PROBABLE ORIGIN OF NEBULJ-J. 313 



marked parallax that we can estimate the distance ; 

 for it is now generally admitted that there is no relation 

 between the apparent magnitude and the real distance 

 of a star. All that we know in regard to the distances 

 of the greater mass of the stars is little else than mere 

 conjecture. Even supposing we knew the absolute 

 distance of a star and could measure its amount of 

 displacement in a given time, still we could not be cer- 

 tain of its rate of motion unless we knew that it was 

 moving directly at right angles to the line of vision, 

 and not at the same time receding or advancing towards 

 us ; and this we could not determine by mere observa- 

 tion. The rate of motion, as determined from its 

 observed change of position, may be, say, only twenty 

 miles a second, while its actual velocity may be ten 

 times that amount. 



By spectrum-analysis it is true we can determine the 

 rate at which a star may be advancing or receding 

 along the line of sight independently of any knowledge 

 of its distance. But this again does not give us the 

 actual rate of motion, unless we are certain that it is 

 moving directly to or from us. If it is at the same 

 time moving transversely to the observer, its actual 

 motion may be more than a hundred miles per second, 

 while the rate at which it is receding or advancing, as 

 determined by spectrum-analysis, may not be 20 miles 

 a second. But in many cases it would be difficult to 

 ascertain whether the star had a transverse motion or 

 not. A star, for example, 1000 times more remote than 

 a Centauri (that is, twenty thousand billion miles), 

 though moving transversely to the observer at the 

 enormous rate of 100 miles per second, would take 

 upwards of 30 years to change its position so much as 

 1", and 1800 years to change its position 1' ; in fact we 

 should have to watch the star for a generation or two 



