278 The Poetry of Physics and The Physics of Poetry
It is believed that the flattening of the galactic disk occurred as a
result of its rotation. The spherical distribution of the globular clusters,
which are among the oldest objects in the galaxy, is thought to demarcate
the original boundaries of the galaxy.
The age of the Sun is 5 billion years, half the age of the galaxy. The
Sun lies in one of the spiral arms where most of the new stars also lie.
Fresh material in the form of gas and dust is pouring out of the center of
the galaxy traveling along the galaxy’s spiral arms with velocities up to
60 kilometers per second. Enough material flows along the arms to
produce one star every year. The flow of this material explains the
presence of the many young stars observed in the spiral arms.
The spiral arms are also the location of vast magnetic fields. The
strength of these fields is only 5 millionths of a gauss which is small
compared with the Earth’s magnetic field (0.5 gauss in strength). High-
energy electrons and protons spiral along the lines of the galactic
magnetic field, focusing cosmic rays in our direction and also producing
radio waves. These radio signals are called synchrotron radiation because
they are produced in almost an identical manner by the man made proton
accelerator, the synchrotron.
The Local Group
The Milky Way Galaxy is surrounded by billions of other galaxies.
Those galaxies lying closest to us form a gravitationally bound cluster
known as the Local Group, which contains about 20 galaxies altogether.
They include our two closest neighbors, the Large and Small Magellanic
Clouds, which are two small angular galaxies that are satellites of the
Milky Way. The two Magellanic Clouds are 40,000 and 30,000 light
years in diameter and have no pronounced structure. Our Local group
contains one other large spiral galaxy like the Milky Way. This galaxy
has three smaller satellite galaxies bound to it. The remainder of the
galaxies of the Local Group are elliptically shaped galaxies of various
size, but all are smaller than the two large spiral galaxies. Some of the
elliptical galaxies are dwarfs about the same size as a globular cluster.
Galaxies
We find a remarkable variety in the size and shapes of the galaxies we
find within our Local Group. Scanning the remainder of the heavens we