Astrophysics Contributions of Indian Scientists INTRODUCTION

Defence Science Journal, Vol 44, No 3. July 1994, pp 207-213 @ 1994, DESIDOC Astrophysics Contributions of Indian Scientists M.S. Vardya Theoretical Astrophysics Group, Tata Institute of Fundamenta[ Research, Bombay-400 005 ABSTRACT A glimpse of astronomy and astrophysics from the earliest times to the end of the nineteenth century is given. This is followed by important contributions to astrophysics in the twentieth century with detailed accounts of the work of MN Saha, OS Kothari and a few others. This review ends with a brief account of the astrophysical activities in which a large number of scientists are presently involved. INTRODUCTION the radial motion of solar gases in the vicinity of Astronomy had an hoary past in India with the oldest sunspots. astronomical text Vedanga Jyotisha dated around However, it was the theory of thermal ionization 1400 BC1. The contribution to mathematical or and its application to the understanding of the stellar Siddhantic astronomy, due to the interaction with spectra by Megh Nad Saha5,6 (1893-1956) in 1920 and Greece in the post-Alexander period, made an impact 1921 which ushered India not only to the forefront of owing to the work of Aryabhata I, Bhaskara I, modern astrophysics, but also left a stamp for all times Bramagupta, Aryabhata II, Bhaskara II, and others, to come. Important contributions of MN Saha, OS starting at the end of the fifth century and going up to Kothari, S Chandrasekhar and a few others are the beginning of the twelfth century .The big event after discussed here, and a brief account of the present-day thi~ was the five masonry observatories built by astrophy~ical activities is given. Raja Jai Singh Sawai in the first half of the eighteenth century; however, ..~ose observatories remained 2. MODI::RN ASTRO~HYSICS architectural show pieces as the invention of telescope Megh Nad Saha was a multifaceted personality who had already taken place in Europe by then. The contributed not only to theoretical astrophysics but also astronomical activities once again started at the end of to atomic and molecular spectroscopy, nuc.l~ar physics, the nineteenth century , when the Europeans, mostly propagation of radio waves, age of rocks, Indian the British, c~me to India for observing the solar eclipses calendar reform, water management and national of 1&68, 1&11 and 1Bn~ thi~ ~tt th~ b'Qll r )lling fQ!" planning 4~ well. Thollgh bc did hij MSc in ilpplib9 astrophysical activities in India, culminating in the mathematics, he was inspir~d to astrophysical problems setting up of the Kodaikanal Observatory in 1899 (this after he came across two popular books on the Sun and has now become the Indian Institute of Astrophysics the stars by Agnes M Clarke. with its headquarters at Bangalore) .This resulted in some important discoveries from India like the Saha's7 first work on astrophysics, a short note on discovery of the helium line during the total eclipse of 'Radiation pressure and quantum theory-a preliminary 1868 by the British and the French scientific team2, the work' which dealt with the formulation of the concept discovery of F-corona by Janssen3 and the discovery of of selective rariiation pressure was very significant the Evershed4 effect by John Evershed when he found though it has not got the credit it deserves; the fact that -~ ed 7 Februar 1994 207 DEF 5( VOL 44, NO 3, JUL y 1994 the detailed papers on the subject was published in an rather than a Boltzmann distribution. It is used not only obscure journal may be the only cause. The main in astrophysical applications but in other fields as well. conclusion of this paper, in his own words, was "that Using this equation, Saha was not only able to explain radiation-pressure may exert an effect on the atoms and many aspects of the solar spectrum, but also the varied molecules which are out of all proportion to their actual nature of stellar spectra and give physical interpretation sizes. It also shows that the radiation-pressure exerts a to the spectral sequence. set of sifting actions on the molecules, driving the active The Henry Draper (HD) system of spectral ones radially outward along the direction of the beam. classification 10introduced by Pickering, Miss Maury and The cumulative effect of the pulses may be sufficiently Miss Cannon was simply an arrangement of spectra in great to endow the atoms with a large velocity-the order of decreasing ratio of intensities of the Balmer velocity with which the tops of solar prominences are lines of hydrogen to intensities of a number of other observed to shoot up". Olin C Wilson9 tried this idea lines. More than 99 per cent of stars fell into a small to explain m~ss loss from cool stars, and this idea is number of groups: 0, B, A, F, G, K, M; of the small very much there in explaining the mass loss from early minority, P, R and N, p refers to spectra of gaseous type stars as we)l. nebula and types R and N overlap with K and M. Some The theory of thermal ionization was proposed by stars had a small c suffix, which was found to be for Saha in an innocuously titled paper 'Ionization in the stars of higher lumminosity or lower density. Saha looked solar chromosphere'5. This was elaborated in two other into the appearance and disappearance of various lines papers, and the fourth paper6 dealt with application to at different temperatures and pressures, and explained the interpretation of stellar spectra. All these four the sequence. Milne extended it by considering where papers were prepared between February and September the peak or maximum of lines occurs rather than the 1919 and communicated from India for publication in marginal appearance/disappearance which Saha Philosophical Magazine between August and adopted; Milne's method is to be preferred for broad September; however, the fourth paper, on the advice classification but Saha's approach is better for further of Professor A Fowler whose laboratory MN Saha was subclassification of spectra. Saha would have further visiting, was withdrawn, completely revised, updated carried his theory of thermal ionization but for the lack using the latest laboratory spectroscopic data and of observational stellar spectra at his disposal, whereas published in the Proceedings of the Royal Society. Henry Norris Russell (1877-1957), the doyen of In trying to understand the solar spectra, he used American astrophysicists, fully realized the impact of the law of mass action or chemical equilibrium, though the equation and exploited it by his regular pilgrimage Saha called it Nerst's formula of 'Reaction-isobar' and to the Mount Wilson Observatory. extended it to ionization in place of chemical Note that though Saha was a theoretician, he kept dissociation. If x is the degree of ionization of a given his work close to observations and experimental results species at temperature T and total pressure P, then and whenever he felt the need, he persuaded others, including some of his students, to 'conduct the x2 U 5 log K = log p = + -log T- 6.5 experiment, In this connection, he proposed)) at the l-x2 4.571T 2 Harvard College Observatory that a stratosphere solar observatory be set up 0to observe the Sun in cthe where K is the chemical equilibrium constant or ultra-violet below 2900 A, This unique proposal was 'Reaction-isobar' and U, the ionization potential or heat more than twenty years ahead of time. of ionization (in place of heat of dissociation) in calories. Saha 12was the first to think of some of the observed The important thing to note is that the degree of interstellar lines being molecular in origin. ionization depends not only on temperature but on pressure as well. The above equation, as given by Saha, Though H C van de Hulstl3 in the Netherlands was has been extended to include the effect of excited states the first to predict the 21 cm line of atomic hydrogen of atoms and ions, to multiple ionization, to a gaseous arising from the hyperfine structure in 1944, Saha14 mixture undergoing ionization, inclusion of interparticle independently predicted it. The line was observed by interaction and treating electron having a Fermi-Dirac three groups in 1951. 208 v ARDY A : ASTROPHYSICS INDIAN CONTRIBUTIONS Though Saha contributed to the growth of A greal deal of workl9.has been done but the last astrophysics in various ways, his equation of thermal conclusion has defied any satisfactory answer so far . ionization has become an indispensable part of physics In a study (jointly with BN Singh20) of the and astrophysics. Bose-Einstein .statistics and degeneracy, Kothari Daulat Singh Kothari (1906-1993), a student of considered non-equilibrium radiation with spectrum Professor MN Saha at Allahabad University and of proportional to r dx 1[exp (x+p)-l] and called it Professor RH Fowler at Cambridge University, made non-degenerate radiation; they conjectured that many important contributions to the development of non-degenerate radiation may be partly responsible for science and education in India, besides making many the departures from black-body radiation in notable contributions to astrophysics. DS Kothari along astrophysical studies. Twenty-five years later, Wildt21 with RC Mazumdar15 computed opacity coefficients for found a similar distribution while studying the electron degenerate matter using a rigorous quantum thermodynamics of gray atmosphere. mechanical treatment and showed161aterthat the energy Kothari had looked into the problem of flow in the degenerate stellar cores .is mainly by the fragmentation in 1 and 3 dimensions, along with his thermal conduction and not by the radiative process.

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