Dr. J. Croll on the Origin of Nebulae. 13 



jecture as to the actual velocity of the star. If we assumed 

 its distance to be 10 times that of a Centauri, this would give 

 a transverse velocity of one mile per second. If we assumed 

 its distance to be 100 times that of a Centauri, this would 

 give 10 miles a second as the velocity, and if 1000 times, the 

 velocity of course would be 100 miles per second. 



As there are but few of the stars which show a measurable 

 parallax, and we have no other reliable method of estimating 

 their distances*, it follows that in reference to the greater 

 number of the stars, neither by spectrum-analysis nor by ob- 

 servation of their change of position can we determine their 

 velocities. There does not, therefore, appear to be the shadow 

 of a reason for believing that none of the stars has a motion 

 of over 30 or 40 miles per second : for any thing that at 

 present is known to the contrary, many of them may possess 

 a proper motion enormously greater than that. 



There is, however, an important point which seems to be 

 overlooked in this objection, viz. that, unless the greater part 

 of the motion of translation be transformed into heat, the 

 chances are that no sun star will be formed. It is necessary 

 to the formation of a sun which is to endure for millions of 

 years, and to form the centre of a planetary system like our 

 own, that the masses coming into collision should be converted 

 into an incandescent nebulous mass. But the greater the 

 amount of motion left unconverted into heat, the less is the 

 chance of this condition being attained. A concussion which 

 would leave the greater part of the motion of translation un- 

 transformed, would be likely as a general rule to produce 

 merely a temporary star, which would blaze forth for a few 

 years, or a few hundred years, or perhaps a few thousand 

 years and then die out. In fact we have had several good 

 examples of such since the time of Hipparchus. Now, 

 although it may be true that, according to the law of chances, 

 collisions producing temporary stars must be far more nume- 

 rous than those resulting in the formation of permanent stars, 

 nevertheless the number of those temporary stars observable 

 in the heavens may be perfectly insignificant in comparison 

 with the number of permanent stars. Suppose there were as 

 many as one hundred temporary stars formed for one per- 

 manent, and that on an average each should continue visible 

 for 1000 years, there would not at the present moment be over 

 half-a-dozen of such stars visible in the heavens. 



4th. " Such collisions as the theory assumes are wholly 



* It is true that we may one day be able to determine by spectrum- 

 analysis the distance of some of the binary stars; but as yet this method 

 has not been applied with success. 



