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 alone there are 18,000,000; they are distinguished by their colors 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, 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 nebulae.