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In eclipsing binary systems, one star periodically passes in front of the other as viewed from Earth, producing characteristic dimming in the observed light curve. Analysis of light curves yields orbital period, stellar radii, and orbital inclination; combining with radial velocity measurements gives both stellar masses directly. Eclipsing binaries provide some of the most accurate measurements of stellar radii and masses and are crucial calibrators of the cosmic distance scale.
Most stars in the galaxy exist in binary or multiple-star systems, orbiting a common center of mass. When the orbital plane is nearly edge-on to our line of sight, each star periodically passes in front of — eclipses — its companion. From Earth, we cannot resolve the two stars separately (they appear as a single point of light), but we can detect the eclipses because the combined brightness drops when one star blocks the other's light. A plot of this brightness over time is called a light curve, and its shape encodes a remarkable amount of physical information.
A typical eclipsing binary light curve shows two dips per orbit. The primary eclipse (deeper dip) occurs when the hotter, brighter star is blocked by its companion; the secondary eclipse (shallower dip) occurs when the cooler star is blocked. The depth of each dip depends on the relative surface brightnesses and sizes of the two stars. If a small, hot star passes behind a large, cool star, the primary eclipse is deep because a large fraction of the system's total light is blocked. The width of each dip tells you how long the eclipse lasts, which depends on the stellar radii relative to the orbital separation and the orbital velocity.
From the light curve alone you can extract the orbital period (the time between successive primary eclipses), the ratio of stellar radii (from the eclipse durations), and the orbital inclination (from the shape of the eclipse ingress and egress — how sharply the brightness drops). If you also have radial velocity measurements from spectroscopy — the Doppler shifts of each star's spectral lines as they orbit — you can determine the actual orbital velocities. Combining period and velocities through Kepler's laws gives you the orbital separation in physical units, and from there you can calculate both stellar masses directly. This is one of the only methods that yields stellar masses without relying on models or assumptions.
Eclipsing binaries are therefore among the most important calibration tools in astronomy. They provide model-independent measurements of stellar mass, radius, and (combined with photometry in different filters) temperature. These measurements anchor the mass-luminosity relationship, test stellar evolution models, and serve as distance indicators. The study of eclipsing binaries illustrates a broader principle in astronomy: since we cannot visit stars, we extract physical properties by carefully analyzing how their light changes over time and wavelength.