Ruprecht 173 & Ruprecht 175 -- Open Clusters in Cygnus
Two Ruprecht Open Clusters and a Cepheid Variable
The star fields of the Swan (Cygnus) are at some locations so complex that they are difficult
to unravel.  The paired Open Clusters Ruprecht 173 and Ruprecht 175 are good examples.  
The milky way star field, in which the clusters reside, blanket the background with countless
stars.  Where the clusters end and the background stars begin cannot be determined visually
but require specific measurements, taken over time of the cluster’s star movements through
space.  In the drawing I have used circles marking the reported boundaries of the two clusters.

At the edge of Ruprecht 173 is a pulsating yellow supergiant star (X Cygni) which was, in part,
responsible for cataloging a class of stars now referred to as Cepheid Variables.  The
significance of Cepheid Variables is that their variable light curve relates to their actual
brightness.  If you know how bright the star is based on its light curve you can then estimate its
distance.  These “light candles” have been used to determine distances even to other galaxies.