Clusters, Galaxies, Black Holes and Stars 277
with cluster of galaxies that orbit the Milky Way galaxy as satellites. The
globular clusters are distributed spherically in the halo of the galaxy
beyond the radii of the spiral arms. These clusters are themselves
spherical in shape because they are very tightly bound gravitationally due
to the high density of stars within them relative to the density of stars in
the main disk of the galaxy. Globular clusters contain hundreds of
thousand stars, which are much older than the stars in the main disk of
the galaxy. To date 158 of these objects have been discovered but it is
believed that there might be some others that have not yet been detected.
In addition to globular clusters there are open or galactic clusters, which
are not tightly bound by gravity and contain only thousands of stars that
are relatively new compared to the more numerous stars of the globular
clusters. Although open clusters do not contain many stars there are
many more of them with well over a thousand having been identified to
date but by some estimates could number as many as 10,000. These
clusters of stars are formed from clouds of gases and dust and as such are
found almost exclusively in spiral and irregular galaxies. One final
structure in the Milky Way that needs to be mentioned is that of gaseous
nebulae. These vast ‘clouds’ of gas and dust are considerably denser than
the normal interstellar medium of gas and dust and are often regions
within the galaxy where new stars are formed. Nebulae appear dark in
the sky unless illuminated by a star located either within the cloud or
nearby.
The Milky Way Galaxy possesses an enormous amount of angular
momentum due to the rotation of the galactic disk about the center of the
galaxy. The galaxy does not rotate like a wheel but each star orbits the
galactic center in the plane of the disk at its own rate, as is also the case
with both globular and open clusters of stars. The period of a star or a
cluster depends on its distance from the center of the galaxy. The closer a
star is to the center of the galaxy, the shorter is the time for it to complete
one revolution just like the planets orbiting the Sun. The reason for this is
that the stars are also gravitationally bound to the galactic center and,
hence, the radius and the period of their orbit are related by Kepler’s
laws of planetary motion. The Sun completes a revolution once every
240 million years and, hence, during its 5 billion years existence it has
completed a little more than 20 galactic orbits during which time the
Earth completed 5 billion solar orbits. Ptolemy and Copernicus were
correct: the Earth does move in an epicycle, but one of galactic
dimensions.