The life and death of stars.

The most obvious part of the Galaxy is stars. Stars contribute to the dynamics of the Galaxy in a way that is interwoven with the gas and dust.

For this course, we don't look at stars in a great deal of detail, but concentrate on a few important points.

Astronomers can't see the inside of stars (except, to some extent, with neutrinos) so they have to use models of what goes on.
- The models are based on gravity, gas dynamics, nuclear physics, etc.
- The models make predictions that can then be checked against what we observe on the outside of stars.
Stars are much hotter and denser in their cores than on the outside.
For most of their lives, their energy source is the fusion of hydrogen nuclei to make helium nuclei.
- Here is what the fuel situation looks like for the Sun.
When the hydrogen runs out in the core, the core shrinks and gets hotter.
Other processes that provide energy can then occur.
- Helium nuclei fuse to make carbon.
- In the biggest stars, even heavier elements can be made, up to iron.
- Here is a picture of what is happening in a very heavy star near the end of its life.
Astronomers name types of stars by their surface temperature: from hottest to coolest the names are OBAFGKM.
The life history of a star depends on its mass.
- The heaviest stars are very hot (O and B) and use up their fuel quickly.
- Medium weight stars like the Sun (G) are not as hot and last a long time.
- The lightest stars are rather cool (K and M). Their fuel lasts a very long time.
To help remember this, visit the star store.
Some protostars are too small to get hot enough to turn into real stars. They become brown dwarfs.

Death of Stars

Sooner or later, stars run out of hydrogen in their cores.

For very heavy stars, it's sooner.
For very light stars, it's later.

After using up the helium in its core, a star burns helium in its core to make carbon. What happens after this depends on the mass of the star.

Light Stars

In a low mass star in its last stage, with a helium burning shell and a carbon core, stellar modelling suggests that the helium burning shell becomes unstable, and burns in bursts. This causes the star to eject much of its outer layers.

The result is a ``planetary nebula'' which may last for 50,000 years before it dissapates into space.

Left behind in the middle is a white dwarf star, small and hot, but with no more nuclear reactions. It starts to cool (but it cools very slowly).

Here is some evidence. (Colors in general aren't exactly real in these pictures.)

The Egg Nebula, where this process seems to be going on now; note the shells of ejected gas. It appears that light gets out near the poles of the star and is reflected from dust.
M57. The ring nebula.
Here are six of them.
NGC6543, the Cat's Eye Nebula, a quite complex planetary nebula, possibly associated with a dying member of a binary system.

Death of big stars and supernovae

In the case of heavier stars, the star makes heavier elements, up to iron, as we have seen.

Consider a massive star like Betelgeuse. This is the one star that has a large enough size and is near enough (~100 pc) to us that we can see it as a disk with the Hubble Space telescope:
Stars that start out very massive as O and B stars don't live very long. In their later stages, they lose a lot of mass through strong ``stellar winds.'' Then, they explode. the explosion is called a Type II supernova.

An exploding massive star (say M ~ 20 M_sun) produces a very large energy release. Some of this comes in the form of visible light, which lasts for several days and then fades away over a period of weeks. If it is in our galaxy and not obscured by dust, a supernova can be recognized as a ``new star.''

A supernova leaves behind gas expanding through space, a ``supernova remnant.''

The most famous example is a supernova in the constellation Taurus that exploded on 4 July 1054. (Actually 1054 minus the 6500 year light travel time to Earth.) Observations of this supernova were recorded by Chinese and Japanese astronomers and also (possibly) in an Anasazi rock painting in New Mexico. Europeans seem not to have paid much attention. The left over remnant is called the Crab Nebula (M1).

The filaments are expanding. Doppler shift measurements of the rate of expansion are consistent with an explosion date around 1100 AD.

There was a supernova in the Large Magellanic Cloud in 1987. Astronomers learned a lot from that. The explosion has illuminated gas lost from the earlier red supergiant star.

Here is a small part of another supernova remnant, the Cygnus Loop. Here is another, N123D in the Large Magellenic Cloud.

The shock waves from supernovae explosions are thought to play a role in triggering star formation by compressing interstellar gas.

The gas/star cycle and the production of heavy elements

Low mass stars last a long time (> 10 x 10⁹ yr) so they do not really contribute to this cycle.
Some heavy elements are made in medium mass stars and spread through the Galaxy by planetary nebulae. The medium mass stars last a few billion years, so they can contribute some.
Heavy elements, especially the heaviest elements, are made in the big massive stars and spread through the galaxy by strong stellar winds and then by supernovae. This happens on a rather short time scale, just a few million years for the heaviest stars.
- Some of the heavy element production is in nuclear fusion before the supernova explosion.
- Additional heavy element production (including elements beyond iron) occur during the explosion, as the shock wave heats the stellar material.

The result should be that the galaxy has been building up its supply of heavy elements. Note that

the Earth is made of heavy elements, iron, silicon, oxygen,...;
you are made mostly of carbon, oxygen and hydrogen;
so presumably the Earth and we are made of stuff that was manufactured in the middle of a star.

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu