AAVSO: What is a Light Curve

Observations of variable stars are commonly plotted on a graph called a light curve, as the apparent brightness (magnitude) versus time, usually in Julian Date (JD). The magnitude scale is plotted so that brightness increases as you go from bottom to top on the Y-axis and the JD increases as you go from left to right on the X- axis.

Information about the periodic behavior of stars, the orbital period of eclipsing binaries, or the degree of regularity (or irregularity) of stellar eruptions, can be directly determined from the light curve. More detailed analysis of the light curve allows astronomers to calculate such information as the masses or sizes of stars. Several years or decades of observational data can reveal the changing period of a star, which could be a signal of a change in the structure of the star.

Phase Diagrams

Phase diagrams (also known as "folded light curves") are a useful tool for studying the behavior of periodic stars such as Cepheid variables and eclipsing binaries. In a phase diagram, multiple cycles of brightness variation are superimposed on each other. Instead of plotting magnitude versus JD as with a regular light curve, each observation is plotted as a function of "how far into the cycle" it is. For most variable stars, a cycle starts at maximum brightness (phase=0), runs through minimum and back to maximum again (phase=l ). With eclipsing binary stars, phase zero occurs at mid-eclipse (minimum).

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