Clusters, Galaxies, Black Holes and Stars 289
It is now believed that this event is associated with planetary nebulae.
Novae occur in binary star systems. It is believed that one of the two
stars is very hot white dwarf and the other is a red giant with a planetary
nebula. It is theorized that gaseous material from the planetary nebula of
the red giant falls upon the surface of the white dwarf causing a
thermonuclear explosion and a bright flash of light, which we observe as
a novae.
White Dwarfs
The carbon core of a red giant can evolve into a white dwarf only if its
mass is less than 1.4 solar masses. Carbon cores with higher mass evolve
into other types of stars. The white dwarf state is the final stage of
evolution of a star with mass less than 1.4 solar masses. The star can no
longer generate new energy. The star contracts into a degenerate gas,
which behaves more like a solid than a gas. The white dwarf reaches a
minimum size approximately the same size as the Earth, which it
maintains even after it, cools. The density of the star is a million times
that of the Sun. The white dwarf has an atmosphere only 100 meters
thick. It loses its energy through emission of radiation until it becomes a
burned-out cinder or a black dwarf hardly radiating at all. Since the
universe is only 10 billion years old a white dwarf created at the very
beginning of the universe would still only have cooled down to 3000 K
which is the lowest temperature ever observed for a white dwarf.
Supernovae, Neutron Stars And Pulsars
The final days of more massive stars are not as peaceful as those of the
white dwarfs. The more massive stars, “rage against the dying of the
light.” They end their existence not with a whimper but with a bang, a
supernovae bang. Supernovae have been observed with the naked eye
three times in the past millennium in our galaxy. Supernovae are
observed by astronomers with their telescopes on a quite regular basis in
distant galaxies. Supernovae in our galaxy occur on average once every
50 years. The first to be recorded in human history took place in the year
- This object, which was so bright it could be seen during the
daytime, was recorded by people all over the world. The gas clouds from
this great explosion are still visible as the Crab Nebula. The second and
third supernovae occurred in 1572 and 1604 and were studied by Tycho