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.

Death of Stars

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

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.)

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 Msun) 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

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

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA