photometry
/ foh-TOM-uh-tree /
Strip astronomy down to one basic question and it is often this: how bright is that object, exactly? Photometry is the craft of measuring the brightness of stars and galaxies precisely — turning a smudge of light on a detector into a hard number. It sounds humble, but tracking how bright things are, and how that changes, underlies an enormous range of discoveries.
In practice, photometry means counting the photons collected from a source. On a CCD image an astronomer adds up the charge in all the pixels covering a star, subtracts the background glow of the sky, and corrects for the detector and the atmosphere, to get the star's flux. That flux is usually expressed as a magnitude — the ancient logarithmic brightness scale where smaller numbers mean brighter and a difference of five magnitudes means a factor of 100 in brightness. Comparing a star's brightness through different filters yields its colour, a clue to its temperature.
Precise photometry is the engine behind much of modern astrophysics. Watching a star dim by a fraction of a percent reveals a planet crossing in front of it; measuring how a Cepheid or a Type Ia supernova brightens and fades lets us use it as a standard candle to gauge cosmic distances; tracking the flicker of a variable star or the slow fade of a supernova maps out stellar physics. Whenever the question is 'how much light, and how is it changing?', the answer comes from photometry.
NASA's Kepler mission found thousands of planets purely by photometry — measuring tiny, periodic dips in starlight as little as a few hundred parts per million when a planet transited its star.
Brightness, measured to a fraction of a percent.
Photometry measures brightness, not distance. A bright star may be nearby and dim, or far and luminous — you can only convert apparent brightness to true luminosity if you know the distance independently.