The Changing Place of Red Giant Stars in the Evolutionary Process

Russell Norris Henry , “On the law of liberation of energy within the stars”, 7 October 1925. Attached to letter Russell Eddington A. S. Jeans J. , 10. Box 104.10, Henry Norris Russell Papers, Princeton University Library (HNR/PUL).

Nielsen Axel V. , “History of the Hertzsprung-Russell Diagram”, Centaurus , ix (1963), 219–52; Sitterly Bancroft W. , “Changing interpretations of the Hertzsprung-Russell Diagram, 1910–1940”, Vistas in astronomy , xii (1970), 357–66; DeVorkin David , “Stellar evolution and the origins of the Hertzsprung-Russell Diagram in early astrophysics”, in Gingerich O. (ed.), Astrophysics and twentieth-century astronomy to 1950 (General history of astronomy , iv A; Cambridge, 1984), 90–108; idem, Henry Norris Russell: Dean of American astronomers (Princeton, 2000); idem , “A. A. Michelson and the problem of stellar diameters”, Journal for the history of astronomy , vi (1975), 1–18.

An earlier version of this paper was written in collaboration with Ralph Kenat in the late 1980s as a review of the literature for the General history of astronomy , ivB, which never appeared. The present paper embodies elements of that review but expands considerably on the topic.

Tassoul Jean-Louis Tassoul Monique , A concise history of solar and stellar physics (Princeton, 2004); Burbidge Margaret E. Burbidge Geoffrey , “Stellar evolution”, in Flügge S. (ed.), Handbuch der Physik 6: Astrophysik II: Sternaufbau (Berlin, 1958), 134–295; Struve Otto Velta Zebergs , Astronomy of the 20^th century (London, 1962); Lang Kenneth Gingerich Owen , A source book in astronomy and astrophysics, 1900–1975 (Cambridge, MA, 1979), chap. 4; Salpeter Edwin , “Energy production in stars”, Vistas in astronomy , i (1955), 283–90; DeVorkin , op. cit. (ref. 2, 1984); Gingerich Owen , “Report on the progress in stellar evolution to 1950”, Astrophysics and space science , cclxvii/1 (1999), 3–21.

Sandage Allan , The Mount Wilson Observatory: Centennial history of the Carnegie Institution of Washington , i (Cambridge, 2004); Osterbrock Donald , The Yerkes Observatory, 1892–1950 (Chicago, 1997), chap. 9.

Meadows A. J. , Science and controversy: A biography of Sir Norman Lockyer (Cambridge, MA, 1972); Osterbrock Donald , Walter Baade: A life in astrophysics (Princeton, 2001), chap. 5; DeVorkin , op. cit. (ref. 2, 2000); Mitton Simon , Conflict in the cosmos: Fred Hoyle's life in science (Washington, DC, 2005), chaps. 6–8; Wali Kameshwar C. , Chandra: A biography of S. Chandrasekhar (Chicago, 1991); Rebsdorf Olling Simon , “The father and the stars: Bengt Strömgren and the history of twentieth-century astronomy in Denmark and in the USA”, Ph.D. diss., University of Aarhus, 2005; Miller Arthur I. , Empire of the stars: Obsession, friendship and betrayal in the quest for black holes (New York, 2005).

The greatest insight into the origin and nature of these controversies resides in the work of Andrew Warwick, Masters of theory: Cambridge and the rise of mathematical physics (Chicago, 2003). See also Stanley Matthew , “So simple a thing as a star: The Eddington-Jeans debate over astrophysical phenomenology”, The British journal for the history of science , xxxix/4 (December 2006, in press).

Hufbauer has explored the importance of robustness for solar physics. See Hufbauer Karl , Exploring the Sun: Solar science since Galileo (Baltimore, 1991), chap. 3. Left unexamined by all of these histories, and left aside for the present, is the question of how evolution became an agenda in astronomy. I am exploring this issue at present.

Russell H. N. , “‘Giant’ and ‘dwarf’ stars”, The observatory , xxxvi (1913), 324–9, p. 328.

Rutherford's comments were made during the discussion following Alfred Fowler, “Spectral classification of stars and the order of stellar evolution”, The observatory , xxxviii (1915), 379–92, p. 390.

Hufbauer Karl , “Astronomers take up the stellar-energy problem, 1917–1920”, Historical studies in the physical sciences , xi (1981), 277–303; idem, “Solutions proposed to the energy problem before 1938”, typescript, American Physical Society, 21 April 1987; idem, “Stellar structure and evolution, 1924–1939”, Journal for the history of astronomy , xxxvii (2006), 203–27.

Hufbauer , op. cit. (ref. 11, 2006).

Russell H. N. Menzel D. H. , 19 December 1927, HNR/PUL. Gingerich , op. cit. (ref. 4).

On this last question, see Allan Sandage Lori M. Lubin VandenBerg Don A. , “The age of the oldest stars in the local galactic disk from Hipparchos parallaxes of G and K subgiants”, Publications of the Astronomical Society of the Pacific , cxv (2003), 1187–206; especially the useful introduction, “The discovery of subgiants (1922–1935)”. I am indebted to Allan Sandage for pointing out the historical significance of the recognition of subgiants (Sandage to author, 30 June 2006).

Eddington A. S. , Report of Commission on the Constitution of the Stars, Transactions of the International Astronomical Union , v (1935), 238.

Eddington A. S. , Report of Commission on the Constitution of the Stars, Transactions of the International Astronomical Union , vi (1938), 291–2. Certainly not known at the time was that the rates, as calculated by the theories of Bethe and von Weizsäcker, were off by a factor of 100. See Schwarzschid Martin Oral History Interview (OHI) #III with Weart Spencer DeVorkin D. , 16 December 1977, 82 (Sources for History of Modern Astrophysics, American Institute of Physics Center for History of Physics (SHMA)).

Milne E. A. , “The analysis of stellar structure”, Monthly notices of the Royal Astronomical Society , xci (1930), 4–55.

Ludwig Biermann OHI, 23 June; 6 July 1978, with Gingerich Owen , 11–14 (SHMA). On mixing length theory see Kippenhahn Rudolf Alfred Weigert , Stellar structure and evolution (New York, 1990), chap. 7.

Cowling T. G. OHI, 22 March 1978, 29–30 (SHMA).

Ibid. , 42–45.

Cowling T. G. , “Note on the fitting of polytropic models in the theory of stellar structure”, Monthly notices of the Royal Astronomical Society , xci (1931), 472–8; idem, “The stability of gaseous stars”, Monthly notices of the Royal Astronomical Society , xciv (1934), 768–82; quote from Cowling , “Convection in stars”, Observatory , lviii (1935), 243–7, p. 247. The full exposition is in Cowling T. G. , “The stability of gaseous stars (Second paper)”, Monthly notices of the Royal Astronomical Society , xcvi (1935), 42–60. See also Cowling T. G. OHI, 22 March 1978 (SHMA), 46–47.

Russell H. N. , “The constitution of the stars”, Journal of the Royal Astronomical Society of Canada , xxvii (1933), 375–426, p. 418.

Cowling OHI , 22 March 1978 (SHMA), 48–49. Strömgren Bengt , “On the interpretation of the Hertzsprung-Russell-Diagram”, Zeitschrift für Astrophysik , vii (1933), 223–48.

Trumpler Robert , “Spectral types in open clusters”, Publications of the Astronomical Society of the Pacific , xxxvii (1925), 307–18, p. 315. See also his “Comparison and classification of star-clusters”, Publications of the Allegheny Observatory of the University of Pittsburgh , vi/4 (Pittsburgh, 1929), 45–74; “On the distances, dimensions and space distribution of open star clusters”, Lick Observatory bulletin , no. 420 (Berkeley, 1930), 154–88.

Strömgren , op. cit. (ref. 23), 247. On Strömgren's life and work, see Rebsdorf , op. cit. (ref. 6); Hufbauer , op. cit. (ref. 11).

Osterbrock , op. cit. (ref. 6).

See Hufbauer , op. cit. (ref. 11).

Kuiper Gerard Otto Struve , 11 February 1936. Kuiper Papers, Box 2, GPK/UA.

Struve Kuiper , 21 March 1936, Kuiper Papers, Box 2, GPK/UA.

Kuiper Gerard , “On the hydrogen content of clusters”, The astrophysical journal , lxxxvi (1937), 176–97, pp. 176–7.

Ibid. 196.

Strömgren Bengt , “On the helium and hydrogen content of the interior of the stars”, The astrophysical journal , lxxxvii (1938), 520–34.

See Hufbauer , op. cit. (ref. 11) for a complete discussion. For an annotated collection of these seminal papers: Lang K. Gingerich O. (eds), A sourcebook in astronomy and astrophysics (Cambridge, MA, 1979), sections 48 and 49. Chandrasekhar S. OHI, 17 May 1977 (SHMA), 76. Bethe Hans A. Critchfield C. L. , “The formation of deuterons by proton combination”, The physical review , liv (1938), 248–54. Bethe Hans , “Energy production in stars”, The physical review , lv (1939), 434–56.

Kuiper Gamow , 10 October 1938, Kuiper Papers, Box 2, GPK/UA.

Chandrasekhar S. Gamow G. Tuve M. , “The problem of stellar energy”, Nature , cxli (1938), 982. Hufbauer Karl has kindly noted that Gamow wrote the report (private communication).

On shell burning, see Gamow G. , “A model with selective thermo-nuclear source”, The astrophysical journal , lxxxvii (1938), 206–8; Critchfield Charles L. Gamow George , “The shell-source stellar model”. The astrophysical journal , lxxxix (1939), 244–54.

Gamow George , “Nuclear reactions in stellar evolution”, Nature , cxliv (issue of 30 September 1939), 575.

Gamow Kuiper , 14 October 1938, Box 2, GPK/UA.

Kuiper Gamow , 27 February 1940, Box 2, GKP/UA. Gamow's English was well known to be idiosyncratic; apparently Kuiper's German was also peculiar, unless he was somehow mocking Gamow here.

Reproduction from: Gamow Kuiper , 4 March 1940, Box 2, GPK/UA. In reply (4 March 1940), Kuiper , correctly interpreting Gamow's allegory, agreed that one of the three “goddesses” had to be smashed, but added that he did not share Teller's “abhorrence of nudism” and “To keep the metaphor one more moment: Some well-dressed ladies might not stand further examination. This does not mean that I doubt Nuclear Physics. But I know to what extent I can trust Astronomical Evidence”. I thank Don Osterbrock Allan Sandage Spencer Weart Teasel Muir-Harmony Kathleen Scorza for many useful and enjoyable discussions of the significance of this cartoon.

Ernst Julius Öpik 1893–1985 , by Bronshten V. A. Pustyl'nik I. B. , ed. by Eremeeva A. I. (Moscow, 2002). Russian passages kindly translated by Rosanne D'Aprile Johnson as part of the Smithsonian's VIARC program. See also Hufbauer , op. cit. (ref. 11).

The Publications de l'Observatoire Astronomique de l'Université de Tartu were regularly sent to major observatories worldwide; the U.S. Naval Observatory's copy of the issue containing Öpik's first paper was received on 28 July 1938 and the second in November 1938. Öpik E. , “Stellar structure, source of energy, and evolution”, Publications de l'Observatoire Astronomique de l'Université de Tartu , xxx/3 (1938), 1–118; “Composite stellar models”, Publications de l'Observatoire Astronomique de l'Université de Tartu , xxx/4 (1938), 1–48.

Gamow George , “Zusammenfassender Bericht: Kernumwandlungen als Energiequelle der Sterne”, Zeitschrift für Astrophysik , xvi (1938), 113–60; Gamow “The energy producing reaction in the Sun”, Astrophysical journal , lxxxix (1939), 130–3; Critchfield Charles Gamow , “The shell source stellar model”, Astrophysical journal , lxxxix (1939), 244–54; Gamow , “Contractive evolution of massive stars”, Astrophysical journal , xcviii (1943), 498.

Chandrasekhar OHI , 17 May 1977, 88 (SHMA).

Chandrasekhar S. Henrich L. R. , “Stellar models with isothermal cores”, Astrophysical journal , xciv (1941), 525–36; Chandrasekhar S. Schönberg M. , “On the evolution of the main-sequence stars”, Astrophysical journal , xcvi (1942), 161–72, p. 172. Chandrasekhar recalls in his oral history that he was indeed aware of the possibility of red giant evolution as a consequence of this work, and regrets not pursuing it at the time. Chandrasekhar OHI , 17 May 1977, 87–88 (SHMA).

Gamow George , “The evolution of contracting stars”, The physical review , lxv (1944), 20–32, p. 27. The first sentence was printed in italics in the original.

Gamow played this up in a popular article, see Gamow George , “Our Sun is bound to explode”, Popular astronomy , xlix (1941), 360–5. See also Gamow George , “Contractive evolution of massive stars” in: Notes, Astrophysical journal , xcviii (1943), 498. In his autobiography Gamow explained that “Urca” was the name of a famous casino in Rio de Janeiro where he first met Mario Schönberg although he was prepared to say it referred to an “unrecordable cooling agent” if pressEd. Gamow George , My world line (New York, 1970), 136–7.

Gamow Adams , 8 March 1944, Box 25, Adams Papers, Huntington Library (WSA/HL).

Ibid. .

Ibid. .

Adams Gamow , 6 April 1944, Box 25 (WSA/HL).

Gamow George , “The red-giant stage of stellar evolution”, The physical review , lxvii (1945), 120–1. Gamow George Keller Geoffrey , “A shell source model for red giant stars”, Reviews of modern physics , xvii (1945), 125–37.

Baade Walter , “The resolution of Messier 32, NGC 205, and the central region of the Andromeda Nebula”, Astrophysical journal , c (1944), 137–46.

Gamow , op. cit. (ref. 47), 148–9.

Chandrasekhar S. , “The structure, the composition, and the source of energy of the stars”, in Hynek J. A. (ed.), Astrophysics: A topical symposium (New York, 1951), 598–674, p. 660.

Ibid. .

Gamow Critchfield , 6 February 1948, Gamow Papers, Library of Congress (GG/LC).

Schwarzschild OHI , 20 April 1983 (rough draft) 18–19 (SHMA). See also Schwarzschild OHI . #III, 16 December 1977, 87–88 (SHMA).

See Schwarzschild M. , “On the helium content of the Sun”, Astrophysical journal , civ (1946), 203–7. Keller completed his Ph.D. at Columbia in 1948 with: “On the physical and chemical composition of the Sun”, Astrophysical journal , cviii (1948), 347–53.

Struve Otto , Stellar evolution (Princeton, 1950), pp. ix–x.

Struve Greenstein , 13 November 1950, Jesse Greenstein Papers, California Institute of Technology Archives (JG/CITA).

Hoyle F. Lyttleton R. A. , “On the internal constitution of the stars”, Monthly notices of the Royal Astronomical Society , cii (1942), 177–93, 215–25; idem , “The structure of stars of non-uniform composition”, cix (1949), 614–30.

For a further exploration of the accretion mechanism in the hands of Hoyle and Lyttleton, see Mitton Simon , op. cit. (ref. 6), 154–8.

Greenstein Jesse L. , “Magnitudes and colors in the globular cluster Messier 4”, Astrophysical journal , xc (1939), 387–413; see Fig. 1 and the discussion on 397.

Baade W. , op. cit. (ref. 53), 137–46; idem, “NGC 147 and NGC 185, two new members of the Local Group of Galaxies”, Astrophysical journal , c (1944), 147–50. See also Osterbrock , op. cit. (ref. 6), chap. 5.

Osterbrock , op. cit. (ref. 6), 136.

Baade Whipple , 2 July 1947, Walter Baade Papers, Huntington Library (WB/HL).

Osterbrock Don to the author, 28 June 2006.

Sandage Allan to the author, 30 June 2006.

Baade Walter , “Galaxies—present day problems”, 11 January 1951. Draft of talk given at Michigan, WB/HL. Osterbrock, op. cit. (ref. 6), 137.

Baade , op. cit. (ref. 70), 7.

Osterbrock , op. cit. (ref. 6), 137.

DeVorkin , op. cit. (ref. 2, 2000), chap. 8. Roman Nancy G. , “A correlation between the spectroscopic and dynamical characteristics of the late F- and early G- type stars”, Astrophysical journal , cxii (1950), 554–8. See commentary by Rubin Vera , “Roman's correlation between spectra and motions of intermediate-type stars”, Astrophysical journal , dxxv (centennial issue, 1999), 401.

Schwarzschild M. Spitzer L. Wildt R. , “On the difference in chemical composition between high- and low-velocity stars”, Astrophysical journal , cxiv (1951), 398–406.

Gamow George , “The red-giant stage of stellar evolution”, Physical review , lxvii/3-4 (1945), 120–1, p. 121. He continued this interpretation in “The origin of elements and the separation of galaxies”, Physical review , lxxiv/4 (1948), 505–6.

In the late 1930s V. Ambartsumian and Spitzer called attention to the existence of rapidly expanding stellar associations, and by the mid-1940s, observational evidence confirmed this. “Reports on the progress of astronomy”, Monthly notices of the Royal Astronomical Society , cv (1945), 124–8, pp. 126–7. By 1948, there was little doubt in Otto Struve's mind that associations were extremely young. Struve Otto , “Reviews”, Astrophysical journal , cix (1949), 80–81.

Eddington A. S. , op. cit. (ref. 15), 238.

Cowling OHI , 22 March 1978, 32–35 (SHMA). Only through an intercession by Sydney Chapman did Cowling again gain Milne's confidence, and only after, following Chapman's diplomatic advice, Cowling learned to acknowledge his debt to Milne more prominently, and emphasize their “points of agreement” rather than their differences. On Öpik's critical edge, see Schwarzschild OHI , 20 April 1983, 12–15 (SHMA).

Schwarzschild oral history interview with Aspray William , 18 November 1986, courtesy Charles Babbage Institute. For instance, Schwarzschild, with Lillian Feinstein, developed special integration methods that could be applied to the IBM punched card machines to compute overtone pulsations. Schwarzschild Martin , “Overtone pulsations for the standard model”, Astrophysical journal , xciv (1941), 245–52; Schwarzschild Feinstein L. , “Automatic integration of linear second-order differential equations by means of punched- card machines”, Review of scientific instruments , xii (1941), 405–8. The former paper garnered 33 citations from authors publishing between 1965 and 1975 (Science Citation Index compilation).

Kidwell Aldrich Peggy , “American scientists and calculating machines: From novelty to commonplace”, Annals of the history of computing , xii (1990), 31–40.

DeVorkin David , Henry Norris Russell (Princeton, 2000), 246.

Schwarzschild OHI , 18 June 1982, rough draft, 21 (SHMA).

Ibid. 20. In November 1936 Milne expressed his frustration at being compelled to confine his work to analytical techniques, feeling that American theorists, not so hampered, were free to use numerical techniques that were bound to prove more powerful and productive, even if less elegant, in the long run. See Milne Russell H. N. , 27 November 1936, and other letters in May 1936, HNR/PUL.

Ibid. 20. What is not fully appreciated at this point but is worthy of future investigation is the application of numerical techniques in the era before reliable high-speed computers. How astronomers initially but slowly converted to hand desk calculators in the 1930s is explored in Kidwell , op. cit. (ref. 80); and how later in the 1950s astronomers resisted high-speed computers is noted in Wrubel Marshal H. , “The electronic computer as an astronomical instrument”, Vistas in astronomy iii (1959), 107–16. In an unpublished study I found useful material covering the issue of computation in astronomy in oral histories such as those cited here by Schwarzschild as well as Paul Herget Dorrit Hoffleit and others held by the American Institute of Physics and at the Babbage Institute, University of Minnesota, as well as various secondary sources on the history of electronic computing.

During the war Schwarzschild participated in high-speed computing at Aberdeen, where he was part of a large team developing real-time ballistics calculations using the ENIAC. Schwarzschild OHI , 18 June 1982, 27 (National Air and Space Museum).

Schwarzschild OHI . #IV, 16 December 1977, with Spencer Weart, 148 (SHMA).

Richardson Robert S. Schwarzschild Martin , “A stellar model for red giants of high central temperature”, Astrophysical journal , cviii (1948), 373–87.

Hen Li Schwarzschild Martin , “Red-giant models with chemical inhomogeneities”, Monthly notices of the Royal Astronomical Society , cix (1949), 638–46.

Ibid. 646.

Gold Thomas soon argued that Schwarzschild's acoustical energy transport model violated the second law of thermodynamics, and he also criticized his work with Li Hen on the basis that their calculations, while numerically correct, did not reproduce the observed distribution of red giants on the HR Diagram. Gold Tom , “Thermodynamic consideration in relation to acoustic energy in stellar models”, Monthly notices of the Royal Astronomical Society , cix (1949), 115–16. See Schwarzschild OHI . #III, 16 December 1977, 98 (SHMA).

Morgan W. W. OHI, 8 August 1978, 60 (SHMA).

Schwarzschild Spitzer Wildt , op. cit. (ref. 70), 398–406. Schwarzschild also interprets this work as highly collaborative: Spitzer was expert in atomic physics and in the physics of the interstellar medium, and Wildt was a seasoned student of molecular physics and chemistry. Schwarzschild OHI . #III, 16 December 1977, 103 (SHMA).

Baade W. , “Basic facts of stellar evolution”, Transactions of the International Astronomical Union , viii (1953), 682–9. See the discussion, p. 689.

Bowen Ira S. , “Survey of the year's work…”, Publications of the Astronomical Society of the Pacific , lxiii (1950), 5–16, p. 15; Baum Williams OHI , 4 January 1997, 2–3 (National Air and Space Museum); Baade Walter , “A program of extragalactic research for the 200-inch Hale Telescope”, Publications of the Astronomical Society of the Pacific , lx (1948), 230–4; see especially p. 233.

Sandage to the author, 30 June 2006. I am indebted to Dr Sandage for his insight into Baade's 1948 paper.

Johnson Harold L. Schwarzschild Martin , “On the color-magnitude diagram for M 15”, Astrophysical journal , cxiii (1951), 630–6. Schwarzschild OHI . #III, 16 December 1977, 126 (SHMA).

The term appears in the early 1950s, although the concept of classes of giants dates back to 1930 and the work of Gustaf Strömberg at Mount Wilson. According to entries scanned by the Astronomy Data Service, the first mention was by Sitterly Bancroft , “The eclipsing variable RS Canum Venaticorum”, Contributions of Princeton University Observatory , xi (1930), 21–41. A more satisfactory account is given in Sandage , op. cit. (ref. 14). See also Strömberg G. , “The distribution of absolute magnitudes among M Stars brighter than the sixth apparent magnitude as determined from parallactic and peculiar velocities”, Astrophysical journal , lxxii (1930), 117–26.

Schwarzschild OHI . #III, 16 December 1977, 116 (SHMA). See also Sandage , op. cit. (ref. 5); Gingerich , op. cit. (ref. 4), 19.

Sandage , op. cit. (ref. 5), 548.

Schwarzschild OHI . #III, 16 December 1977, 116 (SHMA). See also Sandage , op. cit. (ref. 5).

Sandage , op. cit. (ref. 5), 547.

The term ‘zero-age’ appears first in 1956 according to a search of the literature using the Astronomy Data Service. Merle Walker qualified the term with a definition. Walker Merle F. , “Studies of extremely young clusters I: NGC 2264”, Astrophysical journal supplement , ii (1956), 365; Spitzer Lyman , “Reports of observatories”, The astronomical journal , lxi (1956), 343.

Schwarzschild OHI . #IV, 19 July 1979, 91–93 (SHMA).

Schwarzschild OHI . #III, 16 December 1977, 116 (SHMA). Oke J. B. Schwarzschild M. , “Inhomogeneous stellar models. I: Models with a convective core and a discontinuity in the chemical composition”, Astrophysical journal , cxvi (1952), 317–30.

Oke Schwarzschild , op. cit. 327.

Schwarzschild OHI . #IV, 19 July 1979, 127 (SHMA).

Sandage A. R. Schwarzschild M. , “Inhomogeneous stellar models. II: Models with exhausted cores in gravitational contraction”, Astrophysical journal , cxvi (1952), 463–76.

Commentary on Salpeter's development of the triple-alpha process, especially upon being “at the right place at the right time” is given in André Maeder, “Salpeter's nuclear reactions in stars without hydrogen”, Astrophysical journal , dxxv (centennial volume, 1999), 453–4, which follows a reprint of Salpeter's paper, Salpeter E. E. , “Nuclear reactions in stars without hydrogen”, Astrophysical journal , cxv (1952), 326–8. See also Lang Gingerich , op. cit. (ref. 4), 349–52; and Salpeter , op. cit. (ref. 4), 283–90.

Unrecorded conversation with Salpeter Cornell , 29 September 1983. Salpeter also recalls that the 1953 Michigan symposium in astrophysics was pivotal in bringing him to a refined appreciation of the role of helium fusion in explaining red giant luminosities, after he brought out his first arguments in 1952. Salpeter Edwin , “Nuclear reactions in stars without hydrogen”, Astrophysical journal , cxv (1952), 326–8.

Öpik James Small , 6 January 1951, letter published in “Correspondence: Stellar evolution”, Irish astronomical journal , i (1951), 195–6.

Mitton Simon supplies valuable perspective on how questions remaining from Salpeter's work were resolved by Hoyle: Mitton , op. cit. (ref. 6), 219–23.

Ibid. , 162–4.

Hoyle F. Schwarzschild M. , “On the evolution of Type II stars”, The astrophysical journal supplement series , ii/13 (June 1955), 1–40.

Schwarzschild OHI . #IV, 19 July 1979, 120–2 (SHMA).

Schwarzschild OHI . #IV, 19 July 1979, 121–2 (SHMA).

Schwarzschild was sitting in the audience with Rosseland Svein , and asked if he was going to second Eddington, but Rosseland “shook his head with absolute determination”. Schwarzschild OHI . #IV, 19 July 1979, 111 (SHMA).

Chandrasekhar S. , “Commission de la Constitution des Étoiles”, in Oosterhoff T. H. (ed.), Transactions of the International Astronomical Union , viii (1952), 532–46.

Schwarzschild Greenstein , 16 April 1951, JG/CITA.

Greenstein Salpeter , 6 March 1952, Box 6, JG/CITA.

Salpeter Edwin , “Energy production in stars”, Annual review of nuclear science , ii (1953), 41–62, pp. 54, 56.

Salpeter Edwin , “Energy production in stars”, Vistas in astronomy , i (1955), 283–90, p. 288.

Oke J. B. Greenstein J. , “The rotational velocities of A-, F-, and G-type giant stars”, Astrophysical journal , cxx (1954), 384–90.

Schwarzschild Greenstein , 3 June 1954, Box 6 SIII folder, JG/CIT.

Ibid. .

Greenstein Schwarzschild , 11 June 1954, Box 6 SIII folder, JG/CIT.

Letters, Salpeter to/from Greenstein , c. spring 1953, Box 6 SIII folder, JG/CIT.

Salpeter , e-mail notes to the author, July 2006.

Struve Evry Schatzman (with copies to Greenstein and others), 15 March 1952, Box 6, JGP/CITA.

Chandrasekhar OHI , 17 May 1977, 80 (SHMA).