Test of Stellar Structure and Evolution

Testing our ideas about the structure and evolution of stars is very difficult because:

• we cannot see into the interiors of stars (except by using neutrinos and helioseismology [stellar seismology]--techniques which are difficult to stars other than the Sun).
• stellar evolution takes an exceedingly long amount of time and so we cannot watch individual stars run through their lifetimes -- we take snapshots of many stars and try to piece together based on the snapshots.

• Stars and clusters are produced in the Galaxy today and were also among the first objects produced in the Galaxy. As such they offer us glimpses of the stellar evolution process which covers timespans ranging from a few million years (the Pleiades) to more than 14 billion years (the Globular Clusters) and snapshots of stars of all masses (presumably).

• The way the studies are performed as to predict the way in the Hertzsprung-Russell diagrams for clusters of various ages should appear (through theoretical modeling). A typical result can be found in the text (Chapter 21, page 390).

• Given such predictions, clusters of various ages can be observed in order to check on the validity of the theory.

• page 374 Figure 20-13
• page 375 Figure 20-14
• page 391 Figure 21-5 and 21-6

Ages of Clusters

Using the notion that we understand stellar structure then allows us to estimate the ages of clusters of stars. The basic notion is that massive stars evolve faster than low mass stars and so, old clusters will not show hot Main Sequence stars (page 392 Figure 21-7). The hottest Main Sequence stars in the cluster place upper limits on the age of the cluster.

The oldest stellar systems in our Galaxy are the Globular Clusters, however there are also some very old Galactic Clusters (M67). Here is a typical HR diagram for a globular cluster. The point where the stars leave the Main Sequence is known as the turn-off point and is an indicator of the rough age of the cluster. The oldest globular clusters in our Galaxy are estimated to have lifetimes of 14 - 16 billion years.

This is an amusing result because independent measurements of the age of the Universe (based on the standard model for the Universe) give ages of less than 12 billion years and more likely closer to 8 billion years (if some other theoretical prejudices are to be believed).

Hmmm, it appears that the oldest stars in the Galaxy are older than the Universe. The resolution of this dilemma may come from what Einstein sometimes said was his greatest mistake, that is, from the resurrection of something known as the Cosmological Constant.