Astronomy 162: Professor Barbara Ryden

Tuesday, January 28

RED GIANTS


``Computers are useless. They can only give you answers.'' - Pablo Picasso

Key Concepts


(1) When a star exhausts the hydrogen in its core, it becomes a giant or supergiant.

Once a star has used up all the hydrogen in its core, fusion of hydrogen into helium stops. The core starts to contract again (just as it contracted as a protostar, before hydrogen fusion began). As the core contracts, it releases energy. This energy heats up the layer immediately above the contracting helium core.

The layer immediately above the core becomes hot enough to initiate the fusion of hydrogen into helium.

The star now has THREE main layers:

These swollen stars, no longer on the main sequence, are now
giants (if M < 8 Msun) or
supergiants (if M > 8 Msun).

A giant's outer hydrogen envelope cools as it expands. A giant becomes very large in radius and very cool -- hence the name RED GIANT, commonly applied to giant stars.


The layered structure of a red giant looks something like this:

When the Sun becomes a red giant, about 5 billion years from now, its radius will increase to nearly 100 times its present size (engulfing Mercury as it expands!) and its surface temperature will drop as low as 3000 Kelvin (from its present value of 5800 Kelvin).

In a red giant a huge, cool, low-density hydrogen envelope (with a density of about 0.1 kilograms/m3) encloses a small, hot, high-density helium core (with a density of about 1,000 tons/m3).


(2) Supergiants and giants with M > 0.4 Msun become hot enough to fuse helium into carbon.

The helium core is not effectively heated by the fusion shell lying above it (energy tends to flow outward, to the cool surface, remember). The core continues to shrink, becoming denser and hotter as it is compressed by gravity.

Once the central temperature reaches T > 100,000,000 Kelvin, helium can fuse into carbon - a new energy source. Why must the temperature be so high? Because the electric charge of a helium nucleus is twice that of a hydrogen nucleus, it takes higher velocities (and hence higher temperatures) to overcome the electrostatic repulsion and bring helium nuclei together.

Only stars with M > 0.4 Msun become hot enough for fusion of helium to occur. Lower mass stars, once they eventually leave the main sequence, are fated to end up as spheres of helium.
Helium is fused into carbon by the triple alpha process. Just as a hydrogen nucleus has the alias ``proton'', a helium nucleus has an alias as well -- nuclear physicists refer to helium nuclei as ``alpha particles''. Thus, the fusion process which combines three helium nuclei, or alpha particles, into one carbon nucleus is called the triple alpha process.

Step one of the triple alpha process:
4He + 4He --> 8Be

Step two:
8Be + 4He --> 12C + gamma ray photon

Sometimes there is an additional reaction:
12C + 4He --> 16O + gamma ray photon

Thus, the triple alpha process mainly produces carbon (C), but sometimes it ``overshoots'' and produces oxygen (0). The beryllium (Be) that is produced in step one is merely an intermediate.


Cosmic mind-boggling fact for the day:

All the carbon and oxygen in your body was manufactured inside a star.


Once a giant or supergiant begins to fuse helium in its core, it has FOUR main layers:

(3) Supergiants and giants with M > 4 Msun become hot enough to fuse carbon into heavier elements.

Now the carbon-oxygen core continues to shrink, becoming denser and hotter as it is compressed by gravity. Once the central temperature reaches T > 600,000,000 Kelvin, carbon & oxygen can fuse into heavier elements, such as silicon, sulfur, and iron - a new energy source.

Only stars with M > 4 Msun become hot enough for fusion of carbon & oxygen to occur. Stars with 0.4 Msun < M < 4 Msun are fated to end up as spheres of carbon & oxygen. (This will be the fate of the Sun, for instance, which will never become hot enough to fuse into more massive elements.)

IRON is the end of the line where fusion is concerned. The iron nucleus (containing 26 protons) is the most tightly bound of all nuclei. The fusion of iron nuclei ABSORBS energy, instead of EMITTING energy.

Although each fusion process taking you from hydrogen to iron emits energy, each step, as the nuclei become more massive, is less efficient. For instance, fusing 1 kilogram of hydrogen into helium yields 650 trillion joules. Fusing 1 kilogram of helium into carbon by the triple alpha process yields only 60 trillion joules, less than 10 percent as much. Thus, as the star's core becomes hotter, and the fusion reactions powering it become less efficient, each new fusion fuel is used up in a shorter time.

For instance, consider the stages in the life of a 25 Msun star:

The star's core is now pure iron: end of the line as far as fusion is concerned.

A star's lifetime as a giant or supergiant is shorter than its main sequence lifetime (about 1/10 as long). Thus, giants are supergiants are rare compared to main sequence stars of the same mass.

Eventually, every star runs out of fuel for fusion. The energy content of the star drops as it continues to radiate photons into space. Pressure drops in the core. Eventually, one of two things must happen:

(1) The star finds an alternative pressure source to maintain hydrostatic equilibrium. (This means a pressure source which doesn't rely on the random thermal motions of atoms and ions.)
OR
(2) The star collapses into a black hole.


Prof. Barbara Ryden (ryden@astronomy.ohio-state.edu)

Updated: 2003 Jan 29

Copyright 2003, Barbara Ryden