It is then supposed that the contracting mass of gas would begin to rotate and to throw off gigantic streamers, which would in their turn form centres of condensation. The whole structure would thus form a spiral, having a dense region at its centre and knots or lumps of condensed matter along its spiral arms. Besides the formless gaseous nebulae there are hundreds of thousands of "spiral" nebulae such as we have just mentioned in the heavens. They are at all stages of development, and they are visible to us at all angles--that is to say, some of them face directly towards us, others are edge on, and some are in intermediate positions. It appears, therefore, that we have here a striking confirmation of the nebular hypothesis. But we must not go so fast. There is much controversy as to the nature of these spiral nebulae. Some eminent astronomers think they are other stellar universes, comparable in size with our own. In any case they are vast structures, and if they represent stars in process of condensation, they must be giving birth to huge agglomerations of stars--to star clusters at least. These vast and enigmatic objects do not throw much light on the origin of our own solar system. The nebular hypothesis, which was invented by Laplace to explain the origin of our solar system, has not yet met with universal acceptance. The explanation offers grave difficulties, and it is best while the subject is still being closely investigated, to hold all opinions with reserve. It may be taken as probable, however, that the universe has developed from masses of incandescent gas. [Illustration: _Photo: Yerkes Observatory._ FIG. 24.--THE GREAT NEBULA IN ORION The most impressive nebula in the heavens. It is inconceivably greater in dimensions than the whole solar system.] [Illustration: _Photo: Lick Observatory._ FIG. 25--GIANT SPIRAL NEBULA, March 23, 1914 This spiral nebula is seen full on. Notice the central nucleus and the two spiral arms emerging from its opposite directions. Is matter flowing out of the nucleus into the arms or along the arms into the nucleus? In either case we should get two streams in opposite directions within the nucleus.] THE BIRTH AND DEATH OF STARS Sec. 3 Variable, New, and Dark Stars: Dying Suns