Globular clusters are vast spherical collections of hundreds of thousands of stars, all very tightly packed together. They appear as fuzzy, roundish spots in the sky, and are found orbiting around the Milky Way. Observations have shown that most of the stars within a cluster are formed at about the same time. This means that by determining the age of the globular cluster, we also discover the age of its member stars.
The Hertzsprung-Russell diagram is used to derive methods of determining the age of individual stars. The HR diagram plots the brightness (or absolute brightness, luminosity) of a star against its surface temperature (temperature of its outer atmosphere) or spectral type. The H-R diagram also shows the evolutionary paths that star follow as they progress from the main sequence through the stages to becoming white dwarfs. The H-R diagram can also be used to date star clusters. All the stars within a cluster will be at the same stage of evolution, so if we plot their luminosities against their spectral types we obtain the H-R diagram for that cluster. Since some sequences of the diagram depend on the age of the cluster, their presence (or absence) can be used to determine the age.
Open clusters also provide a method for determining the age of individual stars. Stars in an open cluster are formed as part of the same molecular cloud. Because the stars are formed from the same material and at the same time, they start their lives on the main sequence portion of the Hertzsprung-Russell diagram at the same place. Stars burn hydrogen into helium in their cores, and this burns the hydrogen and moves the star to the right on the H-R diagram. The rate of progression depends on the mass of the star. More massive stars move to the right on the H-R diagram more quickly than lower-mass stars. When lower mass stars leave the main sequence, the most massive stars in an open cluster already are red giants or even white dwarfs. In addition, lower mass stars are fainter than higher mass stars, so they become harder to detect as more time passes. As a result, the H-R diagram for any open cluster will contain relatively few low-mass stars. The amount of time after the stars form before the low-mass stars begin to leave the main sequence depends on the cluster's age, so astronomers can determine the age of the cluster by looking at the H-R diagram. By determining the age of an open cluster, astronomers can also determine the ages of the stars within the cluster.
A star's age can be estimated from its position on the Hertzsprung-Russell (H-R) diagram. The H-R diagram is a plot of a star's luminosity (brightness) versus its surface temperature. Stars evolve over time, and their position on the H-R diagram changes as they age. By comparing a star's position on the H-R diagram to the positions of other stars of known age, astronomers can estimate the star's age.
Another way to estimate a star's age is to look at its rotation rate. Stars that are still young rotate very quickly. As stars age, their rotation rates slow down. By measuring a star's rotation rate, astronomers can estimate its age.
Finally, astronomers can also estimate a star's age by looking at its chemical composition. Stars that are still young have a higher abundance of certain elements, such as lithium, than stars that are old. By measuring the abundance of certain elements in a star, astronomers can estimate its age.
By using these methods, astronomers can estimate the ages of stars with varying degrees of accuracy. The most accurate method is to use the H-R diagram, but this method can only be used for stars that are relatively close to the Sun. For stars that are farther away, astronomers must use other methods, such as measuring the rotation rate or chemical composition.
