288 The Poetry of Physics and The Physics of Poetry
The helium core continues to contract achieving a temperature
eventually of 100 billion K at which point helium begins to undergo
thermonuclear fusion forming heavier elements, principally carbon. The
temperatures and pressure become so high in the core that shortly after
the onset of helium burning an explosion takes place in which the helium
core increases in size and the hydrogen envelope decreases in size. The
temperature of the helium core drops because of its increase in size and,
therefore, the rate of helium burning decreases. The hydrogen burning
which continues through all these changes also slows down. The star
now settles into its second but shorter-lived stable phase with a
temperature and luminosity similar to its original main sequence
position. It remains in this state until its helium fuel is exhausted. The
evolution of stars more massive than the Sun is identical to that described
above except the explosion that increases the size of the helium core does
not occur because the core of these stars is rather large to begin with.
Once the star exhausts its helium fuel in its second main sequence
phase its core, which is composed basically of carbon, now begins to
contract. The stars hydrogen envelope expands once again and the star
passes into a red giant phase once again. The carbon core continues to
shrink as the envelope increases in size. Eventually the envelope
becomes so thinned out that neutral atom formation begins to occur
because the number of ionizing collisions is so low. As neutral atoms
form they limit photons which heat the envelope and accelerate the rate
at which it is expanding which produces more neutral atoms which heats
the envelope still more and so on and so forth. The expansion proceeds
so rapidly that the envelope leaves the star completely forming a ring or
planetary nebulae about the star. The planetary nebulae contain perhaps
20% of the stellar materials. It will continue to expand until it dissipates
itself in interstellar space after about 50,000 years. The carbon core left
behind continues to contract evolving into a white dwarf star.
Novae
Once or twice a year somewhere in the heavens the sudden brightening
of a star occurs. The star will shine with a luminosity 10,000 times its
normal output for a few hours. In the days before the telescope this
brightening was thought to be due to the birth of a new star, hence the
name novae.