Description

This simulation illustrates Henrietta Leavitt's measurement of the variability of Cepheid Variable stars in the Small Magellanic Cloud. The left view shows a photographic image of a star field in the SMC. One of the star is a variable. Playing the simulation (or using the step buttons to change the time in one day increments) shows that this star changes in its apparent size in the photograph. The size corresponds to the star's apparent magnitude, and a magnitude measurement tool (red circle) can be positioned so that it is centered on the star and the Apparent Magnitude slider can be adjusted until the circle matches the star's size. This gives a measurement of the star's apparent magnitude at that time. The simulation can be used to measure the period of variability as well as maximum and minimum apparent magnitudes in much the same way that Leavitt would have done using real photographs of the SMC.

The right view shows a plot of the apparent magnitude of the variable star as a function of time, with time taken modulo some trial period. The value of the trial period can be adjusted using the slider control, and max and min values for the trial period can be set to change the range covered by the slider. When the trial period is equal to the variable star's actual period, the data points will all lie close to a single-valued continuous curve with the shape expected for a Cepheid variable light curve.