78 THE UNIVERSE A VAST SYSTEM OF PARTS so bright that they can be seen in the daytime. One of these "novae," 1^ ^>. as such stars are called, appeared in the constellation of Aquila in 1918, and another one was discovered in Cygnus a couple of years later. This increase of brightness, which can be likened to the sudden change from a small one-cell flashlight to that of a huge searchlight, occurs in a cosmic instant, changing from a very faint light to its maximum. The light gradually decreases to its original intensity in about ten years. Such a rapid change can be accounted for only as an explosion. Additional evidence that explosions really take place is ofifered by the expanding shells of nebulous matter that have been observed around several novae. About 150 small blue stars have been observed which are surrounded by shells of luminous, nebulous material. These planetary nebulae, as _ ,,.„ „, they are called, are thought Fig. 13. Dark nebulae in the Milky Way. ^ , ,j r^, ^o be old novae. The cause (Photograph from the Yerkes Observatory, reprinted by permission of the Chicago of these explosions has not University Press.) yet been found.
MANY INSTRUMENTS SUPPLEMENT TELESCOPE 79 considered as a mere stimulant to the imagination as, in the next Section, we look out into space beyond our lens-shaped galaxy. Fig. 14. Great Nebula in Orion. (Courtesy Mount Wilson Observatory.) STUDY QUESTIONS 1. Explain how star distances are determined (1) by methods of parallax and (2) by use of the spectroscope. 2. What are the Fraunhofer lines, and what is their significance? 3. What type of astronomical body is the sun? 4. Name two uses of the spectrometer. 5. What is the Doppler effect? 6. How is the composition of a star determined? 7. How is the intrinsic brightness of a star measured, and why is it needed in measuring star distances? 8. How fast do the stars move? 9. How is the speed of the stars determined? 10. Describe the life cycle of the stars. 11. What is meant by the statement that many stars are multiple?