General Science 2310 - Evolution of Low and Intermediate Mass Stars

Last Update :

THE EVOLUTION OF STARS

EVOLUTION OF LOW AND INTERMEDIATE MASS STARS

Reading from text : 441-449

The main sequence lifetime of a star is over once the Hydrogen in the core has all been fused to Helium.
The core is still leaking energy, so it must contract and heat up
Heat from the core raises the temperature of the surrounding layer to the point where Hydrogen to Helium fusion can take place.
As successive layers are "burned" Helium "ash" is dumped onto the core , causing further contraction, and the gas outside the burning layer is heated so that it expands.
The amount of energy reaching the photosphere is limited by the opacity, so that the surface temperature decreases slightly.
This stage of hydrogen shell burning is characterized by increasing luminosity and slightly decreasing temperatures in the outer layers or "envelope."
These decreasing temperatures raise the opacity to the point where the star transports energy by convection rather than radiation.
This efficient energy transport leads to decreasing surface temperatures and increasing luminosities as the star continues to expand into a red giant.
As the star expands, the core continues to compress and heat up. The pressure becomes so great that the electrons in the core plasma form a degenerate gas.
In a degenerate electron gas, the gas pressure is dominated by electron degeneracy pressure rather than thermal pressure. This means that the temperature can change without any appreciable change in pressure. The only way to increase degeneracy pressure is to increase the particle density.
The details on the interaction between gravitational and degeneracy pressure work out to have the interesting result that as you add matter to a degenerate gas, its volume actually decreases !!
As helium ash is being dumped onto the core of the star, the core contracts, leading to higher core temperatures
Degenerate gases behave a lot like solid metals, Degenerate gases are excellent conductors of heat and electricity.
The shell of burning Hydrogen is heated by the conducting core and by the increased gravitational pressure resulting from the very high core density.
The net result is an ever increasing rate of Hydrogen shell burning, leading to continued expansion of the outer envelope and helium ash being dumped onto the core, making it contract and heat up.
On an HR diagram, the star moves vertically upwards, passing through a sub giant phase into a red giant.
During the red giant phase, stars are so large that they can not hold on to all the gas at their surface. The amount of mass lost during this phase cannot yet be predicted by theoretical models, but the results of mass loss can be predicted and observed.
When the core temperature increases to the point at which it is possible for the core to burn Helium, essentially all the Helium in the core fuses at once, leading to a spectacular Helium flash.
This explosive behaviour occurs because the pressure is dominated by degeneracy pressure so that increasing temperatures will not cause the core to expand and cool as it would in a main sequence star.
The net result of this tremendous outpouring of energy is that the core loses its degeneracy. The core temperatures are high enough that thermal pressure dominates over degeneracy pressure.
The core can now expand, reducing both its own rate of Helium burning and the rate of Hydrogen shell burning that is still going on.
The total luminosity has now been reduced to the point where the star cannot maintain its giant size.
This stage of core Helium burning, shell Hydrogen burning, decreasing size and increasing temperature is called the horizontal branch.
The radius of a horizontal branch star is typically about 5 solar radii. The Hydrogen burning shell is about one tenth of a solar radius from the core.
The envelope of a horizontal branch star is almost entirely radiative.
The precise location of the horizontal branch on an HR diagram depends on both its initial mass and the amount of mass loss during the red giant phase.
In particular, a star which has lost its entire envelope and consists entirely of a Helium fusing core is called a Helium Main Sequence star. Helium main sequence stars are at the hottest (bluest) end of the horizontal branch. The reddest horizontal branch stars are those that have experienced the smallest amount of mass loss.
By the time the Helium in the core has been fused to Carbon and oxygen. The star has completed its horizontal branch phase and is ready to begin its Asymptotic Giant Branch phase.
Lacking this source of energy generation, the core must again contract and heat up causing the free electrons to form a degenerate gas and igniting the layer just outside the core into helium shell burning.
A stellar core of degenerate gas has the property that the degenerate gas pressure depends less sensitively on the density than an ideal gas. The effect of this is that the core contracts more rapidly than before as the carbon-oxygen "ash" is dumped from the Helium burning shell.
This double shell burning phase is characterized by expansion of the envelope, causing the star to again increase in size.
An asymptotic giant typically has a radius of 300 solar radii. The radius of the core is approximately one thousandth of a solar radius, Hydrogen burning is going on in a shell located about one hundredth of a solar radius out from the core.
The envelope of an asymptotic branch star is almost entirely convective.
Having reached the stage of red Supergiant, low mass stars often enter a phase of thermal oscillation, in which the stellar luminosity enters a cycle of bursts and relaxations in which the magnitude of the luminosity can vary by a factor of 100,000.
The period of these thermal oscillations is typically about half a million years.
The high luminosity point of a thermal oscillation is sometimes called a helium-shell flash
This flash occurs because the helium burning shell, although non - degenerate, cannot expand fast enough to compensate for the rising temperatures caused by the energy output from the burning helium. Rising temperatures lead to high burning rates which leads to a chain reaction of ever increasing luminosity.
The chain reaction eventually relaxes when the shell has expanded to the point where their temperature is low enough for convection to transport the heat.
After relaxation, the star readjusts itself to its original configuration of double shell burning and the oscillation begins all over again.
During these thermal oscillations large quantities of the stellar envelope will escape the gravitational pull of the star.
The "final" state of a low to medium mass star comes when all the material outside the degenerate core forming a ring of gas known as a planetary nebula.
The degenerate core eventually loses the planetary nebula and ends its life as a white dwarf, slowly cooling and dimming but maintaining a constant radius.
The precise details of how a star evolves on an HR diagram from the red supergiant to the white dwarf stage is not well understood

Back to index of lectures

Back to The Main Sequence Phase

Forward to Evolution of High Mass Stars

To contact the instructor please email to:

wwall@lakeheadu.ca