2. Ring and Elliptical Nebulae.
3. Spiral or Whirlpool Nebulae.
4. Planetary Nebulae.
Irregular Nebulae.--Of this class the most conspicuous are those in the constellations of Orion and Andromeda. So clearly defined are they, that they are oftentimes seen by the naked eye on a clear night, and are often mistaken for comets.
The great nebula in Orion is one of the most noticeable objects in the heavens. It is noted for its size and brilliancy, and also for the successful observation which it has been subjected to from time to time. This large nebula is situated in that part of Orion which is occupied by several stars known as the Sword Handle. These multiple stars are known by the name of Theta. Around these multiple stars is to be seen the nebula, as though the multiple stars really were enveloped by the nebula extending for a great distance out into space. It is of a faint bluish colour, with the central parts possessing the greatest brilliancy. The suggestion arises in our mind as to whether the nebulae are in any ways indicative of the presence of the electro-magnetic aetherial field that each star undoubtedly possesses. We learned in [Art. 88] that the sun is an electro-magnet, and that it possesses its electro-magnetic field. We have also seen in [Art. 109] that the zodiacal light, which is to be observed in connection with our solar system, is really indicative of the presence of that electro-magnetic field, as it rotates round the central body. Now, if an observer were situated out in space, where the nebula of Orion is situated, and could look at our system with telescopes equally as powerful as those we possess, would not our sun present an equally nebulous light to them because of the presence of its electro-magnetic field?
Conversely, if every star possesses an electro-magnetic aetherial field, as they undoubtedly do, then it seems only reasonable to infer that that electro-magnetic field possesses a nebulosity which corresponds with our zodiacal light. The fact that the spectra of the nebulae are continuous, revealing no dark lines, seems to indicate the purity of some of the nebulae, and that therefore they are free from all known elements.
So that spectroscopic results seem to confirm this hypothesis, as the pure Aether that would surround every star, or multiples of stars, would certainly not reveal any dark lines by means of the spectroscope. Such a hypothesis, as to the real nature of a nebula, is entirely in harmony with the theory of the Aether presented in Chapter [IV]., because being gravitative it will surround each star or multiples of stars, and therefore be denser nearest to those stars, and being atomic, there will be a certain amount of nebulosity manifested by the denser parts of the medium, as is the case in our own solar system.
Ring or Elliptical Nebulae.--These forms of nebulae are so named from their ring-shaped appearance, sometimes being known as Annular Nebulae. The elliptical nebulae are usually classed with them, as they are supposed to be similar kinds of nebulae looked at edgeways. The best known of this class is that found in the constellation of Lyra, and known as 57M, which is the number of the star in Messier's catalogue of stars. It is small but well-defined, so that it looks more like a flat oval solid ring than a nebula.
The central part is not entirely dark, but is filled up with a hazy light. Another annular nebula is that situated to the south-west of Lambda Scorpii. Sir J. Herschel[43] writes of it thus: “It is a delicate but well-defined annulus. The field is crowded with stars, two of which are nebulae. A beautiful delicate ring of a faint ghost-like appearance, about 40" in diameter, in a field of about 150 stars, of 11 and 12 magnitude and under.”
Of the elliptical nebulae the best known is the one in the constellation of Andromeda, which goes by the name of 31M. It is visible on a clear night, and can be seen by the naked eye as a hazy light. There are several other elliptical nebulae, lying to the north-west of this great nebula.