Like the numerous buds and flowers that populate the springtime landscape,
so are the galaxies that make up the Virgo cluster, an enormous cluster of galaxies
that inhabit an area of the constellation Virgo. Virgo also holds the variable star of this season,
W Virginis. W Vir is an exciting yellow giant star that pulsates with a period of about 17 days
and is the prototype of a class of stars known as the Population II Cepheids, also known as the
W Vir variables. In the literature, the term "Cepheids", "Delta Cep stars", "Classical Cepheids",
and "Type I Cepheids" all apply to a group of Cepheids that are physically different from W Vir.
For a review of the Classical Cepheid stars, consult the previous VSOTM articles: Delta Cep,
Zeta Gem, and X Cyg. W Vir is the prototype of the Type II Cepheids, a separate group of stars,
which were the cause of some trouble at the turn of the century. In the process of determining the
extragalactic distance scale, the Population II Cepheids were mistakenly grouped with the
Population I Cepheids and an inaccurate period-luminosity function was determined that
weakened the distance scale. We now know that these are two different types of stars
and that the magnitude differences threw off the distance calibration significantly. W Vir
is an exciting springtime observing target and as a Population II star, is one of the oldest
stars in the Milky Way. These old stars are important to study because they hold clues to stellar and galactic evolution.