1.06.02 Formation of planetary nebula and exposure of core
Mass loss during red giant phase
Throughout the red giant phase the star looses mass from its outer layers. When it enters the red
giant phase the outer layers have expanded so far that the gravitational attraction on them is now
much weaker. Therefore the loss of material due to the stellar wind becomes much greater resulting
in a loss of mass of around 10-7 solar masses a year. A helium flash can also
cause the ejection of stellar material, this which is a violent event that marks the onset of helium
fusion in stars of less than about 2.25 solar masses . More massive stars experience helium shell
flashes which also cause ejection of stellar material. Red giants can also experience a period of
instability between the balance of outward pressure and gravitational contraction. This can lead
to oscillations of the surface of the star resulting in further loss of mass. Finally when the last
stage of nuclear reactions in the core has ended the radiation from shrinking hot core will expel
the remaining outer layers of the star. The net result is that after the red giant phase the core
of the star is exposed. This core is surrounded by layers of gas of about 0.2 solar masses which
were ejected in the final stages of mass ejection. These gases expand into space at a rate of
35,000 - 75,000 miles per hour and are illuminated by the radiation from the central core remnants.
This produces what is called a planetary nebula (so called because early observers mistook these
objects for planets). The planetary nebula will disperse back into the interstellar medium over a
period of about a few thousand years.
The bright core is called a white dwarf star and may have a surface temperature of up to 100,000
Kelvin and a mass of between 0.5 and 1.4 solar masses. The white dwarf will cool down diminishing
in luminosity and will eventually fade from the heavens after about a billion years.
Halt of gravitational contraction of the white dwarf
The gravitational contraction of a white dwarf star is halted by what is called electron degeneracy
pressure. To understand electron degeneracy pressure requires an understanding of quantum mechanics.
However a very simple way to describe it is that the electrons are in their lowest energy levels
and cannot loose any more energy for the core to contract further. The maximum mass that can be
supported by electron degeneracy pressure is 1.4 solar masses. This is called the Chandrasekhar
limit after Subramanyan Chandrasekhar who predicted this limit as a result of his work on
astrophysical models ( for which he was the joint winner of the 1983 Nobel prize for physics).
Therefore if the mass of the remaining core from the red giant is less than 1.4 solar masses it
will end its life as a white dwarf star. The density of a white dwarf star is so great that a spoonful
of material would weigh between 10-100 tons! The fate of core remnants greater than 1.4 solar masses
will be discussed later.