Life on the Main Sequence

Once a star is born, it spends its active life somewhere on the main sequence. Its place on the main sequence is determined by its mass.

Astronomers make computer models of the evolution of stars.

For a star like the Sun .

According to this kind of stellar model, here is where we are now


(The concentrations here are percentages by mass, rather than by numbers of nuclei.)

The same sort of model can be applied to stars with masses that are smaller or larger than that of the Sun.

Evidence for astronomers' model of stars

Astronomers have a model of stars as giant nuclear reactors that burn hydrogen to make helium. This model is backed up with detailed computer models based on physical principles.

One piece of evidence for this picture is that the stellar models do a good job of predicting the existence of the main sequence and where it should be on the HR diagram.

Another kind of evidence comes from looking at clusters of stars. Here is a sketch of what one sees for the HR diagram of a cluster of the type known as a globular cluster.

The evidence from this diagram is that it is about 10 billion years old.

Types of main sequence stars

What about protostars that don't quite make it?

Old age for stars like the Sun.

Old age for stars in general.

Variable stars.

ASTR 122 course home page

Updated 5 December 2007

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