25. The scorecard

As a concluding summary we return to the list proffered at the beginning of the article as criteria that a successful extragalactic distance indicator might aspire to, and try to assess the TRGB in that context.

Luminous:

Indeed the luminosity generated by the helium core flash is enormous, being comparable to the instantaneous output of the rest of galaxy. Sadly, the duration of this burst is only a few seconds, and even more tragically, virtually all of that energy is absorbed internally within the star as it restructures itself en route to quiescent helium core burning on the horizontal branch. Nevertheless, the luminosity at which the upward evolution of the red giant star is terminated by core ignition is still quite luminous by stellar standards. In the I band the TRGB magnitude is as bright as a 5-day Cepheid variable.

Dispersionless:

By extragalactic standards, it is fair to say that any distance indicator that can claim a precision of ±0.20 mag (or 10% in distance) qualifies as being an excellent distance indicator. The TRGB method boasts an externally defined dispersion of less than ± 0.1 mag, comparable to Cepheid distance moduli.

Calibrated:

For the metallicity range -2.2 < [Fe/H] < -0.7 dex, the I-band color-magnitude diagrams for Galactic globular clusters calibrate the zero point of the TRGB method to the same level that the RR Lyrae distance scale is now known. As the RR Lyrae zero point is improved, the TRGB zero point will also get better with time.

Understood:

To first order, nuclear physics determines the bolometric luminosity at which all low-mass stars terminate.

Insensitive:

Theory predicts (and observations confirm) that the bolometric (I-band) magnitude is insensitive to chemical composition variations in the range -2.2 < [Fe/H] < -0.7 dex, and for ages in the calculated range 7-17 Gyr.

Abundant:

The halos of all galaxies are synonymous with Population II stars. If there is a measurable surface brightness defining these halos then the red giant branch stars must be present in abundance.

Ubiquitous:

Every galaxy (elliptical, spiral or irregular) must have had a first generation of stars. The low-mass and low-metallicity members of that generation of stars must now be populating the red giant branch, and defining the tip.

Unambiguous:

At the color and approximate magnitude level expected for TRGB stars other populations of stars do little more than add noise to the distinctive discontinuity in the red-giant-branch luminosity function. However, care should be taken that the TRGB is truly triggering the edge detector by having a fair sample of luminosity function well sampled below the tip before incompleteness in the photometry sets in. All too often that may prove to be a luxury in limiting applications, but the warning still stands.