80 THE UNIVERSE A VAST SYSTEM OF PARTS 12. How is the temperature of stars estimated? 13. Why is the camera of great value in studying the stars? 14. What is the scientific significance of the different instruments used for measuring the temperatures of the stars? 15. Compare the variations of stars in density and mass. 16. Are collisions between stars likely to be of frequent occurrence? Discuss. 17. What is the estimated number of stars in our galaxy? 18. What form does our galaxy take? 19. What is the significance of the Milky Way? 20. What is the size of our galaxy? 21. What is thought to be the nature of a nebular cloud? 22. Give one theory to explain the low-temperature periods in the earth's history.
UNIT II SECTION 4 OUR GALAXY IS BUT ONE OF MANY GALAXIES Introduction. In addition to the nebulae that appear within our own galaxy, there are giant star clouds or irregular exterior nebulae,^ like the Magellanic Clouds, and many spiral nebulae. Our own galaxy is believed to be an unusually extensive nebula about 100,000 light-years, more or less, in diameter. The Great Magellanic Cloud, about 85,000 lightyears away, is only about 21,000 light-years in diameter. The great Andromeda Spiral Nebula is about 1,000,000 light-years distant and is estimated to be about 60,000 light-years in diameter. The light from some of these nearest exterior nebulae started on its long journey to the earth about the time when the mammals were developing in the middle Cenozoic (Courtesy of the Lick Observatory.) Fig. 15. The Great Magellanic Star Cloud. period. During the past 6000 years, which have witnessed the tremendous advance of mankind, this light has been traveling through our own galaxy. If every exterior galaxy had been completely wiped out about 1,000,000 years ago, we would not know it left them before that time. today because the light from these galaxies ' Exterior nebulae are nebulae located outside of our galaxy. 81