Clusters, Galaxies, Black Holes and Stars 291
The material ejected by the Crab supernovae is seen as the Crab
Nebula, which is expanding at the rate of 10,000 kilometers per second
or 1/30 the velocity of light. At the center of the Crab Nebula the remains
of the exploded core have also been found. This tiny object sends pulses
of light and radio waves towards Earth every 0.03 seconds. Other pulsars
have been discovered in the heavens with periods ranging from 0.03 to
1.5 seconds.
Pulsars as we will see are neutron stars that are rapidly rotating.
According to theoretical consideration stars more massive than the 1.4
solar mass limit for white dwarfs can collapse into neutron stars. The
pressures, when stars more massive than white dwarfs collapse, cause the
protons and electrons to combine together to form neutrons. The star
becomes a degenerate gas of neutrons, which behaves like a solid. The
neutron star possesses a very strong magnetic field. The radius of a
neutron star is believed to be about 10 to 100 kilometers. This is the
same size as a pulsar. The small radius of the neutron star enables it to
rotate with extremely high velocities. The neutron star is predicted to
emit pulses of light and radio waves with the period of its rotation. The
pulses emitted by pulsars have the properties of the pulses a neutron star
would produce. Furthermore, calculations show that the remnants of a
supernova could easily form a neutron star. It is, therefore, believed that
pulsars are nothing more than neutron stars. The reason for the rapid
rotation of pulsars and other neutron stars is that as they shrink in size
from the stars they descended from their rate of rotation increases to
conserve angular momentum, as is the case when ice skaters pull in their
arms and rotate at a faster rate.
Black Holes
A black hole is an object whose density is so great that space curves back
on itself and nothing, not even light, can escape the object. Time comes
to a halt within the black hole. There are two basic classes of black holes,
namely stellar black holes due to the collapse of a star that no longer has
nuclear fuel and supermassive black holes containing many multiples of
a solar mass.
The white dwarf and neutron stars are two possible states a star may
collapse into at the end of its existence depending on its mass. The
collapse of stars to these states occur once the star has exhausted nuclear
fuel and can no longer generate the energy necessary to prevent