92 THE UNIVERSE A VAST SYSTEM OF PARTS Sirius wa.s found to shift back and forth every 25 years. In 1844 Bessely on the basis of his computations of the orbit of Sirius, predicted that Sirius was a double star. Later both Sirius and its companion were observed through a telescope. The companion of Sirius is a very interesting star, for although its mass is 4/5 as great as that of the sun, it radiates only 1/360 as much light. It must, therefore, either be cooler or have a much smaller surface than the sun. When it was proved in 1914 that the companion of Sirius is a white-hot star, the conclusion was drawn that it must have a smaller surface. The volume of the companion of Sirius is estimated to be 0.00004 of the volume of the sun. Its diameter is about 1/19 of the sun's diameter, which would make it about the size of the planet Uranus, or three times the size of the earth. Inasmuch as the companion of Sirius is therefore at least 40,000 times as dense as water, a pint of its mass would weigh 20 tons, and a tablespoonful could not be lifted by a strong man. It seems unbelievable that matter in the gaseous state could exhibit such a high density, but such is our confidence in Newton's laws upon which our calculations are based that we must abandon "everyday common sense" for the higher common sense of Science. Accurate checks of Sirius' mass, size, and temperature leave no other conclusion. A number of other white dwarfs (stars resembling the companion of Sirius) are known. One of these white dwarfs has twice the mass of the sun, and yet it is no larger than the planet Mars. The gravitational attraction of this star would be so great that it would cause a man to spread out on the surface like a puddle of water. A cubic inch of matter from the white dwarf star. Wolf 457, would weigh 9000 tons, i.e., just under the weight of a navy cruiser; and if it were placed in a modern skyscraper, it would probably crush its way right through the building and proceed toward the center of the earth as a cannon ball sinks in water. Dwarf stars are now considered to be made up of the nuclei of atoms, which represent most of the mass of atoms but occupy a very small portion of the space taken up by an entire atom. Another theory is that these dwarf stars are made up of very heavy elements not known on the earth. Betelgeuse Exhibits Conditions at the Other Extreme. Betelgeuse is a bright star in the shoulder of Orion, 192 light-years from the sun. This giant red star has a diameter 345 times that of the sun, or about 300,000,000 miles, and its volume is 40,000,000 times that of the sun's volume. Inasmuch as its mass is only twelve times
NEWTON'S LAWS UNIVERSAL IN APPLICATION 93 that of the sun, it water, or 1/1000 that of air. must have a density of only 1/1,000,000 that of The Tides Are Produced by the Gravitational Attraction of the Moon and the Sun. Tides are caused by the gravitational attraction of the moon and the sun. The moon is the principal cause of the tides. The sun's tide-raising force is only about 2/5 that of the moon inasmuch as the sun is 400 times as far from the earth as is the moon and inasmuch as the TO SUN AND MOON < SPRING TIDES TO SUN TO MOON NEAP TIDES Fig. 21. Lunar and solar tides, showing the spring and neap tides. (From the Yerkes Observatory, reprinted by permission of the Chicago University Press.) attraction varies inversely as the square of the distance and only directly as the masses. Twice each lunar day (24 hours, 52 minutes) this attraction acts to increase the depth of the water in the oceans at different times in different localities. The fact that the tides are synchronized with the lunar day rather than with the solar day proves that the moon is chiefly responsible for the tides. The waters on the side of the earth nearest the moon are drawn toward it, while those on the other side are attracted least of all and tend to rise because the earth is attracted more than the water. Thus high tides occur on the sides next to and opposite the moon, while low tides are found at positions just midway between these points.