18 NATURE 

[JANUARY 6, 1923 
The Borderland of Astronomy and Geology. 
By Prof. A. S. Eppincton, F.R.S. 
ae region in which geology and astronomy most 
conspicuously overlap is in the theories of the 
origin of our planet. We have, in fact, two main 
theories—one due originally to an astronomer, Laplace, 
and the other to a geologist, Chamberlin. 
In the last century the evolution of a star seems 
often to have been regarded as something quite de- 
tached from the evolution of the stellar universe. Just 
as the birth and death of a man is an incident which 
can occur at any time in the rise and decline of the 
human race, so it was thought that the birth and 
extinction of a particular star formed merely a detached 
incident in the course of progress of the stellar universe 
—if, indeed, the universe was progressing in any par- 
ticular direction. Thus it was a natural belief that 
the stars died out and were re-formed by collisions of 
extinct stars; and that the matter which now forms 
the sun had undergone many alternations of incan- 
descence and extinction since things first began. But 
this view is quite at variance with the general tendency 
of sidereal astronomy in the present century. We 
have come to recognise that the stellar system is one 
great organisation, and that the stars which are shining 
now are more or less coeval with one another. Every- 
one would admit that Mars and Jupiter were formed 
as parts of one process of evolution—not necessarily 
at the same moment, but each formed as the process 
reached the appropriate stage ; and similarly we now 
believe that it was one process of evolution sweeping 
across the primordial matter which caused it to form 
itself into stars ; and these original stars are the actual 
stars which we see shining now. No doubt the evolu- 
tion did not develop at the same rate in all parts of 
the universe, and there are probably places where 
stars are still being formed; but you will see that 
this view is entirely different from the other view that 
stars were being formed individually .by haphazard 
collisions of dark stars, so that each was an independent 
formation, having no time-connexion with other stars. 
This view has been forced on us partly by direct 
evidence of organisation among the stars, pointing to 
a common origin for large groups of stars. We notice 
scattered groups such as the Hyades, which have almost 
exactly equal and parallel motions. Clearly it would 
be impossible to form such a group if each star were 
the product of an accidental collision. The only way 
in which a common motion like this can arise is by 
associated development from some nebula or other 
diffuse distribution of matter. The connexion is 
clearly a connexion of common origin. Again, practi- 
cally all the bright stars of Orion form a similar group, 
having common motion; and, moreover, they have 
all reached a similar stage of evolution. They are 
connected with the great Orion nebula, the faint exten- 
sions of which fill up nearly the whole constellation. 
It is obvious that here we have to deal with a single 
evolutionary development. But another point which 
militates against a collision theory is the extreme 
rarity of collisions and close approaches. The dis- 
tances separating the stars are enormous compared 
* A lecture delivered before the Geological Society of London on 
November 2r. 
NO./2775, VOL, Ta] 


_ 
with their own dimensions. Sir Frank Dyson once 
used the illustration of twenty tennis-balls, distributed 
at random throughout the whole interior of the earth, 
to give a model of the density of distribution of the 
stars. It has sometimes been objected that we do 
not know how many extinct stars may be wandering 
about and colliding. Dyson’s twenty tennis-balls 
represent only the Jwminous stars ; there may, for all 
we know, be millions of dark bodies ready to be fired 
into incandescence by collision. I think, however, 
that there is now good evidence, based on the dynamics 
of stellar motions, that the dark stars cannot greatly 
outnumber the luminous stars—probably not ten times 
and certainly not a hundred times. (If they were 
more numerous than that, the average velocities of 
stars would, owing to the gravitational attraction, 
be much higher than is observed.) That argument, 
then, is no longer valid. Taking a very liberal view of 
the kind of approach that can be held to constitute a 
collision, it is estimated that a star would only suffer 
collision once in 10! years. 
Thus the astronomer is not predisposed to look 
favourably on a hypothesis of the origin of the solar 
system which postulates anything of the nature of a 
collision. He has the conception of an orderly develop- 
ment of the stars crystallising out of the primordial 
material, and, unless perhaps in exceptional cases, 
following an undisturbed course of development. We 
; hope for a theory that will show us the star after its 
first isolation from surrounding material spontaneously 
developing the system of planets. 
It now appears almost certain that, whether the 
original matter was gaseous or whether it was composed 
of meteors, it must at an early stage in the star’s 
history have been completely volatilised into gas. This 
was while the star was extremely diffuse, and, for 
example, before the planets separated from it. This 
means that the material now forming a planet has at 
one time passed through the furnace, and has cooled 
down from a gaseous stage. How far that has a direct 
bearing on geology I cannot say, since I have nothing 
to guide me as to the course of its subsequent chequered 
history. I do not say that the earth was a gaseous 
body when it first became recognisable as an inde- 
pendent planet, but I am convinced that its material 
was at one time merged in a completely gaseous sun. 
It may be of interest to indicate why it seems so 
probable that a star in its early diffuse state is gaseous 
and not meteoric. The stars are known to be of 
closely similar mass. There are occasional exceptions, 
but probably 90 per cent. of them are between one-half 
and five times the sun’s mass. We have no explana- 
tion of this uniformity if they are initially merely 
aggregations of solid meteors; but we have a very 
exact explanation if they are gaseous. In fact this 
critical mass round which the actual masses of the 
stars cluster so closely is predicted by the theory of 
equilibrium of spheres of gas, using only well-known 
physical constants determined in the laboratory. The 
crucial factor is radiation-pressure, which is inappreci- 
able in smaller masses, and almost suddenly takes 
control between one-half and five times the sun’s mass. 
