Lighthouse Stars in Distant Galaxies express
In distant galaxies, old stars swell and redden. Among them are two groups that act like lighthouses: some have just ignited helium burning, while others are already burning carbon and oxygen. Using the Hubble Space Telescope, astronomers looked into 92 galaxies and compared the brightness of these two types of stars.
It turns out that stars on the asymptotic giant branch are, on average, 0.589 magnitudes brighter than stars at the tip of the red-giant branch. It's like having a ruler with two marks, and the distance between them is always the same — in any galaxy, regardless of dust and distance. Now, by measuring the brightness of such a star, you can calculate how far away its galaxy is.
This opens the way to measuring distances to galaxies that were previously too faint for such methods. Already, archival Hubble data allow finding such lighthouse stars in galaxies up to 9 megaparsecs away — about 30 million light-years.
Original abstract
as written by the authors arXiv:2602.14304
Hubble Space Telescope (HST) observations of 92 galaxies that have a strong showing of I-band Asymptotic Giant Branch (IAGB) stars in their color-magnitude diagrams (CMDs), are used to measure the relative offset between the mean apparent I-band magnitudes of the IAGB population and the corresponding apparent I-band magnitudes of the TRGB as measured in the same frames (and CMDs) of those individual galaxies. This first exploratory, large-sample comparison is independent of any extinction (foreground or internal) that may be shared by these two populations. The marginalized luminosity functions used to determine the modal value of the {\it IAGB } population are well fit by a single, symmetric Gaussian. The difference in the two apparent magnitudes (in the sense IAGB minus TRGB) is -0.589 mag, with a combined standard deviation of +/- 0.119 mag. Adopting M_I = -4.05 mag for the TRGB stars, the modal absolute magnitude of the IAGB is then calculated to be M_I(IAGB) = -4.64 +/- 0.12 mag. The ensemble dispersion quoted above gives a standard error on the mean of +/- 0.012 mag (based on the full sample of 92 galaxies). Independently, the three geometry-based zero points for I-band AGB stars are found (in Paper I) to be M_I = -4.49 +/- 0.003~mag in the LMC (4204 stars), M_I = -4.67 +/- 0.008 mag, for the SMC (916 stars) and M_I = -4.78 +/- 0.030 mag for NGC4258 (62 stars), leading to a global zero-point (weighted) average of
Stars on the asymptotic giant branch pulsate with periods of hundreds of days, and their brightness changes like a slowly blinking lightbulb.
In the novel 'Light of Distant Galaxies,' astronauts use such stars as natural navigation buoys to stay on course in intergalactic space.