Variable stars are stars that vary in brightness, or magnitude. There are many different types of variable stars. One group of variable stars is the pulsating variables. These stars expand and contract in a repeating cycle of size changes. The change in size can be observed as a change in apparent brightness (apparent magnitude.) Cepheid variables are one type of pulsating variable stars. Cepheids have a repeating cycle of change that is periodic - as regular as the beating of a heart. Observations of the changes in apparent magnitude of variable stars - including Cepheids - are plotted as the apparent magnitude versus time, usually in Julian Date (JD). The resulting graph is called a light curve.
The light curve for the Cepheid variable star X Cyg (located in the constellation Cygnus) is shown below. Each data point represents one observation. Once many observations have been plotted, important information can be obtained from the resulting pattern of changing magnitudes. The period for X Cyg is the amount of time it takes for the star to go through one complete cycle from maximum magnitude (brightness), through minimum magnitude (dimmest), and back to maximum magnitude (brightness.)
Analysis of the light curve for X Cyg shows that the magnitude ranges from an average maximum magnitude of 6.0 to an average minimum magnitude of 7.0 with a period of approximately 16 days. X Cyg exhibits periodic behavior - it is a Cepheid variable star with a predictable cycle of changing magnitudes, a stellar heart that beats once every 16 days.