Cepheids are semi-predictable. They do loose mass, and the amount of fusionable helium gets less as fusion progresses to heavier materials in the
core. I will briefly try to describe what we know about Cepheids down below:
For stars to be stable, heat must flow from layer to layer at a constant rate, equal to the energy production in the core, and the loss of heat at the
surface. This requires a balance between temperature and density in a given region, and in the layers above and below that region. If, for example, a
region is too low in density to effectively block the flow of radiation, so that heat flows through the region faster than it can be replaced, the
region will cool and contract, causing an increase in its density, which allows it to more effectively block the outward flow of radiation.
Conversely, if a region has too high a density, and heat cannot pass through the region fast enough, the gas in that region will heat and expand,
causing a decrease in its density, which allows the heat to escape more easily. Fluctuations in these densities cause vibrations to pass through stars
, but they do not usually produce substantial changes in the star's structure, because any variation from a stable structure is kept small, by a
continual adjustment of density and temperature.
Cepheids are relatively massive, young stars which are in the last stages of their life, and passing through a yellow-giant stage. When the
surface temperature is in a temperature range similar to that of the Sun, there is a region not far below the surface where single-ionized helium ions
are close to the temperature at which they become double-ionized.
Suppose that in the region where temperatures are slightly above the double ionization temperature, some fluctuation in conditions causes
temperatures to become a little too high, so that the gas ought to expand and cool off. The outer layers of the star start to expand and cool, as
energy stored in the kinetic energy of the gas is transformed into gravitational potential energy (the star gets larger). Under normal circumstances,
as the star expands, the temperature drops; but in Cepheids, as the temperature in the region where helium is doubly ionized reaches the double
ionization temperature, the temperature stops going down, even though the star continues to get larger. Heat is still being converted into
gravitational potential energy, but temperature stops dropping, because the recombination of electrons with the doubly ionized helium atoms releases
large amounts of energy, and keeps the temperature at a constant value.
The expansion continues, without any change in temperature in the region where helium is recombining, until all the helium has recombined; but by
then, the star is much larger and brighter than it was, so it starts to cool, and has to contract again. Everything now runs in reverse, with
gravitational potential energy being converted into kinetic energy as the star gets smaller and hotter; but just as before, when the temperature
reaches the double ionization temperature, things get "stuck", as the singly ionized helium ions lose their remaining electron, and gravitational
energy which ought to go into heating up the gas is used to ionize the atoms, instead.
In other words, the ionization of helium prevents the temperature from rising until all the helium is doubly ionized, while the star contracts;
while the recombination of the ions prevents the temperature from dropping until all the helium is singly ionized, while the star expands. The
transfer of heat to ionization during contraction pushes the star to too small a size, while the transfer of heat from recombination during expansion
pushes the star to too large a size, and thus the star is oscillating, and that is your Cepheid variable.
In multiple star systems, more variables comes into play, as some of the outer layers of the star are drawn towards the companion stars.
edit on 17/12/2012 by Hellhound604 because: (no reason given)