On examining the heavens on a clear evening, when the Moon is not shining, we find here and there groups of stars which seem to be compressed together in such a manner as to present a dull cloud-like appearance: these are termed clusters and nebulæ, and are usually classed as follows: —

1. Irregular groups, visible more or less to the naked eye.

2. Clusters resolvable into separate stars with the aid of a telescope.

3. Nebulæ, for the most part irresolvable.

The latter are subdivided into: —

1. i. Annular Nebulæ.

2. ii. Elliptic Nebulæ.

3. iii. Spiral Nebulæ.

4. iv. Planetary Nebulæ.

5. v. Nebulous Stars.

Of the 1st class, there are several examples to be found, with all of which the reader is probably more or less familiar. The cluster of the Pleiades in Taurus is probably best known. When examined directly, few persons can see more than 6 stars, but by turning the eye sideways, we discover there are many more, and between 50 and 60 are visible in a telescope. The most brilliant star in the group is Alcyone, or η Tauri, of the 3rd magnitude ; next in order come Electra and Atlas of the 4th; Maia and Taygeta of the 5th; Pleione and Celeno, which are between the 6th and 7th; Asterope between the 7th and 8th; and finally, a great number of smaller stars.