CHAPTER XX
The Visible Universe
Some researches on the distribution of stars in the sky have recently been made at the Harvard Observatory (U.S.A.). The principal results are:—(1) The number of stars on any “given area of the Milky Way is about twice as great as in an equal area of any other region.” (2) This ratio does not increase for faint stars down to the 12th magnitude. (3) “The Milky Way covers about one-third of the sky and contains about half of the stars.” (4) There are about 10,000 stars of magnitude 6·6 or brighter, 100,000 down to magnitude 8·7, one million to magnitude 11, and two millions to magnitude 11·9. It is estimated that there are about 18 millions of stars down to the 15th magnitude visible in a telescope of 15 inches aperture.[456]
According to Prof. Kapteyn’s researches on stellar distribution, he finds that going out from the earth into space, the “star density”—that is, the number of stars per unit volume of space—is fairly constant until we reach a distance of about 200 “light years.” From this point the density gradually diminishes out to a distance of 2500 “light years,” at which distance it is reduced to about one-fifth of the density in the sun’s vicinity.[457]
In a letter to the late Mr. Proctor (Knowledge, November, 1885, p. 21), Sir John Herschel suggested that our Galaxy (or stellar system) “contained within itself miniatures of itself.” This beautiful idea is probably true. In his account of the greater “Magellanic cloud,” Sir John Herschel describes one of the numerous objects it contains as follows:—
“Very bright, very large; oval; very gradually pretty, much brighter in the middle; a beautiful nebula; it has very much the resemblance to the Nubecula Major itself as seen with the naked eye, but it is far brighter and more impressive in its general aspect as if it were doubled in intensity. Note—July 29, 1837. I well remember this observation, it was the result of repeated comparisons between the object seen in the telescope and the actual nubecula as seen high in the sky on the meridian, and no vague estimate carelessly set down. And who can say whether in this object, magnified and analysed by telescopes infinitely superior to what we now possess, there may not exist all the complexity of detail that the nubecula itself presents to our examination?”[458]
The late Lord Kelvin, in a remarkable address delivered before the Physical Science Section of the British Association at its meeting at Glasgow in 1901, considered the probable quantity of matter contained in our Visible Universe. He takes a sphere of radius represented by the distance of a star having a parallax of one-thousandth of a second (or about 3000 years’ journey for light), and he supposes that uniformly distributed within this sphere there exists a mass of matter equal to 1000 million times the sun’s mass. With these data he finds that a body placed originally at the surface of the sphere would in 5 million years acquire by gravitational force a velocity of about 12½ miles a second, and after 25 million of years a velocity of about 67 miles a second. As these velocities are of the same order as the observed velocities among the stars, Lord Kelvin concludes that there is probably as much matter in our universe as would be represented by a thousand million suns. If we assumed a mass of ten thousand suns the velocities would be much too high. The most probable estimate of the total number of the visible stars is about 100 millions; so that if Lord Kelvin’s calculations are correct we seem bound to assume that space contains a number of dark bodies. The nebulæ, however, probably contain vast masses of matter, and this may perhaps account—partially, at least—for the large amount of matter estimated by Lord Kelvin. (See Chapter on “Nebulæ.”)