Determining Ages of Star Clusters Using Color Magnitude Diagrams
________________________________________
The “Determining Cluster Ages Using CMDs” pre-lab exercise is a
prerequisite for this exercise. It should be tu
...
Determining Ages of Star Clusters Using Color Magnitude Diagrams
________________________________________
The “Determining Cluster Ages Using CMDs” pre-lab exercise is a
prerequisite for this exercise. It should be turned in before beginning.
Lab Exercise
Learning Goals
Describe what is meant by a color-magnitude diagram (CMD).
List the information that can be inferred from a CMD.
Itemize the assumptions made when working with a CMD.
Qualitatively state the relationship between the color of a mainsequence star and its main-sequence lifetime.
Explain the fundamental basis for determining the age of a cluster given the cluster’s “turnoff” B – V.
Determine cluster ages using their color-magnitude diagrams.
Summarize the complete process of how astronomers estimate the ages of star clusters.
Introduction
Importance of determining the ages of star clusters
Clusters are groupings of stars that were all born at roughly the same time. When we speak of the
age of a cluster, we are equivalently asking the ages of the stars.
So why do we care how old clusters are? The age of a cluster can provide clues to the formation
of galaxies and therefore the Universe. For example, stars in globular clusters appear to be the
oldest in our galaxy. We can then infer that globular clusters were some of the first structures to
evolve during the formation of our galaxy.
Effect of cluster distance
If one star in the sky looks brighter than another, can we say that star is more luminous?
When we look up to the sky at night, we see stars of varying brightness; that is, their apparent
magnitudes are different. This difference is caused in part by the varying luminosities of the
stars. But it could also be caused by the distance the star is from Earth. For example, the Sun
appears brighter to us than Vega, but Vega is more than 50 times as luminous. Thus, we usually
cannot use apparent magnitude of a star as an estimate of its luminosity.
In a cluster of stars, however, we can use the apparent magnitude of its constituent stars as a
relative measure of their luminosities. Since a cluster is localized to one relatively small volume
of space, all of the stars within a cluster are roughly at the same distance from Earth. Because the
stars are at roughly the same distance, differences in apparent magnitudes are due only to
differences in the stars’ luminosities.
Adapted from University of Washington
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