176 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1928 



but stars are visible. Our galactic system of stars is probably the 

 final product of just such a transformation, the Milky Way still 

 recording the position of the equatorial plane of the original nebula. 



Stars born in this way may meet with a variety of accidents, and 

 these result in different observed astronomical formations. A star 

 may rotate too fast for safety, just as a flywheel may; when this 

 happens it breaks into two, and the two stars so formed revolve 

 endlessly about one another as a binary system. Two stars may run 

 jnto one another, although this is very rare. A more common occur- 

 rence is for two stars to escape running into one another by a narrow 

 shave. When this happens, huge tides are raised on the two stars 

 involved, and these may take the form of long streamers of gas, which 

 ultimately condense into '' drops " just as did the gas in the outlying 

 regions of the spiral nebulae. It seems reasonably certain that the 

 planets were formed in this way. 



Tlie birth of the solar system, then, resulted from the close ap- 

 proacli of two stars ; if a second star had not happened to come close 

 to our sun, there would have been no solar system. It may be thought 

 that with a life of millions of millions of years behind it, one star 

 or another would have been certain to come near enougli at some 

 time to tear planets out of the body of our sun. Calculation shows 

 the reverse; even after their long lives of millions of millions of 

 years, only about 1 star in 100,000 can be surrounded by planets born 

 in this way. A quite unusual accident is necessary to produce planets, 

 and our sun with its family of attendant planets is rather of the 

 nature of an astronomical freak. 



In the 1,000 million stars surrounding our sun there are, at a mod- 

 erate computation, not more than 10,000 planetary systems, because 

 there has not been time for more than this number to be born. The}^ 

 are of course still coming into existence; calculation suggests a birth- 

 rate of about 1 per 1,000 million years. Thus we should have to visit 

 thousands of millions of stars before finding a planetary system of 

 as recent creation as our own, and we should have to visit millions 

 of millions of stars before finding a planet on which civilization and 

 interest in the outer universe were as recent a growth as are our 

 own. We are standing at the first flush of the dawn of civilization 

 and are terribly inexperienced beings. 



It may be suggested that the creation of planetary systems is also 

 only beginning, and that in time every star will be surrounded, like 

 oui" sun, by a family of planets. But no ; the stars will have dissolved 

 into radiation or disappeared into darkness before there is time for 

 this to happen. So far as we can judge, our part of the universe has 

 lived the more eventful part of its life already; what we are witness- 

 ing is less the rising of the curtain before the pla_y than the burning 



