282 The Poetry of Physics and The Physics of Poetry
this process because the dust serves as a catalyst for the condensation.
Once a sufficient amount of condensation has occurred and the density of
the cloud is higher than the surrounding interstellar material the cloud
will begin to collapse due to the gravitational pull of its own parts. The
typical cloud or protostar is a few million light years across. The
gravitational collapse due to the gravitational pull of its own parts takes
millions of years depending on the mass of the protostar. As the protostar
collapses the temperature and pressure within it increases. The collapse
stops once the gravitational force is counter balanced by the outward
force of the pressure within the cloud. At this point the cloud has
achieved stellar dimensions, which are of the order of a million
kilometers.
Because of the large extent of the protostar gas cloud and the overall
rotation of the galaxy the edge of the cloud away from the galactic center
will have a larger velocity than the edge of the cloud closer to the center
of the galaxy. As a result the gas cloud has a net rotation or angular
momentum. Since the angular momentum or rotational motion of an
object is conserved, the rate of rotation of the protostar increases as the
cloud shrinks in size. The velocities that develop can become quite large
so that if the star retained all its angular momentum it would break apart
from the centrifugal forces generated by the spinning motion. Instead
there is a gradual loss of material and angular motion as the protostar
shrinks in size. Some of the material that is spun off as the protostar
shrinks forms the planets and their moons, which also take up a great
deal of the angular momentum that is lost.
Stellar material is lost through solar winds and/or the formation of
planets. It should therefore not be a surprise to discover that many stars
have been observed to have planets given the mechanism for star
formation. Stars with masses similar to the Sun are very likely to have
planets. The solar wind is a steady stream of gases lost by the Sun and
other stars. The Sun has lost almost all of its angular momentum by both
planet formation and the solar wind. The planets and their satellites
possess 99% of the angular momentum of the solar system. The Sun has
the remaining 1%.
If the protostar is much more massive than the Sun then a single star
system cannot form and instead a multiple star cluster will form. Stars
are never found with masses greater than 50 solar masses. Multiple star
systems are as common as single star systems with two star systems or
binaries composing almost half the stars in our galaxy. The formation of