When we discussed the face of the sky in our first chapter, the stars were nothing to us but a distant background of points of light. This background enabled us to fix our bearings in space, and we saw how we could pick out our near neighbours, the planets and other members of the sun's family, by their rapid motion against it.

Since then we have examined what the stars really are, and have discussed their various physical characteristics. Amongst other things, we have found that they shew great variety in their candle-powers. While some are thousands of times more luminous than our sun, others are thousands of times fainter. If we compare our sun to an ordinary candle, some stars must be compared to searchlights, and some, at the other end of the scale, to glow-worms or fireflies.

It has only recently been discovered how great a range there is in the candle-powers of the stars. For a long time it was supposed that the stars all had pretty much the same intrinsic brightness—like a row of street lamps—so that when a star looked very dim, it was only because it was very distant. In 1761 the astronomer Lambert argued that as all the stars had been made to serve the same purpose, there was no reason why some should have been made fainter than others; if some appeared fainter it could only be that they were more distant.