Light curve is a term for a graph used in astronomy when the light output of an object (a variable star, a supernova, or an AGN) is measured and plotted against time. The light curve is plotted in Cartesian coordinates, with time on the X-axis and flux or magnitude on the Y-axis. Light curves are like an electrocardiogram, taking the pulse of the sky1.

Different classes of objects will have distinctive light curves relating to their physical properties. Eclipsing variable stars will show very regularly-spaced dips in flux as the stars eclipse one another. Some pulsating stars will show very regular peaks and troughs; others, like white dwarfs may have what appear to be chaotic light curves, only to be revealed as superpositions of hundreds of independent pulsation periods by Fourier analysis. Accreting stars and active galaxies with accretion disks, may have truly chaotic or quasi-chaotic light curves, sometimes showing excess power at long periods (called red noise), sometimes showing brief moments of quasi-periodic variability (often called quasi-periodic oscillations or QPOs). Burst sources, flare stars, and supernovae may show sudden, dramatic changes in flux which decay over time.

Light curves were first (and still can be) measured visually, with the observer peering through a telescope. They estimate the brightness of an object based upon pre-determined "comparison stars" of known, (hopefully) fixed brightnesses. Later, light curves could be measured photographically. Photographs were later replaced by photomultiplier tubes and strip chart recorders, and still later by CCD cameras and computers. With the development of other branches of astronomy -- radio astronomy, X-ray astronomy,and gamma ray astronomy -- light curves are no longer limited to the optical spectrum, and many sources are known to be variable at high and low energies.

1The clever people on the Rossi X-Ray Timing Explorer (XTE) project actually thought up this comparison first -- I have a promo poster of the XTE satellite with Taking the Pulse of the Universe! in bold letters across the top.

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