CLASSIFICATION OF STELLAR SPECTRA.
When the spectra of thousands of stars were recorded for study, such a variety was found that some system of classification was necessary. The commencement of such a system was made by Secchi in 1863. It was based on the observed relation between the color of a star and the general character of its spectrum.
Arranging the stars in a regular series, from blue in tint through white to red, it was found that the number and character of the spectral lines varied in a corresponding way. The blue stars, like Sirius, Vega and α Aquilæ, though they had the F lines strong, as well as the two violet lines H, had otherwise only extremely fine lines. On the other hand, the red stars, like α Orionis and α Scorpii, show spectra with several broad bands. Secchi was thus led to recognize three types of spectra, as follows:
The first type is that of the white or slightly blue stars, like Sirius, Vega, Altair, Rigel, etc. The typical spectrum of these stars shows all seven spectral colors, interrupted by four strong, dark lines, one in the red, one in the bluish green, and the two others in the violet. All four of these lines belong to hydrogen. Their marked peculiarity is their breadth, which tends to show that the absorbing layer is of considerable thickness or is subjected to a great pressure. Besides these broad rays, fine metallic rays are found in the brighter stars of this type. Secchi considers that this is the most numerous type of all, half the stars which he studied belonging to it.
Fig. 1. Spectrum of Sirius.
Fig. 2. Spectra of α Aurigæ and Sun.
Fig. 3. Spectra of α Bootis and β Geminorum.
The second type is that of the somewhat yellow stars, like Capella, Pollux, Arcturus, Procyon, etc. The most striking feature of the spectrum of these stars is its resemblance to that of our sun. Like the latter, it is crossed by very fine and close black rays. It would seem that the more the star inclines toward red, the broader these rays become and the easier it is to distinguish them. We give a figure showing the remarkable agreement between the spectrum of Capella, which may be taken as an example of the type, and that of the sun.
The spectra of the third type, belonging mostly to the red stars, are composed of a double system of nebulous bands and dark lines. The latter are fundamentally the same as in the second type, the broad nebulous bands being an addition to the spectrum. α Herculis may be taken as an example of this type.
Fig. 7. Spectrum with both Bright and Dark Lines.
It is to be remarked that, in these progressive types, the brilliancy of the more refrangible end of the spectrum continually diminishes relatively to that of the red end. To this is due the gradations of color in the stars.
To these three types Secchi subsequently added a fourth, given by comparatively few stars of a deep red color. The spectra of this class consist principally of three bright bands, which are separated by dark intervals. The brightest is in the green; a very faint one is in the blue; the third is in the yellow and red, and is divided up into a number of others.
To these types a fifth was subsequently added by Wolf and Rayet, of the Paris Observatory. The spectra of this class show a singular mixture of bright lines and dark bands, as if three different spectra were combined, one continuous, one an absorption spectrum, and one an emission spectrum from glowing gas. Less than a hundred stars of this type have been discovered. A very remarkable peculiarity, which we shall discuss hereafter, is that they are nearly all situated very near the central line of the Milky Way.
Fig. 4. Spectra of α Cygni and α Tauri.
Fig. 5. Spectrum of α Orionis.
Fig. 6. Spectrum of γ Cassiopeiæ
Vogel proposed a modification of Secchi’s classification, by subdividing each of his three types into two or three others, and including the Wolf-Rayet stars under the second type. His definitions are as follows:
Type I is distinguished by the intensity of the light in the more refrangible end of the spectrum, the blue and violet. The type may be divided into three subdivisions, designated a, b and c:
In Ia the metallic lines are very faint, while the hydrogen lines are distinguished by their breadth and strength.
In Ib the hydrogen lines are wanting.
In Ic the lines of hydrogen and helium both show as bright lines. Stars showing this spectrum are now known as helium stars.
According to Vogel, the spectra of type II are distinguished by having the metallic lines well-marked and the more refrangible end of the spectrum much fainter than in the case of type I. He recognizes two subdivisions:
In IIa the metallic lines are very numerous, especially in the yellow and green. The hydrogen lines are strong, but not so striking as in Ia.
In IIb are found dark lines, bright lines and faint bands. In this subdivision he includes the Wolf-Rayet stars, more generally classified as of the fifth type.
The distinguishing mark of the third type is that, besides dark lines, there are numerous dark bands in all parts of the spectrum, and the more refrangible end of the latter is almost wanting. There are two subdivisions of this type:
In IIIa the broad bands nearest the violet end are sharp, dark and well-defined, while those near the red end are ill-defined and faint. In IIIb the bands near the red end are sharp and well-defined; those toward the violet faint and ill-defined. The character of the bands is therefore the reverse of that in subdivision a.
This classification of Vogel is still generally followed in Germany and elsewhere. It is found, however, that there are star spectra of types intermediate to all these defined. Moreover, in each type the individual differences are so considerable that there is no well-defined limit to the number of classes that may be recognized. At the Harvard Observatory a classification quite different from that of Vogel has been used, but it is too detailed for presentation here. The stars of type II are frequently termed Capellan stars, or Solar stars. Certain stars of type I are termed Orion stars, owing to the number of stars of the type found in that constellation. The stars which show the lines of helium are known as helium stars. We mention these designations because they frequently occur in literature. It would, however, be outside the object of the present work to describe all these classifications in detail. We therefore confine ourselves to a few illustrations of spectra of the familiar types described by Secchi and Vogel.
There are many star spectra which cannot be included in any of the classes we have described. Up to the present time these are generally described as stars of peculiar spectra.
As the present chapter is confined to the more general side of the subject, we shall not attempt any description of special spectra. These, especially the peculiar spectra of the nebulæ, of new stars, of variable stars, etc., will be referred to, so far as necessary, in the chapters relating to those objects.
The most interesting conclusion drawn from observations with the spectroscope is that the stars are composed, in the main, of elements similar to those found in our sun. As the latter contains most of the elements found on the earth and few or none not found there, we may say that earth and stars seem to be all made out of like matter. It is, however, not yet easy to say that no elements unknown on the earth exist in the heavens. It would scarcely be safe to assume that, because the line of some terrestrial substance is found in the spectrum of a star, it is produced by that substance. It is quite possible that an unknown substance might show a line in appreciably the same position as that of some substance known to us. The evidence becomes conclusive only in the case of those elements of which the spectral lines are so numerous that when they all coincide with lines given by a star, there can be no doubt of the identity.