1.06.03 Characteristics of red giant phase related to differences in mass of stars
Stars of mass between 0.1 and 0.5 solar masses
These stars reach the red giant phase due to the pressure from the increased reactions in the hydrogen
burning shell. This is due to the increased temperature as a result of core contraction. However they
never produce core temperatures sufficient to burn helium. They eventually end up as a white dwarf
star composed of helium.
Stars of mass between 0.5 and 4 solar masses
These stars go on to burn helium in the core and the eventually end life as white dwarf stars composed
of carbon and oxygen. The lower mass stars have lower surface temperatures and have much longer life times
as main sequence stars and red giants.
Stars of mass less than about 2.25 solar masses
Helium burning in the core starts when the core has become compressed so much that it is in a
degenerate condition. This results in a violent helium flash.
Stars of mass greater than about 2.25 solar masses
Core temperatures rise to produce helium burning before the core has become compressed to degenerate
conditions and therefore no helium flash occurs.
Stars of mass between 4 and 8 solar masses
In these stars temperatures increase sufficiently during the contraction of the carbon core to produce
a further stage of carbon burning. These stars can also experience helium shell flashes. These shell
flashes are not related to the degenerate conditions that lead to the helium core flashes experienced
by lower mass stars. In this case the helium shell is too insubstantial to lift the material above it
therefore it cannot expand to moderate its temperature. Subsequently the shell temperature increases
causing an increase in the reaction rate until the energy is released in an explosive reaction. The
subsequent expansion, cooling and then contraction of the star can produce periodic helium shell flashes
over periods of around 100,000 years. These stars end their lives as a white dwarf composed of oxygen neon
and magnesium.
