What is the meaning of these different types of stars—the blue, the yellow, and the red? and can a star change its colour and type? Yes, we believe that all the blue stars, if they follow the normal course, will in countless ages become red, and though the life of the whole human race may perhaps not be long enough to witness a single instance, we feel confident that this process is going on. One walk through a forest is enough to teach us that a tree grows from seed to sapling, from maturity to decay, because we see trees around us in all these stages. So with the stars. Here the process of development is far more difficult to understand, and we are still ignorant concerning the birth and death of stars; but it is clear that we see a series of stages which pass gradually into one another, and that the cause of a star’s growing old is a gradual loss of heat by radiation. The blue Orion stars are the brightest and hottest of all those about which we have spoken, and are generally thought to have reached the climax of a star’s career. They are great globes of thin gas, much less dense than water, but glowing hot through and through. The outer layers naturally part with their heat first, and in cooling they absorb more and more light from the radiant centre. As the blue rays are always first and most powerfully affected, the blue tint is soon lost, and the colour slowly changes through white, yellow, and orange to deep-red. At the same time changes take place which cause different gases to become most conspicuous in the star’s spectrum at different stages. But what are the early stages which precede the brilliance of an Orion star? Arguments have been brought forward to show that young stars also are comparatively cool, but even less dense than Orion stars, that as they grow denser and smaller through gravity, which causes all the particles of a star to condense towards its centre, they must at first grow gradually hotter and brighter until a certain point is reached, after which they will grow cooler and fainter again until they become quite cold and dark; and so there is a double progression, viz. from red towards blue, and back again to red, some stars changing in one direction and some in the other. On the other hand, the stage which immediately precedes the Orion type somewhat resembles a peculiar class of nebulae, so the upward progression of the Orion stars may have been from nebulae through this stage. Let us see what these stages are.