New Investigations of the Spectra of Gaseous Nebulae.

(From Publications of the Lick Observatory, vol. XIII, 1918.)

Despite the very high development of the spectroscopy of stars, and especially of the Sun, our knowledge of the spectra of nebulae has been very limited. For this reason the appearance in 1918 of volume XIII of the Publications of the Lick Observatory (Mt. Hamilton, California), devoted entirely to investigations—among them spectrographic investigations—of gaseous nebulae, constitutes a very important event.

The investigations of W. Wright, with which we shall concern ourselves here, represent a considerable step forward both with respect to the number of nebulae studied and the number of lines measured in them, and also because they bring forward for the first time a number of such spectral features of gaseous nebulae as are of enormous interest from a purely physical point of view.

If one does not count the measurements of the spectra of nebulae by Campbell¹), Lockyer²), and Max Wolf³), which, although relating to a fairly considerable number of lines in the spectra of the brightest representatives of these objects, give only approximate positions of the lines, then we possessed exact data only for two known nebular lines, \(N_1\) and \(N_2\) (\(\lambda\) 4960 and 5007 Å), measured visually by Keeler⁴) and photographically by Hermann⁵), Campbell and Moore⁶), and Wright, and also for a doublet near 3727 Å in the spectrum of the Orion Nebula, measured by the interference method by Fabry, Buisson, and Bourget⁷). Wright’s investigations, which interest us here, cover 50 nebulae and give exact positions of the lines in the spectra of 9 of these objects. Eleven combinations of spectrographs were used in this work, in conjunction with the 36-inch refractor and the 36-inch Crossley reflector of the Lick Observatory. The most interesting results were obtained with the aid of a quartz spectrograph (its first application to nebulae), used both with and without a slit, the latter for the study of the distribution in nebulae of monochromatic radiations.

In the table given below are listed the wavelengths obtained by the investigator on the basis of all the material studied for 9 nebulae (see p. 282).

In addition to the positions, the intensities of the lines were also measured. These are, in general, the first quantitative determinations of this kind for nebulae.

They were based on a calibrated wedge in Hartmann’s microphotometer. Corrections were introduced for selective absorption in the Earth’s atmosphere [with the aid of Fowler’s formula⁸)] and for selective reflection from the mirror of the reflector⁴); unfortunately, however, no rigorous reduction for the chromatic curve of the photographic plate was made.

It is remarkable that, also for the first time, the spectra of the nuclei and gaseous envelopes of planetary nebulae were studied. Further, the distribution of matter in nebulae was studied by means of a slitless quartz spectrograph, and an attempt was made to classify gaseous nebulae.

¹) The intensities of the lines were determined from spectrograms of the quartz spectrograph used on the Crossley reflector.

* Lines observed for the first time in this investigation.
$^{1}$ These are the final values of $\lambda$ for the N$_1$ and N$_2$ lines—means obtained by Campbell and Moore from the results of various investigations.
$^{3}$ The nearest to the most intense line in the second spectrum of hydrogen observed by Dufour. This is the only case of a close coincidence with a line of this spectrum.

Not being able to dwell here on the details of these remarkable investigations, we shall confine ourselves to presenting only their results in the form of brief statements:

1) The continuous spectrum of the nuclei of planetary nebulae (otherwise, their “central stars”) shows a considerable intensity in its ultraviolet part, which compels one to ascribe to the nuclei a very high temperature.

2) Half of the nuclei studied show emission bands characteristic of Wolf-Rayet stars (class O according to the Harvard classification) and must be assigned to stars of this type.

A definite connection is thereby established between planetary nebulae and stars.

3) There is no essential spectral difference between the so-called irregular Orion nebula and planetary nebulae. Since the spectra of nebulae are evidently sufficiently sensitive to changes in the physical conditions within them, the similarity of these spectra compels one to regard as unacceptable the former division of gaseous nebulae into irregular and planetary.

4) Many of the planetary nebulae show, on spectrograms obtained with an objective spectrograph, an extraordinary variety in the forms corresponding to various lines (even of one and the same element). The nature of this phenomenon remains unclear. In part it is due to the different distribution of gases in the nebula; in the case of the lines 4686 Å and some others, it is almost certainly due to local physical conditions. Selective absorption of the lines themselves is also possible.

5) Thus, a continuous spectrum is observed in the gaseous envelopes of planetary nebulae, linearly extending to the limit of the Balmer series and continuing into the region of ultraviolet rays.

This, of course, is a spectrum analogous to that observed by Evershed\(^{9}\) in the solar chromosphere and prominences, corresponding to ultraviolet absorption, first discovered by Huggins\(^{10}\) in the spectrum of Vega (α Lyrae). It is probably closely connected with the Balmer series.

6) The possibility of the presence of carbon and nitrogen in gaseous nebulae has been established.

7) New confirmations have been given of the very close similarity of the spectra of Novae in the so-called nebular*) stage of their development with the spectra of gaseous nebulae.

8) A classification of gaseous nebulae\(^{12}\) has been proposed, based on the relative intensity of the lines with λ 4686 and 3869 Å. Class I includes objects showing 4686 Å in the nebula itself; class II includes those in which 4686 Å is absent in the nebula, but 3869 Å is present; and class III includes those in which both 4686 and 3869 Å are absent. Finer subdivisions are given by letters.

Wright’s attempts to find regular relationships in the lines of nebular spectra are also of interest. Here is a curious little table showing the behavior of the intensity of certain lines near 4686 Å in different nebulae:

*) T. A. Tikhov distinguishes 5 stages in the development of Novae; the nebular stage is the 3rd according to his division. See bibliographic index No. 11.

In the following summary table are given groups of close lines revealing, according to Wright, mutual dependences (placed in vertical columns) in the photometric relation.

One of the most interesting results of Wright’s investigations should be considered the fact that the continuous spectrum in the nuclei of gaseous nebulae proves to be extraordinarily intense in its ultraviolet part and increases in intensity up to the very limit that Wright was able to reach, i.e. approximately to 330 μμ. On the one hand, this effect may be ascribed to the high temperature*) of the nuclei of planetary nebulae, while on the other hand one may suspect that the continuous spectrum exhibited by the gaseous shells surrounding the nuclei may be superposed on the spectrum of the nucleus and, at least in part, produce the observed effect. This latter spectrum, breaking off sharply at the limit of the Balmer series (namely at 3650 Å, whereas the end of the Balmer series of hydrogen is at 3647 Å), is of special interest; apparently it was observed by Dufour³) in the laboratory in his investigations of hydrogen, although this investigator was inclined to connect it not with the Balmer series but with the second spectrum of H. Here one cannot fail to recall the continuous absorption spectrum observed by Wood¹⁴) in studying absorption in sodium vapor, which began precisely at the boundary of the Balmer series of this element and extended far into the ultraviolet end of the spectrum.

It should also be noted that the limit of the Balmer series of H lies very close to one of the $L$ lines in the X-ray spectrum of hydrogen, and that the absorption observed in the spectrum of Sirius (α Canis Majoris) corresponds exactly to the absorption bands found by Barkla and Sadler and by De-Broglie in the $K$ and $L$ groups of lines of X-ray spectra.

From a purely physical point of view, the principle of the spectral classification of gaseous nebulae, based on the relative intensity of certain lines, is of enormous interest. Special attention is attracted by 4686 and 3426 Å. The former is known, since Fowler’s experiments¹⁵), for its sensitivity to conditions of luminosity; the latter shows, to an even greater degree, the peculiarity inherent in the line 4686 Å of becoming localized in definite parts of nebulae, apparently in accordance with the specific conditions of luminosity in these parts.

Literature.

1) Astronomy and Astrophysics 13, 384 and 494, 1894.
2) Phil. Transactions 186 A, 73, 1894.
3) Sitzb. Heidelb. Akad. Wiss. 35 Ab., 1911.
4) Lick Observ. Publikations 3, 165, 1894.
5) Astroph. Journal 15, 291, 1902.
6) Lick Observ. Bulletin 9, 6, 1915.
7) Astroph. Journal 40, 241, 1914.
8) Smitson. Phys Tables, 6-th Ed. 162, 1914.
9) Phil. Trans., 197 A, 399, 1901.

*) It is, however, as the investigator himself notes, absurdly high, about 50,000 C.

10) An Atlas of Representative Stellar Spectra, p. 85.

11) Proceedings of the Petrograd Scientific Institute, vol. IV, 1921.

12) W. Wright. Proceed. Nat. Acad. Sci. Am., 1, 269 and 590, 1915; see also Cannon. Annals of Harvard Coll. Observatory, 76, 1916¹).

13) Ann. Chim et Phys. (8), 9, 361, 1906.

14) Astroph. Journal, 29, 100, 1909.

15) Month. Notices R. Astr. Soc. London, 73, 63, 1912.

P. Davidovich.

¹) This is the first attempt at a classification of the spectra of gaseous nebulae, having only historical interest.