Great Nebula in Orion (M. 42). Visible to the naked eye just below a line connecting β and ζ Orionis, and involving θ Orionis. It exhibits an extremely complicated structure, and many of its irregular branches and condensations may be discerned in small instruments. Sir W. Herschel failed to resolve this object into stars with his 4-foot reflector; but Lord Rosse, in 1844, thought he had effected it with his 6-foot mirror, though the conclusion was premature. The spectroscopic researches of Huggins have shown this nebula to be composed of incandescent gases, so that the stars telescopically observed in it are probably in the foreground and entirely disconnected from the nebulous mass. Effective photographs have been taken of it by Draper, Common, and Roberts. It certainly forms one of the grandest objects in the heavens.
The Planetary[68] Nebula (M. 97). Discovered by Mechain in 1781. In small telescopes it looks like a rather faint, round mass of nebulosity, somewhat brighter in the middle than at the edges. In Lord Rosse’s telescope it shows many details, including a spiral arrangement and two dark spots in the middle inclosing bright, eye-like condensations. The margin is fringed with protuberances, and from its peculiar aspect this object has been called the “Owl” Nebula. Diameter between 155″ and 160″. It may readily be picked up 2-1/4° S.E. of β Ursæ Majoris. It yields a gaseous spectrum.
In Draco at R.A. 17h 58m 36s, Dec. +66° 38′ there is a pretty small, but exceedingly bright planetary nebula. With a low power it looks like a star out of focus, but a high power expands it into a well-defined planetary disk. As observed in Lord Rosse’s 3-ft. reflector on Sept. 17, 1873, this nebula exhibited “a round, well-defined disk of a full blue colour, light very equable, diameter 22″·4, surrounded by an extremely faint nebulosity.” This is an excellent object of its class.
Spiral Nebula (M. 51). Discovered by Messier on Oct. 13, 1773. It is situated in Canes Venatici, and 4° S.W. from ζ Ursæ Majoris. An ordinary instrument will reveal it as a double nebula, and the two parts will be seen to differ greatly in size. Messier gave the distance separating them as 4′ 35″. Sir J. Herschel drew this object as a bright, centrally condensed nebula, surrounded by a dark space and then by a luminous ring divided through nearly one half of its circumference. Closely outlying this he placed a bright round nebula. Lord Rosse’s 6-foot showed something very different. In April 1845 its spiral character was discovered; coils of nebulosity were observed tending in a spiral form towards the centre, and the outlying nebula was seen to be connected with it. Some striking drawings have been published of this object. Those by Sir J. Herschel and Lord Rosse differ essentially, and would scarcely be supposed to represent the same nebula; but when we reflect that the instruments used were respectively of 18 inches and 72 inches aperture, the cause of the disparity becomes evident.
Another fine example of a spiral nebula is M. 99, in the northern wing of Virgo, and 8° E. of β Leonis. This object was discovered by Mechain; its spiral form of structure was detected by Lord Rosse in 1848. Diameter 2½′ Like M. 51 it gives a continuous spectrum and is resolvable into stars.
1. Nebula with bright centre. 2. Planetary Nebula.
3. Ring-nebula in Lyra. 4. Star-cluster in Hercules.
The Crab Nebula in Taurus (M. 1). Discovered by Bevis in 1731, and situated 1° N.E. of ζ Tauri. Its diameter is 5½′ by 3½′. An early drawing with Lord Rosse’s telescope shows it with numerous radiations; whence it was termed the Crab Nebula, from the supposed resemblance: but later observations have given it quite another form. In 1877 there was no trace of the nebulous arms: it appeared as a well-defined, oval nebula with some irregularities of structure. This object is very plain in small telescopes, and may be readily picked up from its proximity to ζ Tauri; but in such instruments it is void of detail, and merely presents a pale, oval nebulosity. It has not been clearly resolved, though it has a mottled appearance, indicating a stellar composition, in large apertures.
The Dumb-bell Nebula (M. 27). Discovered by Messier in 1779, and situated in Vulpecula—a region very rich in small stars. Diameter about 7′ or 8′. Its general form resembles a dumb-bell or hour-glass; hence its name. Struve, Lord Rosse, and others have seen many stars in the nebulous mass, but the latter is not resolvable. I have seen seven stars in the nebula with a 10-inch reflector. Its peculiar shape is perceptible in a small instrument. This object frequently serves to illustrate books on Astronomy; but the drawings by Sir J. Herschel, Lord Rosse, and others are curiously discordant, and show how greatly differences in telescopic power may affect the observed appearance of an object.