Celestial Bodies 161 
multiple systems of stars are visible in the sky with only 
a small glass. As many as 50,000 stars may be grouped to- 
gether in a single system of stars—such as the Great Globu- 
lar Cluster in Hercules. 
Not all stars maintain the same brightness. Some (Mira) 
vary their brightness in regular intervals, and are known as 
variables. Other stars have been known to explode with a 
blinding flash that increases their brightness many thousand- 
fold, and then in a few days they fade from sight. Explosive 
stars are called novae; their explosions cannot be predicted. 
Someday even our sun might explode, a catastrophe which no 
earthly life could survive. 
Great clouds of luminous gas, the nebulae, abound in 
space. They take on many strange shapes and forms such as 
the North American Nebula (shaped like the geographic out- 
line of North America), the Ring Nebula of Lyra (a celestial 
smoke ring), or the Horse-head Nebula of Orion. 
UNIVERSES WITHIN UNIVERSES 
The stars, nebulae, clusters, etc., are all gathered together 
in great super-systems known as galaxies. A galaxy may con- 
tain several billions of stars. The solar system, all the visible 
stars, and many millions of stars invisible to the naked eye 
are in the Milky Way Galaxy, to which we belong. By means 
of the largest telescopes we can see millions of other galaxies 
beyond our own. Because they all appear to be moving away 
from us, it is supposed that our universe is rapidly expanding. 
Just as atoms are the smallest particles of matter in the 
universe, galaxies are the largest aggregates of matter known. 
