Photometry is the measurement of the intensity of light from celestial objects. It is one of the oldest techniques in astronomy. Even before telescopes, astronomers estimated the brightness of stars by eye. Today, photometry uses sensitive detectors and filters to measure light with extraordinary precision.

The technique is simple in principle. Point a telescope at an object. Measure how much light reaches the detector. Compare that to a standard star of known brightness. Repeat through different filters to get a spectrum of brightness across wavelengths. The result is a set of magnitudes or fluxes that describe the object's color and energy output.

Photometry is used for many purposes. It tracks variable stars, measures the light curves of supernovae, detects transiting exoplanets, and monitors the brightness of quasars. It is also used to classify stars by their color, which correlates with temperature. The Sloan Digital Sky Survey and the Kepler mission are examples of large photometric surveys.

Modern photometry is automated. Robotic telescopes observe thousands of objects per night, and software pipelines reduce the data. The challenge is no longer collecting photons. It is making sense of the flood of data.