Determining Solar Age
How do we know the sun's age?
There are two important ways that the age of a star, such as our
Sun, can be determined. The first and more well known process is by
observing other stars and making a kind of tally as to its mass,
brightness, chemical composition, etc. From so many observations, a
"stellar evolution" pattern can be developed.
For example, since we understand that the main source of solar
energy is hydrogen fusion, it follows that a young star should have
quite a bit of hydrogen in it, whereas an older star would have more
helium (the product of hydrogen fusion). If we then arrange the
observed stars in terms of how much hydrogen, helium, and other
chemicals it might have, we also see other trends - such as stars
with originally about the same mass as the Sun tend to become red
dwarfs when most of its hydrogen gets used up.
So, in the first process, we make a very strong estimate of the age
of a star by observing a lot of other stars and using that data to
develop a "life-cycle" of stars. Then all we have to do is determine
where in that cycle is our own Sun and that tells us its age.
In the second, newer, process, we use our knowledge of radioactivity
and the patterns of production and development of radioactive
materials (we know for example that a sample of C-14 will become
half its current concentration after 5730 years) to get an estimate
about the age of objects in our solar system. It turns out that the
oldest known objects have the same approximate age as that of the
best estimates of the Sun's age.
So these two processes come up to the same conclusion that the Sun
is about 4.5 billion years old.
Greg (Roberto Gregorius)
We can estimate the Sun's age by studying its behavior. It takes
about 5 billion years for a star like ours to behave as our Sun
does. If it were much younger, it would be much hotter; cooler as
it gets much older. We can also compare the Sun to other stars like it.
Studying the Sun's spectrum also gives us an idea of its age.
Hope this helps!
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Update: June 2012