276 The Poetry of Physics and The Physics of Poetry
mutual gravitational attraction. Many of these clusters form superclusters
where once again gravity binds them together. We begin our story with a
description of galaxies beginning with our very own Milky Way galaxy
in which our Sun resides after which we will describe a number of
different types of galaxies. Once this task is completed we will begin to
describe the cluster that contains the Milky Way, the Local Group, and
the supercluster, the Virgo Supercluster also known as the Local
Supercluster that contains the Local Group and hence the Milky Way.
After describing our local cluster and supercluster we will go on to
describe the other clusters and superclusters of our universe. We will
leave the description of the birth and the evolution of stars to the next
chapter.
The Milky Way Galaxy
The Milky Way Galaxy is a collection of approximately 10^11 stars with a
total mass of 10^41 kilograms. The shape of the galaxy is essentially that
of a disk with a central spherical bulge. When we view the Milky Way in
the night sky we are looking into the plane of the disk. The galaxy
appears in the sky as a band of closely clustered clouds, which was given
the name the Milky Way and hence became the name of our galaxy. The
term galaxy is related to the term Milky Way as the Greek word for
milky is γαλαξίας (galaxias).
If viewed from the top of the plane of the disk of stars, one would
see that the Milky Way is not a homogeneous distribution of stars, gas
and dust, but rather forms a spiral structure and would look very much
like the spiral galaxy M81 located in the Ursa Major constellation. The
radius of the Milky Way’s galactic disk is about 50,000 light years. The
Sun is about 26,000 light years from the center located in one of
the galaxy’s spiral arms. The thickness of the galactic disk varies
from 16,000 light years at the center to 6,000 light years at the edge. The
very center of the Milky Way galaxy consists of a very large compact
object, a supermassive black hole, with a total mass of approximately
4.1 million solar masses. This is typical as most galaxies have a
supermassive black hole at their centre.
Radiating out from the centre of the Milky Way galaxy in the plane of
the disk are the spiral arms consisting of two major arms and several
minor arms. The Sun sits in one of the minor arms. In additions to the
spiral arms of the galaxy there are clusters of stars not to be confused