Within this use case you recognize a physical association of stars
close in space as opposed to a superpositon created by projection
effects of stars very far one from the others. The key measure is
distance derived from parallax. With the true members of the
association (open cluster) you create a Herzsprung-Russell diagram as
in the tutorial “the stars” (ivo://edu.gavo.org/eurovo/aida_stars).
The Herzsprung-Russell diagram of stellar clusters is very important
because of the low noise of few unrelated stars.
If used in the classroom this advanced use case requires a reasonable
understanding of histograms and bidimensional scatter diagrams. It is
also rather long to perform from beginning to end.
