She concluded that hydrogen and helium dominate the Sun and the stars.

Russell, an outside examiner of the thesis, objected, and the accounts differ on what she did next. APS News says he convinced her to leave the conclusion out. Perkowitz says the final text called the enormous abundance “almost certainly not real”, and that Russell reached the same abundances by another method in 1929.

Who wrote the equation first

Saha credited an earlier writer in his first paper. Choudhuri found that John Eggert, a physical chemist who worked under Walther Nernst in Berlin, wrote the relation in a 1919 paper, and that FA Lindemann wrote it the same year in a Philosophical Magazine note on magnetic storms. Eggert could do the numbers only for hydrogen, having no way to get the energy figure for other elements, and Saha named him as the source of the equation.

Saha did not know of Lindemann’s note until he met him in London in 1921, he wrote to Plaskett. Plaskett replied that physicists at Oxford felt Lindemann never got enough credit, but said he did not share the doubt: “it was you who showed how fruitful this concept was”.

Choudhuri’s conclusion is that Saha was the first to see that the equation, combined with atomic physics data, could explain stellar spectra. He has found no statement in the literature that questions naming the equation after Saha. On Fowler, Saha wrote to Plaskett in 1946 that he was a guest and colleague in Fowler’s laboratory, never a student, and that his first three papers were written in India.

Allahabad, Calcutta and the lab he never had

Saha went on to Nernst’s laboratory in Berlin in February 1921 to test the theory by experiment. Choudhuri says that work produced no published paper, though Saha told Ashutosh Mookerjee in August 1921 that he had shown gases can be ionised by heat alone. He returned to Calcutta as Khaira Professor and moved in October 1923 to Allahabad, Banglapedia says, where he taught until 1938. The furnace he built there still stands in the basement of Allahabad University’s physics department, Choudhuri writes, and it yielded one short paper in 1927 with his students.

The Royal Society elected Saha a Fellow in 1927, only the fourth Indian elected in the early twentieth century, Choudhuri writes, after Ramanujan, Jagadish Chandra Bose and CV Raman. Citing the historian David DeVorkin, he says the Society first made discreet enquiries about Saha’s youthful links with revolutionaries.

Choudhuri also records a quarrel. SN Bose said Saha lacked good experimental skills, which displeased the Palit Professor, CV Raman. In a 1946 letter to Plaskett, Saha denied ever being Raman’s pupil: “I never owed anything to him in life, except persistent ill-will and attempt to harm me whenever possible.” That is Saha’s side only.

Science and Culture, planning and the Damodar

In 1935 Saha founded the Indian Science News Association in Calcutta, and his colleague DM Bose says he used its journal, Science and Culture, from 1935 to 1953 to argue for applying science to national development.

In October 1938 Subhas Chandra Bose, then Congress president, convened a meeting of state industry ministers that set up a National Planning Committee. Jawaharlal Nehru was named chairman, and DM Bose says Saha was a member, chaired its Power and Fuel Sub-Committee and sat on the one for River Transport and Irrigation. Bose adds that Saha fought the Gandhian plan of self-sufficient villages, which he called the “spinning-wheel and bullock-cart cult”.

The Damodar flood of 1943 gave him a specific project. The Damodar Valley Corporation’s own history page says the Bengal government appointed a Damodar Flood Enquiry Committee with the Maharaja of Burdwan and Saha as members. The committee proposed an authority modelled on the Tennessee Valley Authority in the United States and dams with a total capacity of 1.5 million acre feet. WL Voorduin, a senior engineer from that authority, submitted a preliminary memorandum in August 1944, and the corporation came into existence on 7 July 1948 as the first multipurpose river valley project of independent India.

DM Bose says Saha’s series in Science and Culture after the flood proposed that scheme, and that Voorduin was brought in on Saha’s suggestion. The corporation’s page credits BR Ambedkar, then in the pre-independence cabinet, with driving the central government’s side. Bose gives Saha the credit for a blueprint adapting the Tennessee model to India, but thought combining irrigation and hydroelectric power in a coal-rich valley a mistake and wanted an audit.

A seat in Parliament

The Election Commission’s report on the first general election lists “Meghnath Shah” of the Revolutionary Socialist Party as the winner in Calcutta North West, with 74,124 votes (53.05 per cent) against 51,168 (36.62 per cent) for Prabhu Dayal Himatsingka of the Congress, on a turnout of 36.52 per cent. Shyamal Bhadra, writing in The Statesman, says Saha won as an independent.

DM Bose quotes Saha explaining, when he entered Parliament in 1952, that apart from the political movement of his youth “I lived in the Ivory Tower till 1930”. Bose says Saha wrote critical editorials on the first Five-Year Plan, and that Nehru’s decision to put atomic energy under Homi Bhabha must have disappointed him. Saha died on 16 February 1956 in New Delhi, Banglapedia says, during a visit for a Planning Commission meeting.

The Institute of Nuclear Physics

Saha took the Palit Chair in 1938. The Saha Institute of Nuclear Physics says he saw the promise of nuclear science in the fission discovered by Otto Hahn and Fritz Strassmann in 1939, put nuclear physics in the postgraduate syllabus by 1940 and decided to build a small cyclotron, a machine that accelerates charged particles, instead of buying one. DM Bose says Nehru’s mediation brought Rs 60,000 from the Dorabji Tata Trust.

Syamaprasad Mookerjee laid the foundation stone, and the Institute says it was founded in 1949. Irène Joliot-Curie inaugurated the building on 11 January 1950, a date now kept as Foundation Day. DM Bose and Banglapedia instead give 1948 for the foundation stone and 1951 for the opening. The Institute took Saha’s name after his death.

Seven nominations and no prize

The archive lists his nominations as two in Physics 1930 (from Dehendra Bose and Sisir Mitra), then Arthur Compton in 1937, Mitra in 1939, Compton in 1940, Mitra in 1951 and Mitra in 1955. Saha himself nominated Arnold Sommerfeld in 1951.

Singh and Riess, who worked from the Nobel Committee’s papers, give the 1930 detail. Thirty-nine people were invited to nominate and 37 did, with 21 valid recommendations. Raman received 10 and Saha two, both from Calcutta, in a joint letter dated 25 January 1930. The authors say no Western scientist nominated Saha that year, though he had written to Niels Bohr on 30 September 1929 that Compton planned to. The archive shows Compton nominating him in 1937 and 1940, not 1930.

The geophysicist Carlheim-Gyllensköld wrote a report of a little over four pages on Saha’s work. The committee called it very important for astrophysics but, in Singh and Riess’s translation, said it could “hardly be seen as a new physical discovery”, more an application of known data. It judged his work on selective radiation pressure a logical consequence of the ionisation equation. The authors add their own reading: Raman’s effect needed only a mercury lamp and a spectroscope, while Saha’s work called for large telescopes and high-temperature experiments that few laboratories could run. Choudhuri notes that Saha was one of three candidates considered closely in 1930, and that the prize had not gone to an astrophysicist in the early twentieth century until Hans Bethe won it in 1967 for, in the Nobel citation’s words, “discoveries concerning the energy production in stars”.

The archive lists nominators, not reasons, and none of the sources used here reports the committee’s reasoning after 1930. The last nomination for Saha came from Sisir Mitra in 1955.