Beyond Maturity

Stellar Evolution Beyond the Main Sequence

1. After Hydrogen Burning

What happens to a star roughly five times as heavy as the Sun - astronomers say five solar masses - after it burns out its hydrogen? Five solar masses is a moderately massive star. The two main sequence stars in the binary system U Ophiuci are about 5 solar masses, so this is about what will happen to them as they grow older. We tell the story using an HR diagram (Figure 1).

When hydrogen finishes burning in the center of our 5 solar mass star [C], after about 56 million years, it leaves behind a helium core which is roughly one tenth the total mass of the star. Hydrogen burning shifts to an onion-like shell surrounding the core, which feeds fresh helium into the core.

With no nuclear reactions going on inside it, this helium core shrinks under its own gravity, and heats up. At the same time, the energy produced in the hydrogen-burning shell increases very rapidly. This has the effect of making the outer layers of the star expand and, within a few million years, the star has left the main sequence to become a red giant [D], completely crossing the HR diagram. Since this phase of evolution is fast, we don't see many stars here, resulting in a gap, the Hertzsprung gap, on HR diagrams of galactic clusters. In colour photographs, these giants appear yellow or red.

The star now has a radius 65 times the Sun, a brightness 1,000 times the Sun, and a surface temperature of about 4,000 degrees. But although the star is very large, most of the mass of the star is contained inside a very small central sphere.

While our five solar mass star was on the main sequence, heat was carried outwards from central the core by massive currents of hot plasma moving up, cooling and moving down again. This process is known as convection, and is familiar to anyone who has boiled water in a saucepan.

Convection is very important for stellar evolution because it mixes up different layers of the star. If the region where nuclear reactions take place is convective, it increases the amount of fuel available. If the surface layers are convective, processed material can be dredged up from the deep interior to become visible at the surface.

For the outer 80% of our main-sequence star, however, the heat was carried outwards by radiation, mostly X-rays and gamma rays, until it reached the surface where it was converted into ultraviolet and visible light.

As a red giant, our star now looks quite different. Although the core is hot, not much energy is produced there, so the convection currents switch off. The outer layers of the giant star are now very cool and quite opaque to radiation. Deep down, bubbles of plasma heat up and expand until they are quite buoyant relative to surrounding material. They rise up through the outer layers of the star, until they reach a region nearer the surface where they are able to release their heat and cool, before sinking down again. The outer 50% of red giant stars are deeply convective.

The hydrogen shell feeds helium into the core, which grows until it is nearly one fifth of the mass of the star. When the temperature at the center reaches about 100 million degrees, nuclear reactions between three helium atoms switch on, creating carbon and releasing energy [E]. With this new source of heat, the core expands and becomes convective.

The energy produced in the hydrogen shell drops and the surface of the star gets slightly hotter. The star becomes a yellow giant with a surface temperature between 6000 and 8000 degrees. Our five solar mass star will burn helium steadily in its core for about ten million years, creating carbon and, later, oxygen [F].

Pulsations

At some point, the star is likely to cross a part of the HR diagram where the outer layers become unstable - and dramatic changes can take place. Energy trying to escape from the star is blocked (or absorbed) by cool material near the surface. This blocked energy heats up the cool layers just below the surface and makes the star expand. When it has expanded enough, the blocking layers start to let energy escape and cool. The star contracts until the process repeats itself. This is called pulsation and the stars are called cepheid variables after the first one discovered, Delta Cephei.

Classical cepheids expand and contract with periods between 1 and 50 days. Over a period of 5.4 days, Delta Cephei changes in radius from 40 to 55 times solar and back, and brightens from 1,500 to 3,000 times solar and back. The way that the brightness of a star changes with time is known as its light curve.

The light curve of a cepheid is quite regular, repeating faithfully every cycle, but not all cepheids pulsate with the same period, or have identical light curves. In 1912, H. Leavitt made the remarkable discovery that the periods of cepheid variables are linked to their luminosities. That means that if you discover a cepheid variable and measure its period, you know its luminosity.

The difference between the apparent brightness of a cepheid in the sky and its luminosity then tells you how far away it is. This property is terribly useful, because cepheids are bright enough to be seen over great distances, enabling Edwin Hubble to make his discovery in 1924 that the spiral nebulae are distant galaxies similar to our own. Cepheids were used in early studies of cosmology to establish the cosmic distance scale and, more recently, Hubble Space Telescope measurements of cepheids in Virgo and Fornax cluster galaxies have provided the best ever measurements of the Hubble constant, the rate of expansion of the Universe.

As well as cepheids, pulsations are found in many types of star across the HR diagram, from white dwarfs to red supergiants. Even the Sun and other main-sequence stars pulsate, although the driving mechanism is not always the same.

Double-shell burning

When the supply of helium in the core runs out, our star is in the same predicament as when hydrogen ran out. With no heat sources left, the core shrinks and helium-burning moves to an onion-like shell around it [G]. The core is now a mixture of carbon and oxygen at over 100 million degrees.

The heat produced by the star comes from two shells, an outer one burning hydrogen to helium, and an inner one burning helium to add fresh carbon and oxygen to the core. As before, the star becomes cooler and larger, returning towards the giant branch.

To distinguish it from stars on the first giant branch, the star is now described as an asymptotic giant branch star [H]. These stars turn out to be extremely important for stellar evolution and for the existence of life on Earth.

The star is now more luminous than ever, some 2,000 times brighter than the Sun. Convective mixing in the outer layers extends through 80% of the star, sometimes reaching down to the hydrogen-burning shell and below. As a consequence, hydrogen atoms - protons - are brought into contact with helium and carbon at very high temperatures. A network of new nuclear reactions may be set up; protons and neutrons are captured by heavier atoms to create exotic elements such as barium, strontium and yttrium. These, along with copious amounts of carbon and oxygen, are dredged up to the surface of the star.

Being so luminous, the radiation leaving the star is actually able to push material off the star. Vigorous stellar winds are seen in all very bright stars, they strip the outer layers of the star away, causing it to lose mass and, in the case of our asymptotic branch giant, spewing out masses of carbon, oxygen and exotic elements, mostly in the form of small dust grains. These grains are thrown out into interstellar space until, much later, they may be gathered up into an interstellar cloud and the star formation process can begin all over again. Most of the carbon and oxygen on Earth once came from stars like this.

Approaching the end

Deep inside the star, two things could now happen. If the star does not lose some of its outer layers, it will behave like a more massive star. We'll see what happens to these later. However, our five solar mass star will probably lose up to four fifths of its mass as a stellar wind, mostly during its asymptotic giant evolution. It will eventually become an extremely luminous carbon star, forty thousand times brighter and one thousand times larger than the Sun [K]. Compared with the Sun, its surface will be very rich in carbon and oxygen.

Eventually, the processes removing the outermost layers of the star will become so effective that nearly all the remaining hydrogen outside the hydrogen/helium boundary will be blown off, almost at one go. This cocoon of gas and dust will detach itself from the star and start to expand into space.

Initially this cocoon may be dense and very cool, and can shroud the visible light from the star. Then the only way to see the star is in infrared light. The IRAS spacecraft discovered many such stars during the 1980's. The star at the center of the Egg nebula is another example. The cocoon has been thrown outwards at high speed and will continue expanding as the central star evolves.
 
 

Planetary nebulae

As the hydrogen-burning shell burns its way towards to the stellar surface it starts to switch off. The helium shell does likewise. Without heat sources, the heat leaving the star is not replaced. With nothing to hold the surface of the star up, it starts to shrink and go out.

Since the star is bright, the heat leaves quickly, and the shrinking can be rapid. In fact, it can shrink from being 1000 times larger than the Sun, to 100 times smaller than the Sun in a few tens of thousands years. It crosses back from the cool side of the HR diagram to the hot side, paradoxically becoming even hotter and bluer than massive young main-sequence stars [L].

When it becomes hotter than about 30 thousand degrees, ultraviolet light strips electrons from hydrogen atoms in the expanding cocoon around the star. This cocoon, now almost a light year across, starts to glow as a planetary nebula. Famous examples include the ring nebulae in Lyrae, and the Helix nebula.

Stellar cinders

The central stars of planetary nebulae are quite extraordinary objects. Their surfaces can be over 100 thousand degrees. Some central stars have surfaces in which no hydrogen remains, some have no helium either. With their original hydrogen stripped away, they are the bare remnants of stellar evolution.

These stars have no nuclear fuel reserves left. They cannot become much smaller because the electrons inside them can't be forced any closer together - they are electron degenerate. The carbon-oxygen mixture inside them is no longer a gas like the hydrogen in young stars, but is now a very hot crystalline or metallic solid, at about 100 million degrees.

The star is more than ten times smaller than the Sun, but it is still shining 1000 times more brightly. However the light radiating from the star is draining heat away from the center. Gradually, that 100 million degrees will fall, quickly at first, and then more slowly as the star gets dimmer and cooler, to become a white dwarf, 100 times smaller than the Sun [M].

 Most of the stars in the sky will end their lives as white dwarfs. It is a startling fact that most white dwarfs have almost the same mass, six tenths the mass of the Sun. Our five solar mass star might end up slightly heavier, maybe nine tenths of a solar mass, but it is clear that these stars return a lot of the material they started with back into space.

Born-again stars

It seems that this is not quite the end of the road for all white dwarfs. Several types of star just don't fit this picture. Nearly a century ago, FG Sge was a faint blue star similar to a hot white dwarf or planetary nebula central star,. Today it is a luminous red giant. Another star called Sakurai's object has had a similar rebirth within the last three years! It seems that nuclear reactions can be switch back on for a short time, so that they are born again. This is only a temporary respite. The white dwarf state always beckons.

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