Location of Cepheid variable stars on the HR diagram. Notice Cepheids are luminous stars (up to 104 Lsuns), making them useful for extragalactic distance estamates.
A Cepheid in M100, measured as part of the HST key project.
Period Luminosity relation for Cepheids. As expected, more luminous (larger) stars have longer periods.
Two illustrative light curves. The lower image shows a double exposure of a star field ("blink comparator"). The Cepheid is easily identified.
Figure from Seeds