280 The Poetry of Physics and The Physics of Poetry
galaxies. Halton Arp explains the diversity of forms of galaxies in terms
of the initial mass and spin of the galaxy. He argues that as the cloud,
which formed the proto-galaxy, began to shrink under the force of
gravity, the proto-galaxy began to spin more rapidly in order to conserve
angular momentum like the figure skater who spins more rapidly as she
draws in her arms. If this spin becomes too great, the galaxy will lose
mass because of the large centrifugal forces that are generated by the
spin. This explains, he claims, why the largest galaxies are elliptical or
spherical and not rotating as much as their smaller counter-parts, the
spiral and irregular galaxies. The spiral and irregular galaxies were
originally high spin objects, which, as a consequence, could not form
very massive states because of their spin. Some of the mass they lost
might very well have formed the smaller satellite galaxies often
associated with these larger high spin galaxies.
Active Galaxies
All the objects we have discussed so far are normal galaxies. We shall
now draw our attention to a class of objects known as active galaxies.
These include radio or exploding galaxies, quasars (quasi-stellar radio
sources), and Seyfert galaxies. Active galaxies share a common property,
namely, that at their center is an active galactic nucleus, which is a
massive black hole with a mass somewhere between 10^6 and 10^10 Sun
masses. This active galactic nucleus radiates much more electromagnetic
radiation than any other object in the sky. In fact the massive amount
of energy active galaxies radiate cannot be accounted for by the
thermonuclear fusion reactions that take place in the Sun and other
stars. The energy these objects generate is from the collapse of huge
amount of matter from other parts of the active galaxy into the active
galactic nucleus, i.e. the massive black hole that sits at the centre of
the galaxy. These objects generate some or all of the full range of
electromagnetic radiation including radio, infrared, optical, ultraviolet,
x-ray and gamma rays. What distinguishes the different members of the
class of active galaxies is the mix of radiations they emit. These objects
are extremely red shifted indicating that they are very far from us and
represent events that took place in the early history of the universe.
Radio galaxies consisting of quasars and blazers radiate enormous
amounts of radio waves.