The students are given a copy of HR diagram with isochrones
(file isochrones.ps), and 3 datasets (age1, age2 and age3).
The isochrones on the diagram are labelled with numbers indicating
age in millions of years.
Their job is to mark (T,L) points on the diagram and estimate
the age of the cluster for each data set.

The file isochrones_solutions.ps contains the HR diagram
with all data points marked on it. 
Age1, age2 and age3 are 8, 200 and 3000 millions of years.