QUESTION:
How do astronomers determine the age of a star?

ANSWER by Scott Sandford on November 11,1995:
Now... on to the question!  Astronomers can estimate the age of a star 
in a number of ways.  The accuracy of the age determination depends 
on the type of star and the technique used.

Stars near the beginnings and endings of their lives are relatively easy 
to date.  Obviously stars that have just formed from ice, dust, and gas 
in dense interstellar clouds are very young.  This youth is often given 
away by the star's association with the dust cloud from which it was 
born.  Stars that are so old that they are beginning to exhaust their 
nuclear fusion fuel supply are generally also fairly easy to date 
because we know reasonably well how fast different types of stars 
burn their fuel.  Very large stars burn their fuel at an enormous rate 
and quickly exhaust their supply.  Exceptionally large stars can burn 
up all their available fuel in as little as 10 million years.  Smaller stars 
burn their fuel at much more modest rates and consequently last much 
longer.  Thus, when we see smaller stars (like our Sun) in the final 
stages of their lives, we know they are billions or tens of billions of 
years old, the exact age depending on the exact size of the star.

The biggest difficulty comes in trying to determine the ages of stars 
that are in the middle of their normal life spans.  During this period of 
their lives stars tend to be very stable and do not change much with 
time, so it is more difficult to date them.  During this portion of their 
evolution the stars are said to "be on the Main Sequence."  One of the 
ways you can determine the age of stars on the main sequence is to 
look at how quickly they are spinning.  Most stars spin and their rate 
of spin decreases as they age.  For example, our Sun spins on its axis 
once every 30 days or so.  In comparison, the star Epsilon Eridani, 
which is a nearby star similar to our Sun, but younger, spins on its 
axis once very 11.1 days.

There are several other ways to date stars.  Sometimes it is difficult to 
date an individual star but you can infer its age by looking at its 
neighbors.  Some stars belong to star clusters, groups of stars that 
were born together from the same dust cloud and that travel together.  
In these cases, if you can determine the age of the cluster you have 
also narrowed down the ages of all the stars in the cluster.

One special example of determining the age of a star has to do with 
our own Sun.  Our Sun and all the planets, comets, asteroids, etc. that 
orbit it were formed at essentially the same time.  This means that if 
we can determine the age of the oldest rocks in our Solar System we 
will also have determined the age of the Sun.  Fortunately, we have 
very old rocks that date back to the beginning of our Solar System, 
namely meteorites.  The ages of meteorites can be determined using 
'clocks' based on the decay of radioactive elements.  At this point 
scientists have determined the ages of many, many meteorites and the 
oldest ages always turn out to be about 4.55 billion years.  Thus, in 
this special case we can state with a high degree of assurance that our 
star, the Sun, is presently 4.55 billion years old.  That's old enough 
that the candles on the Sun's birthday cake might rival it for 
brightness!