Determining the age
of a star is not as easy as you might think. Since we can't ask a star how old
it is, we have to guess the stars age by its appearance. And just like with
humans sometimes looks can be deceiving!
There are many ways
to determine the age of a star, and today we will discuss stellar models. Like
we've discussed before, stars can be placed on an Hertzsprung-Russel (HR)
diagram. To do this, you need to measure the stars brightness, or luminosity,
and you also need to know what type of star it is. Is it a big, hot blue star,?
A cool, small, red? Somewhere in between? Astronomers can determine this by
looking at a star's spectrum, or distribution of light, with a telescope. Once
we know these two things, we can place the star at the proper position on the
HR diagram. Astronomers have been hard at work modeling how stars form, and how
their size, temperature, and brightness changes as they age. They have
developed paths or lines that are placed on the HR diagram which show a stars
path on the graph as it ages. There are models for before the star has reached
the main sequence, and after. Basically what you do, is place the star on the
HR diagram, see which line it is closest too, and that tells you the stars size
and age. Here is an example of how this works. The graph above shows brightness
vs. temperature, and models (solid lines) for stars of different masses. Stars,
in theory, follow one solid line path going right to left as it ages. The star represents the spot
on the diagram where some arbitrary star's properties are. Based on its
position, the star is probably about 4 times the mass of the sun, and about
200,000 years old! This is before it has started hydrogen burning, and is still
a "baby" star. You can follow the same method with different models
and estimate the age of a star that is burning hydrogen, or on its way towards
death.
