tip of the red-giant branch

The phrase tip of the red-giant branch (abbreviated TRGB) refers to a standard candle consisting of the brightest red-giant branch (RGB) stars observed within a galaxy. RGB stars are presumed to have a maximum luminosity and when viewing a sufficient number within a galaxy, the brightest should be close to that maximum. Using that luminosity, the distance to the RGB stars (and their galaxy) can be determined. The maximum occurs right before the star undergoes a helium flash, after which the star leaves the RGB of the H-R diagram. The galaxy must close enough that RGB stars can be discerned, giving the technique a similar range to that of using Cepheid variables. Having the two methods improves the accuracy and confidence in their reliability because they offer the chance of independent confirmation and calibration, but TRGB may offer some additional range because it is based upon especially bright stars. The RGB stars' spectral energy distributions peak in the near infrared and the method works better with space telescopes since portions of the relevant wavelength range are blocked by the atmosphere. The method has been used with HST and presumably when additional infrared-capable space telescopes come into use, the method will become more common.