As the star contracts under the gravitation of its parts and increases in temperature and density there comes more and more into play an important factor that has a great effect upon its future development. This is light-pressure or radiation pressure which acts in opposition to gravity and exerts a strong outward pressure upon matter within the depths of the star, tending to push it outward from the center where the temperature is greatest and the light is most intense. It is a most interesting fact that if the mass of a star, that is the quantity of matter that it contains, exceeds a certain value the pressure of light or radiation within it overbalances the gravitational attraction that draws matter towards its center and the star disintegrates or ceases to exist as a star. This accounts for the fact that the stars differ very little among themselves in the quantity of matter that they contain, that is, in their masses, though they may differ enormously in size. Stars that exceed a certain mass will become unstable and this may account for the association of luminous nebulæ with the hottest of all stars, the nebulæ possibly being puffed off from the surfaces of these stars under the action of radiation pressure. After a star has reached the height of its development as a bluish-white helium star with a temperature of something like 10,000° Centigrade and a density about one-tenth that of the sun, it begins to lose heat and to cool gradually though it continues to contract and increase in density. It is now on the descending scale of evolution and is to be counted among the dwarfs instead of the giants. A brilliant blue-white helium or Orion star is about one hundred times more luminous than the sun, and its diameter is about ten times that of the sun. Our own sun, we find, is on the descending scale of stellar evolution. It is a yellow dwarf star of temperature about 6,000° Centigrade and density one and one-fourth that of water, which is probably about as great a density as is attained by any star since even the non-luminous planets Jupiter and Saturn have lower densities than the sun. The last stage in the development of a star is represented by the dwarf red star of high density and low temperature. The diameter of the dwarf red star probably averages about five hundred thousand miles and its temperature is 3,000° Centigrade or less. After this we have the extinct stars, similar possibly to our planet Jupiter, though considerably larger, with a dense gaseous atmosphere and a certain degree of internal heat.