Clusters, Galaxies, Black Holes and Stars 281
The classical mode for the observation of the heavens has always
been the detection of visible light, that rather narrow spectrum of
electromagnetic radiation that we are able to see with our naked eye.
Visible light is not the only type of electromagnetic radiation emitted by
the heavens. In fact, the entire range of the spectrum is radiated and we
are just beginning to take advantage of this fact. The first type of non-
visible sequels to be exploited was radio waves. Work in this area began
shortly after World War II, perhaps prodded by the development of
radar. A great number of radio sources were discovered. Stars and
galaxies that were known from visual sightings were found to be also
prodigious emitters of radio waves. In addition to these familiar objects,
new objects were discovered that were never observed before. These
included quasars and blazers, which form the class of radio galaxies.
Stars
We live in the stellar era. Aside from dark matter and energy stars are the
basic building blocks of our universe. There are approximately 10^22 stars
in the universe. They compose 90% of the galactic non-dark material.
Stars do not exist in isolation. They are found in galaxies, which provide
the concentrations of gas and dust necessary for their formation.
Stars are self-illuminating objects that generate their own energy
through thermonuclear fusion. The Sun is a typical star creating its own
light and heat. The Sun represents only one of the many types of stars
found in the universe. Red giants, white dwarfs, novas, super novae, and
pulsars are among the wide variety of stars known to astronomers. The
mass of stars range from 0.01 to 50 solar masses where one solar mass
is the mass of the Sun or 2 × 10^30 kilograms (kg). The great variety of
stellar types is due partly to stellar evolution and partly to the differences
in stellar masses. Stars do not remain in the same state throughout their
existence but evolve through a series of stages beginning with their birth
as protostars and ending with their death as either black dwarfs, neutron
stars or black holes. We shall describe the evolution of stars in this
chapter noting the differences due to their masses.
Star Formation
Star formation takes place in galaxies as a result of the condensation of
gas clouds. The presence of dust particles in the gas cloud is essential for