The Development of Research in Interstellar Absorption,c. 1900–1930

Kapteyn J. C. , “On the Absorption of Light in Space”, Astrophysical journal , xxix(1909), 46–54.

We shall use “absorption” to mean extinction as is normally done in astronomical literature. Under absorption of light we shall include scattering as well as true absorption.

For a good account of the historical development of Olbers's paradox, see: Jaki Stanley L. , The paradox of Olbers' paradox (New York, 1969).

Jaki , The paradox of Others' paradox 78.

Ibid. , 89–90.

Ibid. , 136–7.

Barnard E. E. , “The Bruce Photographic Telescope of the Yerkes Observatory”, Astrophysical journal , xxi (1905), 35–48.

Barnard E. E. , “On a Nebulous Groundwork in the Constellation Taurus”, Astrophysical journal , xxv (1907), 218–225.

Barnard E. E. , “On a Great Nebulous Region and On the Question of Absorbing Matter in Space and the Transparency of the Nebulae”, Astrophysical journal , xxxi (1910), 8–14, p. 13.

Barnard E. E. , “On the Dark Markings of the Sky with a Catalogue of 182 Such Objects”, Astrophysical journal , xlix (1919), 1–23.

The dark areas noted by Barnard should not be confused with those reported by Hagen Father J. G. , c. 1920. Hagen's dark areas were spurious visual observations that photographs failed to confirm.

Op. cit. (ref. 7), 46–7.

Several astronomers at the time, notably Ranyard A. C. , held an alternative view. They considered the dark areas to be obscuring matter between us and the Milky Way star field. Barnard admitted that there were cases that tended to support their obscuration hypothesis but he did not think the evidence in their favour was very strong.

This nebula is located in the constellation Taurus at α = 4^h15^m, δ = + 28°·2.

Op. cit. (ref. 8), 221.

Op. cit. (ref. 9), 13.

Luminous, diffuse nebulae are similar to dark nebulae except for the chance occurrence of being close enough to a star to be illuminated by reflected light. The relation between the luminous nebulae and neighbouring stars was first pointed out by Slipher in 1912.

Op. cit. (ref. 10), 1.

Letter, Russell H. N. Shapley H. , 13 March 1919, in the Archives of Harvard University.

Harlow Shapley , “Studies Based on the Colors and Magnitudes in Stellar Clusters”, Astrophysical journal , xlv (1917), 118–41, 164–81; xlvi (1917), 64–75; xlviii (1918), 89–124, 154–81, 279–94; xlix (1919), 24–41, 96–107, 249–65, 311–36; 1 (1919), 42–49, 107–40.

The diagram reproduced is Figure 5 in Shapley Harlow Shapley Betz Martha , “Studies Based on the Colors and Magnitudes in Stellar Clusters. Paper Fourteen: Further Remarks on the Structure of the Galactic System”, Astrophysical journal , I (1919), 107–40, p. 118.

Letter, Russell H. N. Shapley H. , 20 March 1919, in the Archives of Princeton University.

While globular clusters are found at high galactic latitudes, open clusters are found only near the galactic equator.

Letter, Shapley H. Russell H. N. , 18 May 1919, in the Archives of Princeton University.

Letter, Russell H. N. Shapley H. , 9 June 1919, in the Archives of Harvard University.

Curtis Heber D. , “A Study of Absorption Effects in the Spiral Nebulae”, Publications of the Astronomical Society of the Pacific , xxix (1917), 145–46.

Ibid. 146.

Definite proof that spiral nebulae were comparable to our Milky Way galaxy did not come until 1924 when Hubble was able to deduce the distances of M31 and M33 using the period-luminosity relation of Cepheid variables.

Ritchey G. W. , “Novae in Spiral Nebulae”, Publications of the Astronomical Society of the Pacific , xxix (1917), 210–12.

Ritchey G. W. , “Another Faint Nova in the Andromeda Nebula”, ibid. 257.

Curtis H. D. , “New Stars in Spiral Nebulae”, Publications of the Astronomical Society of the Pacific , xxix (1917), 180–2.

Curtis H. D. , “Novae in Spiral Nebulae and the Island Universe Theory”, ibid. , 206–7.

Kapteyn J. C. , “On the Absorption of Light in Space”, Mt Wilson contributions , no. 31 (1909), 3; Astrophysical journal , xxix (1909), 46–54, p. 48.

Ibid. .

Slipher V. M. , “Peculiar Star Spectra Suggestive of Selective Absorption of Light in Space”, Lowell Observatory bulletin , ii (1909), 1–2.

Hartmann J. , “Investigations on the Spectrum and Orbit of δ Orionis”, Astrophysical journal , xix (1904), 268–86.

Ibid. 275.

Loc. cit. (ref. 35).

Ibid. 2.

By selective absorption, Slipher means line absorption, i.e., absorption over a narrow band of the spectrum. We prefer to call this phenomenon “discrete” absorption.

Slipher also predicted the existence of stationary sodium D lines, which were later discovered by Miss Heger at Lick Observatory. Slipher had claimed in recent years that he discovered the interstellar sodium D lines, according to J. S. Hall of Lowell Observatory, although neither Hall nor we have been able to find any confirmatory evidence. It is evident from notes written by Slipher in 1911 that he had been searching for the sodium D lines but without success.

Letter, Kapteyn J. C. Slipher V. M. , 30 October 1909, at Lowell Observatory.

Plaskett J. S. Pearce J. A. , “The Problem of Diffuse Matter in the Galaxy”, Publications of the Dominion Astrophysical Observatory , v (1933), 167–237, p. 169.

Dr George Ellery Hale was one of the most influential persons during the development of modern astronomy. Not only was he important in organizational decisions, but his personal advice was often sought and heeded by individual astronomers.

Letter, Plaskett J. S. Kapteyn J. C. , 31 July 1918, at Dominion Astrophysical Observatory, Victoria.

Letter, Plaskett J. S. Slipher V. M. , 7 December 1922, at Dominion Astrophysical Observatory, Victoria.

Slipher V. M. Plaskett J. S. , 18 December 1922, at Dominion Astrophysical Observatory, Victoria.

Eddington A. S. , “Diffuse Matter in Interstellar Space”, Proceedings of the Royal Society , Ser. A, cxi (1926), 424–56.

The necessary stellar radial velocities were found several years later in a binary stellar system during a study conducted by Pearce J. A. ( Pearce J. A. , “The Coexistence of Stellar and Interstellar Calcium Lines in the Massive B9 Star H.D. 698”, Monthly notices of the Royal Astronomical Society , xcii (1932), 877–85). The binary system consisted of a massive B9 star and a smaller B5 star. The two stars reached a maximum relative velocity difference of about 300 km/sec. The large velocities Doppler-shifted the stellar spectra at maximum displacement to such an extent that the interstellar calcium lines were clearly observable between the stellar calcium line components. The maximum displacement of the interstellar lines from the B9 stellar lines of about 100 km/sec clearly indicated the coexistence of the lines with the spectra of lines later than type B3.

See Oort J. H. , “Observational Evidence Confirming Lindblad's Hypothesis of a Rotation of the Galactic System”, Bulletin of the Astronomical Institutes of the Netherlands , iii (1927), 275–82.

Plaskett J. S. , “The Rotation of the Galaxy”, Monthly notices of the Royal Astronomical Society , lxxxviii (1928), 395–403.

Otto Struve , “Interstellar Calcium”, Astrophysical journal , lxv (1927), 163–99.

Ibid. 198.

Otto Struve , “Further Work on Interstellar Calcium”, Astrophysical journal , lxvii (1928), 353–90.

Plaskett J. S. Pearce J. A. , “The Motions and Distribution of Interstellar Matter”, Monthly notices of the Royal Astronomical Society , xc (1930), 243–68.

Letter, Struve O. Oort J. H. , 29 August 1929, at Leiden Observatory.