(by Amara Graps)
Dating the Sun is an indirect process. There are several independent ways of estimating the age and they all give nearly the same answer: about 5 billion years.
The age of the Sun can be estimated from the ages obtained from radioactive dating of the oldest meteorites. This may seem odd at first, but in fact it is extremely likely that the solar system (i.e. th Sun, planets, asteroids etc.) formed as one unit. Therefore the age of the Sun should be close to the age of the meteorites, which can be found using the method of radioactive dating.
G.J. Wasserburg obtained a meteoritic age of (4.57 +/- 0.01) x 10^9 years and D.B. Guenther (1989, Astrophysical Journal 339, 1156) estimated that hydrogen burning started shortly thereafer (40 million (0.04 +/- .01) x 10^9 years later).
Additional evidence comes from the Earth. The oldest Earth rocks are also about 4.6 billion years old. The oldest fossils, found in Australia, are about 3.5 bilion years old. The presence of fossils in rocks indicates that the Earth was a suitable place for life when the fossils formed. This implies that the Sun was luminous at that time. [Of course we can't say exactly how long before the fossil formed the Sun was like it is today, but it does give us a lower bound.]
What is meant by "luminous?" We mean that the Sun was at or near the stable part of its lifetime called the "main sequence" more than 3.6 billion years ago. Viewing the Sun as a star on the main sequence, is very useful and important for astronomers because they have a model called "The Standard Solar Model" that views the Sun at stages in its life while it is burning hydrogen and converting that to helium. The model can be run forward and backward in time, and the astronomers can check the observable quantities in the model like luminosity, solar radius, composition, solar p-mode frequencies, and so on with our real Sun. They can stop the model at any time during its main sequence. If what we see from our Sun matches the quantities in the model for a specific age, then we have one more piece of information of what we think that the age of the Sun is.
One complication of checking the Solar Model with our real Sun is the quantity of helium: the "helium abundance." That is rather difficult to obtain. According to the Dalsgaard article (see below), the solar spectrum is too complicated to accurately measure the helium abundance, so that one parameter has to be estimated (one infers the helium abundance by matching the observed solar radius and luminosity in the solar models). It turns out this affects the estimated age very little.
Two useful references for more detailed discussion:
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An article entitled
"The Current State of Solar Modeling"
by J. Christensen-Dalsgaard et al. in the May 31, 1996 issue of
Science talks about the age
of the Sun and the Standard Solar Model.
Chapter 8: The Origin of the Solar System in the book The Sun As Star by Roger Taylor, Cambridge University Press, 1997 describes the radioactive dating of meteorites and chemical evidence from the formation of the solar system.
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