Through these researches, it has been determined that the Milky Way is, so to speak, the foundation upon which the star system, visible to us, is built. All kinds of stellar bodies have been studied and their distribution has proved to be symmetrical with reference to the plane of the Galaxy. The majority is greatly concentrated around the Milky Way. To these belong the new stars which occasionally blaze into existence, as the well-known new star in Perseus, 1901, and nearly all of which have appeared in the Milky Way or in its immediate vicinity. We also find there the irregular nebulæ, enormous, vastly diffused volumes of gas, among which the best known is the Orion nebula, and which seem to constitute the primeval matter out of which the stars are born. We might further mention the star-clusters, dense, ball-shaped agglomerations of stars, and the so-called planetary nebulæ, which—at least in their visible outer shell—also consist of gas accumulations with a spherical or ellipsoid conformation. The numerous spiral nebulæ on the other hand, those strange stellar bodies to which we later shall have occasion to return, are beyond comparison more frequent in the regions surrounding the poles of the Galaxy than in the rest of the heavens.

Many astronomers have considered the Milky Way itself a nebula. The most common theory doubtless is that it closely conforms to a spiral nebula—an opinion that has found a particularly warm advocate in the Dutch astronomer, Easton (see [Figure 1]). A few years ago Prof. Bohlin expressed the view that it is most akin to a planetary nebula, or more precisely to a ring nebula which is supposed to grow out of a planetary ellipsoid nebula by the gaseous matter being driven from its poles toward its equator. It is of a certain interest that this theory lends itself to the support of Swedenborg’s—nevertheless improbable—hypothesis about the origin of the planets in the solar system. As we later shall see the Easton conception has the better reasons in its favour.

If classified according to age the stars are again distributed with the Milky Way as a reference point. Thus, let us consider their evolution, which for various reasons is assumed to take the following course. We may commence when the star-matter existed on the nebula stage. It then radiated the light characteristic of certain incandescent gases, principally the lightest two, hydrogen and helium, and further of an otherwise unknown gas called nebulium (nebula-substance). These gases were later condensed and dark spectral lines commenced to appear beside the bright lines of the aforesaid gases. Stars on this stage, named after their discoverer Wolf-Rayet stars, occur only in the immediate vicinity of the Milky Way. A later stage in their evolution is represented by the so-called helium stars in whose spectrum the dark helium lines preponderate. They are considerably concentrated around our Galaxy. Somewhat more evenly distributed and yet of decidedly greater frequence in the neighbourhood of the Milky Way, the hydrogen stars appear, characterized by strongly developed hydrogen lines and somewhat retreating helium lines. These stars are more developed than the helium stars and form with them the group of white stars so named after the colour of their light. Next in evolution follow the yellow stars, to which our Sun belongs. Dark metal lines appear in their spectrum. They are more evenly distributed than the groups mentioned before. Still further is this true about the red stars whose spectra contain the characteristic bands of chemical compounds and therefore betray comparatively advanced cooling. They are fairly uniformly spread over the heavens but are still somewhat more numerous in the vicinity of the Milky Way than further therefrom.

Fig. 1. The Milky Way, pictured as a spiral nebula
by Easton

These facts are demonstrated in the statistics by E. C. Pickering, Director of the Harvard Observatory, who divided the firmament in four equal zones, the first of which is nearest to the Milky Way (and includes it) and the last of which contains the Galactic poles. His table shows the percentages of different stars in each of the four zones.

Galactic
Latitude		Helium
Stars		Hydrogen
Stars		White
Yellow
Stars		Yellow
Stars		Red
Stars
±8.1°51.237.429.729.426.7
±21.631.728.627.926.727.6
±39.811.918.321.121.923.6
±62.35.215.721.322.022.1
Number of
stars observed		716188513291719457

The difference is most pronounced in the two first groups; in the three last it is small but unmistakable. An even distribution would correspond to 25 per cent. in all four divisions of the heavens.

These comprehensive statistics, embracing 6106 stars, seem to indicate that the stars in their first stage were within the Milky Way but subsequently drifted away with increasing age. This leads us to the thought that they originated from the irregular, nebulous accumulations which occur in the Milky Way and in its vicinity, or more correctly from similar formations which formerly existed in these regions but which now have clustered into stars. This agrees very well with another observation. With the help of the spectroscope the motion of different stars has been determined with reference to the point where the sun now is. The velocities have been found greater the older the stars are as shown in the table below taken mainly from the investigations of the renowned astronomer, Campbell.

Mean velocity of:
Irregular nebulæ 0Km.( 0 Miles )per sec.
Wolf-Rayet stars 4.5”( 2.8 ”  )” ”
Helium stars 6.5”( 4.0 ”  )” ”
Hydrogen stars11”( 6.8 ”  )” ”
Yellow stars15”( 9.3 ”  )” ”
Red stars17”(11.5 ”  )” ”
Planetary nebulæ25”(15.5 ”  )” ”