1919 .] 
Astronomical Notes. 
385 
The third body is therefore as hot as a body formed by the complete union 
of two stars. Its subsequent life-history is absolutely different, however. 
The one is enduring, the other evanescent. The one is so hot that it takes 
millions of years to cool; the other, though really at the same temperature, 
is too hot to cool at all. It cannot hold together. The molecular velocity 
is greater than the critical. The partnership of the molecules is for ever 
dissolved. Each goes on its independent course, carrying its energy far 
- away into the comparatively empty regions of space. 
To the eye, aided even by the greatest telescope, stars appear as points. 
We are too apt to consider them as points when discussing their encounters. 
To avoid this error let us picture two stars similar to our Sun passing one 
another with their centres three and a half times as far apart as the Moon is 
from the Earth. They graze one another. The third body formed by the 
collision, though tiny compared to either of the suns, is in truth gigantic 
and constituted in a very remarkable way. The suns before they meet 
are moving in slightly curved parabolic or hyperbolic orbits. Each is rotating 
as it flies, and, before striking, both are distorted by tidal action. At the 
beginning of the great encounter the light elements in the atmospheres of 
the suns meet first. They are forced in upon one another bv the pressure 
of the advancing bodies, and are soon partially enwrapped by heavier 
materials torn from deeper portions of the wounds. 
At first the third body is by no means spherical. While the collision 
is in progress the least resistance to the explosive energy is presented in 
the regions farthest from the plane of relative motion. Some of the arrested 
mass is drawn back by the retreating suns. The rest, drawn to spindle- 
shape, is left to work out its own destiny. It is all intensely hot. To 
measure its temperature we should need to take as our unit at least a 
million degrees centigrade. Its volume is for the moment small. The 
pressure corresponds to the temperature. The result is the greatest explosion 
known in the whole realm of nature. The turbulent agitation of the mole¬ 
cules represents an energy of tens of millions of calories per gramme. The 
pressure is intense. The molecules are trying to move in every direction, 
and each is incessantly in violent collision with its neighbours. It is clear 
that to a distant observer the spectrum of this body will be continuous. 
The retreating stars, with their relative velocity of some 400 miles per 
second, will be well apart before the jumble of vibrations in the arrested 
mass can settle down into general outward rush. The growth of the third 
body will be comparatively slow at first, but will be continually accelerated 
as long as the radial pressure lasts. In a day or two the outer gases will 
overtake the retreating suns, but having now a velocity of 1,000 miles or 
more per second a very small portion of their mass will be captured. 
One most important fact in this connection is that, directly after the 
collision, the different elements of which the third body is composed have 
widely different temperatures. This follows from the fact that all the 
substances have the same molar velocity transformed, and therefore have 
the same molecular velocity produced. The heavy elements will thus have 
much higher temperatures than the light ones. Oxygen will be four times 
and lead fifteen times as hot as hydrogen. In the intense struggle that 
follows, the light elements gain energy from the heavy ones, and, as their 
velocities increase, tend to find their way through the seething mass towards 
the outside. Selective molecular escape begins. The expanding nucleus 
is surrounded by shells of outrushing gas, whose motion it accelerates by 
its tremendous pressure. The distant observer finds every stage of the 
brief but stirring life-history of the nova recorded in the complexities of 
its characteristic spectrum. C G- 
28—Science. 
