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visible to the naked eye. Those in and near the Milky Way frequently contain hundreds or even thousands of stars too small to be seen separately without a telescope. The stars differ from each other in colour, although not in so marked a degree as terrestrial objects. The most casual observer cannot fail to note the difference between the bluish white of Alpha Lyrae and the reddish light of Arcturus. There seems to be a regular gradation in the colour of the stars from blue, through yellow, to red. These differences of colour are connected with differences in the spectra of the stars. As a general rule, the redder a star is, the greater the number and intensity of the dark lines that can be seen in the green and blue parts of its spectrum.

A slight examination of the heavens shows that the stars are not scattered equally over the sky, but that there is more or less of a tendency to collect into constellations. This is especially the case with the brighter stars. But no well-marked dividing line between the constellations is possible; that is, we cannot draw a line showing exactly where one constellation ends and another begins. Nevertheless a division into constellations was made in ancient times and has been followed by astronomers down to the present time.

How and by whom the constellations were first mapped out and named no one knows. The Chinese had their asterisms—collections of stars smaller than what we call constellations—in the earliest years of their history.