W Virginis variable
A W Virginis variable is a variable star similar to a Cepheid variable. However, using the same formula to calculate its distance will yield a value that is too small, a mistake that Edwin Hubble made while observing M31.
W Virginis stars differ from Cepheids because they are Population II stars and so have a lower metallicity than our Sun. Their luminosity is, on average, less than classical Cepheids (about 1.5 magnitudes). The period varies between 1 and 60 days.
W Virginis stars are also called Type II Cepheids.
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