327. _Star-Clusters._--Besides this gradual and regular condensation towards the Galaxy, occasional aggregations of stars into _clusters_ may be seen. Some of these are visible to the naked eye, sometimes as separate stars, like the "Seven Stars," or Pleiades, but more commonly as patches of diffused light, the stars being too small to be seen separately. The number visible in powerful telescopes is, however, much greater. Sometimes hundreds or even thousands of stars are visible in the field of view at once, and sometimes the number is so great that they cannot be counted. 328. _Nebulæ._--Another class of objects which are found in the celestial spaces are irregular masses of soft, cloudy light, known as _nebulæ_. Many objects which look like nebulæ in small telescopes are shown by more powerful instruments to be really star-clusters. But many of these objects are not composed of stars at all, being immense masses of gaseous matter. [Illustration: Fig. 369.] The general distribution of nebulæ is the reverse of that of the stars. Nebulæ are thickest where stars are thinnest. While stars are most numerous in the region of the Milky-Way, nebulæ are most abundant about the poles of the Milky-Way. This condensation of nebulæ about the poles of the Milky-Way is shown in Figs. 367 and 369, in which the points represent, not stars, but nebulæ. II. THE STARS. The Constellations. [Illustration: Fig. 370.] [Illustration: Fig. 371.] 329. _The Great Bear._--The Great Bear, or _Ursa Major_, is one of the circumpolar constellations (4), and contains one of the most familiar _asterisms_, or groups of stars, in our sky; namely, the _Great Dipper_, or _Charles's Wain_. The positions and names of the seven prominent stars in it are shown in Fig. 370. The two stars Alpha and Beta are called the _Pointers_. This asterism is sometimes called the _Butcher's Cleaver_. The whole constellation is shown in Fig. 371. A rather faint star marks the nose of the bear, and three equidistant pairs of faint stars mark his feet. 330. _The Little Bear, Draco, and Cassiopeia._--These are all circumpolar constellations. The most important star of the Little Bear, or _Ursa Minor_, is _Polaris_, or the _Pole Star_. This star may be found by drawing a line from Beta to Alpha of the Dipper, and prolonging it as shown in Fig. 372. This explains why these stars are called the _Pointers_. The Pole Star, with the six other chief stars of the Little Bear, form an asterism called the _Little Dipper_. These six stars are joined with Polaris by a dotted line in Fig. 372. [Illustration: Fig. 372.] The stars in a serpentine line between the two Dippers are the chief stars of _Draco_, or the _Dragon_; the trapezium marking its head. Fig. 373 shows the constellations of Ursa Minor and Draco as usually figured. [Illustration: Fig. 373.]