Celestial Bodies 159 
colors in the stars. Observe a blue star like Vega, or a red star 
like Antares, and you will come to realize what remarkable 
differences in color the stars do exhibit, even though at first 
glance they appear colorless. 
WHY IS A STAR? 
Stars are the fundamental building blocks of the universe. 
Planets like our earth are mere fragments of matter without 
much cosmic significance, but a star is sufficiently large to 
rank as a distinct member of the universe. Curiously, there 
is not a very great range in the amount of matter making up 
different stars. There are big ones and small ones to be sure, 
but only rarely is a big one more than ten times larger than 
the average size, or a small one more than ten times smaller. 
We may well inquire, why this tendency of cosmic matter to 
form stars all more or less of a standard size? In way of ex- 
planation, Sir Arthur Eddington has pointed out that there 
are two conflicting forces at work in the universe: (1) grav- 
itation and (2) radiation pressure. Gravitation is the mys- 
terious force which attracts every particle of matter in the 
universe to every other particle. Radiation pressure is the 
pressure due to intense heat and light radiation. Stars; he 
Says, occur in sizes just large enough to bring about an 
equilibrium between these two conflicting forces of grav- 
itation and radiation pressure. If they were much smaller 
they would collapse under gravitational stress; if they were 
much larger they would explode because of excess radiation 
pressure. 
THE HOSTS OF HEAVEN 
Stars occur singly in space, or in systems of two, three, or 
more as you may readily verify by observation. Many 
