Are mostly suns, but being, the nearest of them, at a distance from us more than 500,000 times our distance from the sun, are of a size we cannot estimate, but are believed to be 300 times larger than the earth; they are of unequal brightness, and are, according to this standard, classified as of the first, second, down to the sixteenth magnitude; those visible to the naked eye include stars from the first to the sixth magnitude, and number 3000, while 20,000,000 are visible by the telescope; of these in the Milky Way (q. v .) alone there are 18,000,000; they are distinguished by their colours as well as their brightness, being white, orange, red, green, and blue according to their temperature and composition; they have from ancient date been grouped into constellations of the northern and the southern hemispheres and of the zodiac ( q. v .), the stars in each of which being noted by the Greek letters, as [Greek: alpha], [Greek: beta], according to their brightness; they all move more or less, and some go round each other, and are called double according as there are two or more of them so revolving; besides stars singly visible there are others called clusters or nebulæ ( q. v .).