Cepheid Variable Period-Luminosity

What you are seeing: a Type I Cepheid variable star pulsating with period $P$ . Left: the star (size + color from radius + $T_{\rm eff}$ ). Middle: the V-band lightcurve. Right: the Leavitt Law $M_V = -2.78 \log_{10} P - 1.35$ with the current star and known Galactic calibrators

Figure 1. Pulsating Cepheid + V-band lightcurve + Leavitt period-luminosity diagram. Method: kinematic radial pulsation R(t), PL fit.

period P (days)10.0

animation speed2

presetzG

distance (pc)360

WHAT TO TRY

Step through the Cepheid presets: longer-period stars are intrinsically brighter, the Leavitt period-luminosity law. The mean absolute magnitude M_V in the readout tracks the period.
Watch the star pulsate: it swells and cools then shrinks and heats, and the light curve traces the characteristic sawtooth. The radius and T_eff readouts oscillate together.
Set the distance: because the period fixes the true luminosity, comparing it to the apparent brightness yields the distance, the rung that calibrates the cosmic distance ladder.