Hubble's Classification of Non-Galactic Nebulae,1922–1926

The proceedings of the debate may be found in the Bulletin of the National Research Council , ii (1921), 171–217. A detailed study of the events surrounding and including the debate is being made by Berendzen R. Hoskin M. A. .

Hubble's Paper , “Cepheids in spiral nebulae”, was read at the 1924 Washington meeting of the American Astronomical Society, and was reprinted in Observatory , xlviii (1925), 139–42. Actually, Hubble made the discovery of a Cepheid in M31 in 1923, and he announced the discovery to Shapley in a letter of 19 February 1924 (Lick Observatory Archives). A discussion of Hubble's classic 1925 paper is given in the Leaflets of the Astronomical Society of the Pacific, June and July 1971, by Berendzen R. Hoskin M. A. .

The details of William Herschel's scheme are described by Curtis H. D. in “Nebulae”, Handbuch der Astrophysik , v (1933), 919.

Wolf M. , “Die Klassifizierung der Kleinen Nebelflecken”, Publikationen des Astrophysikalischen Instituts Koingstuhl-Heidelberg , iii (1909), 109–12.

William Herschel's classification system was revised by his son, John, who introduced five categories, each with five sub-divisions. A nebula was then described by a set of five numbers that corresponded to the characteristics given on a key of categories.

The types of nebulae are indicated by the letters from a to w with j, x, y, z omitted and the symbol h₀ included.

Hubble E. , “Photographic investigations of faint nebulae”, Publications of the Yerkes Observatory , iv (1920), 69.

Hubble E. , “A general study of diffuse galactic nebulae”, Astrophysical journal , lvi (1922), 162.

Ibid. 166.

Curtis H. D. , “The nebulae”, The Adolpho Stahl Lectures in Astronomy (San Francisco, 1919), 98.

Hubble E. , “A general study of diffuse galactic nebulae”, 168.

Ibid. .

More information about the meeting can be found in Observatory , xlv (1922), 176–90; and in the Publications of the Astronomical Society of the Pacific , xxxiv (1922), 275–85.

“The American Section of the International Astronomical Union—Report of the Committee on Nebulae”, submitted in March, 1922. The Members of the American Section were Barnard E. E. Hubble E. Lampland C. O. Slipher V. M. Wright V. H. . (Lowell Observatory Archives).

The report is published in the International Astronomical Union Transactions , i (1922), 91–4.

Bigourdan had published this scheme previously in Comptes rendus de l'Académie des Sciences , clviii (1914), 1949–57.

Letter, Lundmark K. Campbell W. W. , 13 June 1922 (Lick Observatory Archives).

Hubble received his Ph.D. from the University of Chicago in 1917. Background material on Hubble is given by Mayall N. U. , “Edwin Powell Hubble”, National Academy of Sciences Biographical Memoirs , xli (1970), 175–214; also, Mayall N. U. , “Edwin Hubble, observational cosmologist”, Sky and telescope , Jan. 1954, 78–85.

Hubble E. , “A general study of diffuse galactic nebulae”, 167.

“American Section Report”, 5.

Slipher V. M. was president of the Commission on Nebulae for the 1925 IAU meeting. At the 1922 meeting in Rome it was decided that presidents of commissions would collect ideas and suggestions to be discussed at the next meeting; hence Slipher V. M. became the centre for all communication concerning nebulae. From 1916 to 1952, Slipher was Director of the Lowell Observatory.

James Jeans , Problems of cosmology and stellar dynamics (Cambridge, 1919).

Letter, Hubble E. Slipher V. M. , 4 April 1923 (Lowell Observatory Archives).

Ibid. .

Jeffreys H. , “On Jeans' theory of the origin of spiral nebulae”, Monthly notices of the Royal Astronomical Society , lxxxiii (1923), 449–453. The purpose of this paper was to criticize a mathematical point of Jeans's theory. At the end of the paper he admits, however, that, having seen Jeans's reply, the criticism was not valid and the theory as stated was correct.

Letter, Hubble E. Slipher V. M. , 24 July 1923 (Lowell Observatory Archives).

Hubble E. , “The classification of nebulae” (Lowell Observatory Archives). This is the manuscript that Hubble sent to Slipher V. M. in 1923. It was later distributed to the members of the IAU Commission on Nebulae.

Ibid. .

Ibid. .

The eccentricity of elliptical nebulae was to be found by comparing the photographic image with a series of ellipses of eccentricities 0·0, 0·2, 0·4, 0·6, 0·8. The decimal point was dropped and n was expressed as an integer between 0 and 8.

The logarithmic spiral is now called the “normal” spiral.

The barred spiral was the ϕ shaped spiral previously mentioned by Hubble. This shape was first mentioned by Curtis H. D. in Publications of the Lick Observatory , xiii (1918), 12.

Hubble E. , “The classification of nebulae”.

Ibid. .

Letter of 24 July 1923 (see ref. 26 above).

Letter, Hubble E. Slipher V. M. , 9 February 1924 (Lowell Observatory Archives).

Holetschek , Annuales der Wierner Sternwurk , xx (1907), a catalogue of total luminosities for 417 extra-galactic nebulae.

Hubble explained this relation more fully in his 1926 paper (see ref. 40 below):

The distribution of magnitudes appears to be uniform throughout the sequence. For each type or stage in the sequence, the total magnitudes are related to the logarithms of the maximum diameters by the formula.

m_T = C — 5 log d.

where C varies progressively from type to type, indicating a variation in diameter for a given magnitude or vice versa. By applying corrections to C, the nebulae can be reduced to a standard type and then a single formula expresses the relaxation for all nebulae from the Magellanic clouds to the faintest that can be classified…. The coefficient of log d corresponds with the inverse square law, which suggests that the nebulae are all of the same order of absolute luminosity and that apparent magnitudes are measures of distance.

Letter, Hubble E. Slipher V. M. , 22 June 1926 (Lowell Observatory Archives).

Hubble E. “Extra-galactic nebulae”, Astrophysical journal , lxiv (1926), 321–369.

The changes were trivial: n was allowed to take the integral values between 0 and 7 instead of between 0 and 8 as in the 1923 manuscript, and a few of the examples were changed.

The “tuning fork” diagram, which was a slightly revised version of the 1926 ordering, was used by Hubble in The realm of the nebulae (New Haven, 1936), 45. Actually, the “tuning fork” closely resembles the Y diagram that had been used by Jeans in Astronomy and cosmogony (Cambridge, 1928), 324, in which he attempted to show that his theory of evolution for spirals was related to Hubble's observations.

Hubble E. , “Extra-galactic nebulae”, 346.

Ibid. 324.

It was decided at the 1922 Rome meeting that the commission should also consider star clusters.

International Astronomical Union Transactions , ii (1925), 206.

Private communication, 13 April 1970. From its origin until quite recently, Dr Mayall was Director of the Kitt Peak National Observatory.

James Jeans , “An evolving universe”, Carnegie Institution of Washington, News service bulletin, staff edition, no. 23 (1931), 157.

Recent investigations indicate that some peculiar objects that are not yet well understood—like exploding galaxies, Seyfert galaxies, and so on—do not seem to fit the Hubble system. Aside from these few peculiarities, the Hubble classification is today's standard, as has been noted by many recent authors, including: De Vaucouleurs, Handbuch der Physik , liii (1959), 275; Lequeux J. , Structure and evolution of galaxies (New York, 1969), 92; Baade W. , Evolution of stars and galaxies (Cambridge, 1963), 16; Unsold A. , The new cosmos (New York, 1969), 285.

Some important revisions and classifications include: Shapley H. , “On the classification of extra-galactic nebulae”, Harvard bulletin , no. 849 (1927) [it is interesting to note that while in the 1927 paper Shapley severely criticized Hubble's scheme and proposed one of his own, in “Second note on the relative number of spiral and elliptical nebulae”, Harvard bulletin , no. 876 (1930), he used Hubble's morphology in his discussion]; Morgan W. W. , “A preliminary classification of the forms of galaxies according to their stellar population”, Publications of the Astronomical Society of the Pacific , lxx (1958), 394; De Vaucouleurs G. , “Classification and morphology of external galaxies”, Handbuch der Physik , liii (1959), 275–372; Van den Bergh S. , “A preliminary luminosity classification of late-type galaxies”, Astrophysical journal , cxxxi (1960), 215; Sandage A. R. , The Hubble atlas of galaxies (Washington, 1961).

Russell H. N. , “The nebulae”, lecture given at the University of Toronto (Feb. 1924), 260 (from the Russell Collection in the Princeton University Archives).

See, for example, Motz Duveen , Essentials of astronomy (California, 1967), 569; or Walter Baade , Evolution of stars and galaxies (Cambridge, 1963), 12.

Lundmark K. , “A preliminary classification of nebulae”, Arkiv för Mathematik, Astronomi och Fysik , xix B (1926), no. 8. This paper was published several months before Hubble's article of 1926 appeared in print.

Letter, 22 June 1926 (see ref. 39 above).

Hubble E. , “Extra-galactic nebulae”, 323.

Letter, Lundmark K. Campbell W. W. , 28 May 1922 (Lick Observatory Archives).