Two brightness readings through coloured filters can reveal a star's temperature
Measure a star through a blue filter, then a yellow-green one, and subtract. That single number, the colour index, tells astronomers how hot the star's surface is. For 51 Pegasi it works out to roughly 5,768 kelvin. Photometry, the careful measurement of light, quietly underpins much of what astronomy knows.
The name joins the Greek for light and measure. A photometer on a telescope collects light, passes it through a filter and records how much arrives. Early instruments, photoelectric photometers, turned light into an electric current and measured one object at a time. CCD cameras, effectively grids of tiny photometers, have largely replaced them because they capture many objects at once, although photoelectric devices still earn their keep where very fine timing matters. The best can detect changes of around 0.001 magnitude.
To compare results between observatories, astronomers agree on sets of filters called photometric systems, among them UBV, its extension UBVRI, the near-infrared JHK and the Strömgren system. Capital letters label magnitudes from standard instruments, such as V; lower-case letters mark older estimates by eye or photographic plate, which is why a sixth-magnitude star might appear as 6.0V, 6.0v or 6.0p and the numbers need not match. Strömgren filters have the extra advantage of helping correct for dust that reddens starlight.
Colour is where physics enters. The difference between blue and visual magnitudes, B minus V, tracks temperature. For 51 Pegasi, 6.16 minus 5.46 gives +0.70, a yellow star consistent with its G-type spectrum. Plotting many stars' brightness against colour gives the colour-magnitude diagram, the observed version of the Hertzsprung-Russell diagram, which in an open cluster can indicate its age.
Brightness combined with the inverse-square law yields luminosity when distance is known, or distance when luminosity is. Tracking light over time produces light curves, which reveal the orbits and sizes of eclipsing binary stars, the spin of asteroids, the energy of supernovae and the dimming caused by planets crossing their stars. Researchers choose among differential photometry, which compares nearby objects and is usually the most precise, relative photometry, and absolute photometry on a standard scale, which is the hardest to do well and ideally needs a clear sky.
Source: Photometry (astronomy)
