Island Universes 215

give the photographs of several stellar systems of this kind,
taken through the powerful telescope of the Mount Wilson
Observatory. These photographs make apparent the len-
ticular shape of the extragalactic "nebulae" and also in-
dicate the presence of somewhat irregular spiral arms
winding around the central elongated bodies. Not all extra-
galactic nebulas possess these spiral arms, however, and
a number of them have regular shapes of more or less
flattened ellipsoids.

Elliptical "Nebulae'

Barred Spirals

FIGURE 56
Bubble's classification of extragalactic "nebulae."

In Figure 56 the reader will find a schematic represen-
tation of the various observed shapes of these nebulas,
based on the observations of the Mount Wilson astronomer,
E. Hubble, to whom we owe most of our information
concerning these distant island universes. When observed
through not very strong telescopes, they seem to be continu-
ous luminous masses of gas (hence their name "nebulas");
but the loo-inch telescope of the Mount Wilson Observa-
tory reveals that the outer arms at least are actually made
up of billions of separate stars, very similar to the mem-