Surface brightness fluctuation (SBF) is a secondary distance indicator used to estimate distances to galaxies. The technique uses the fact that galaxies are made up of a finite number of stars. The number of stars in any small patch of the galaxy will vary from point to point, creating a noise-like fluctuation in the surface brightness distribution. While the various stars present in a galaxy will cover an enormous range in luminosity, the SBF can be characterized as if all stars had the same brightness, which is the luminosity-weighted integral over the luminosity distribution of stars. Older elliptical galaxies have fairly consistent stellar populations, thus the typical ``fluctuation star" closely approximates a standard candle. In practice, corrections are required to account for variations in age or metallicity from galaxy to galaxy.

The SBF pattern is measured from the power spectrum of the residuals left behind from a deep galaxy image after a smooth model of the galaxy has been subtracted. The SBF pattern is evident as the transform of the point spread function

Point spread function

The point spread function describes the response of an imaging system to a point source or point object. A more general term for the PSF is a system's impulse response, the PSF being the impulse response of a focused optical system. The PSF in many contexts can be thought of as the extended blob...

 in the Fourier domain. The amplitude of the spectrum gives the luminosity of the fluctuation star. Because the technique depends on a precise understanding of the image structure of the galaxy, extraneous sources such as globular cluster

Globular cluster

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...

s and background galaxies must be excluded from the analysis. Areas of interstellar dust absorption must also be excluded. In practice this means that SBF works best for elliptical galaxies or the bulges of S0 galaxies, as spiral galaxies generally have complex morphologies and extensive dust features.

SBF is calibrated by use of fiducial Cepheid Period-Luminosity relation (P-L) based on observations of variables

Cepheid variable

A Cepheid is a member of a class of very luminous variable stars. The strong direct relationship between a Cepheid variable's luminosity and pulsation period, secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales.Cepheid...

 located in the Large Magellanic Cloud

Large Magellanic Cloud

The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...

.

SBF is an indicator that uses stars in the old stellar populations (Population II).