Beyond Maturity

Stellar Evolution Beyond the Main Sequence

3. Lightweight stars

At the other end of the scale, most stars in the Galaxy are like the Sun or smaller. The transformation from main-sequence star to red giant at the end of hydrogen burning is a more gradual process. There is no Hertzsprung gap in the HR diagrams of old clusters. The helium core starts out small, supported by incompressible electrons. Conditions are like those inside a white dwarf.

After half a billion years, the helium core grows to about half a solar mass. Eventually, it becomes hot enough for helium-burning to begin. Because the core is so dense and at uniform temperature, the ignition of helium causes a runaway explosion throughout the core. It is not well understood what changes are caused inside the star, but the star certainly loses mass between leaving the main sequence and becoming a helium-burning horizontal branch star.

 When core helium burning ends, the low mass star rejoins the asymptotic giant branch, where the hydrogen and helium burning shells interact, causing a series of thermal pulses. These create new chemical species, dredge up new material to the surface of the star, and throw puffs of gas and dust into space. Concentric shells seen by the Hubble Space Telescope in the Egg nebula are probably be connected with such a puffing star. Eventually it will eject a planetary nebula and collapse, as before, to become a white dwarf.

 Stars less than one third of a solar mass will not ignite helium at all. When hydrogen-burning finishes, they will simply fade away to become a helium white dwarf. However, it will be many billions of years before they reach this stage.

Binary Stars

There is not space here to look at what happens to stars with companions, binary stars. Since stars can grow in size by a huge amount during their evolution, the presence of a companion can have a profound effect. As the surface of one star expands, it may come under the gravitational influence of its companion. Depending on the initial masses and separation of the two stars, an almost infinite number of outcomes are possible.

First one star may spill over onto the other. Then the second star may spill back again. One star may expand to engulf the other. The masses of both stars can flip-flop. Fexample, Sirius now consists of a brilliant 2.3 solar mass A1V main- sequence star, and a very faint 1.05 solar mass white dwarf. To become a white dwarf before its main-sequence companion, it must once have been the more massive star.

 Binary stars are responsible for cataclysmic variables or dwarf novae, which contain a white dwarf and a main-sequence star, and for X-ray binaries, which contain a neutron star or black hole. Mass spilling over from one star to another can also lead to supernova explosions, often referred to as Type I supernovae.

Variable Stars

Stellar evolution involves change over thousands, millions and billions of years. These changes are usually too small to be observed in the short time that astronomers have been watching the skies. But many stars also change on timescales anywhere between seconds (pulsars), days (cepheids) and years (long period variables).

We have already mentioned pulsations, but flares and spots connected with magnetic fields on the stellar surface can be observed in cool stars, such as the BY Draconis and RS Canis Venaticorum variables. Nuclear explosions on the surfaces of accreting white dwarfs are seen as dwarf novae. Puffs of ejected dust obscuring the normally visible star are seen in R Corona Borealis stars. Binary stars vary because of their orbital motion and eclipses.

A hot star can heat its cool companion so that it becomes hot on one side and cool on the other. New types of variability are discovered almost every year. Each one helps astronomers to learn more about the real physics going on in stars, and hence to build better models of stellar evolution.

The end

After hydrogen burning, a star passes through many transformations. Understanding how a star evolves allows us to see physics in action on a huge scale. Different parts of a star experience extremes of temperature, density and change. Stars are the factories where hydrogen is transformed into the elements that provide us with life and luxury. There is still lots to learn about where these come from.

• Table of Contents
• 1. After Hydrogen Burning
• 2. Heavyweight Stars
• 3. Lightweight Stars
• The end