In astronomy, an isochrone is a theoretical construct used to
determine the age of a star cluster (open clusters and globular clusters
both).
The path that an individual star will follow in the Hertzsprung-Russell
Diagram differs depending upon the mass of the star. The length of a star's
lifetime will also differ, with massive stars only living maybe a few tens
of millions of years, and the lowest-mass stars lasting as long as a hundred
thousand million years.
If we were to look at the H-R diagram of a star
cluster when it is very young, nearly all of the stars would lie on the
zero-age main sequence, the diagonal line running from upper left to
lower right in madvid's diagram under
H-R diagram. As time passes, the more massive
stars (at high temperatures and high luminosities) evolve very
quickly to the
right in the diagram toward cooler temperatures, while the smallest and
coolest stars barely evolve at all. As more time passes, smaller and smaller
stars evolve off the main sequence toward the red giant branch.
Thus, if we were to take snapshots of the distribution of stars in the
H-R diagram, we would see the distribution change over time.
One way to determine the ages of star clusters is to build computer models of
stars with different masses (say 0.1 to 10 solar masses in steps of
0.1 solar masses), and evolve these models in time. At
certain
time intervals (say every five million years), you record the temperature and
luminosity of each model. You then connect the temperatures and luminosities
of the stars with different masses at the same age. This will tell
you what the theoretical Hertzsprung-Russell diagram of a cluster of
stars at a given age should look like. Each snapshot in time is called
an isochrone. By fitting these isochrones to the observed
H-R diagrams of real star clusters, you can estimate the ages of the
clusters.
In reality, isochrone fitting is a tricky science. For one, isochrones depend
not only on the masses of the individual stars, but on their
chemical abundances (the amount and ratios of hydrogen, helium, and
other elements) as well. For another, it
assumes that we fully understand the physics of stellar evolution, and while
we have a very good idea of how stars work, our knowledge is incomplete. It
also requires that we translate our understanding of stellar interiors
(where the evolution occurs) to an observable quantity -- the
stellar spectra of cluster stars.
Finally, this method also assumes that stars are all born at the same time
in a given cluster. This is not a bad approximation, as the period of star
formation in a single star-forming region is at most a few tens of
millions of years. Since most observed clusters are much older than this
anyway, it does not result in a large error.
The science of isochrone fitting first developed around the time when
computer simulations of stellar evolution were first practical
-- in the
late 1950's and early-to-mid 1960's. The first specific reference to this
sort of thing I could find was the paper by P. Demarque and R. Larson
(both then at the University of Toronto) on "The Age of Galactic Cluster
NGC 188" in 1964 (Astrophysical Journal volume 140, 544).
(They computed their models on an IBM 7090!)
Probably the earliest paper to actually discuss the problem of isochrone
fitting in detail was by Allan Sandage and Olin Eggen in 1969
(Astrophysical Journal volume 158, 685).
Isochrones have been an
important topic of study since then, particularly as our theoretical
understanding of stellar evolution has increased. Today, many research
groups compete (in a mostly friendly way) to develop isochrones with improved
treatments of stellar physics (for example, improved treatments of
convection, rotation, opacities, and so on). One important
topic has been the ages of the oldest globular clusters in the universe,
and the age of the universe itself. Improvements in our
understanding of stellar physics have
placed the ages of some of the oldest globular clusters at about
12 gigayears (12,000,000,000 years), in reasonably good agreement with
age of the universe determined by cosmologists.
The papers mentioned above (and many others) may be found
at the NASA ADS abstract service: adsabs.harvard.edu/abstract_service.html