R Apodis

Lore & Background

HD 131109 was the first star observed to be variable in the constellation Apus, discovered in 1873 by Benjamin Apthorp Gould. It was hastily given the variable star designation R Apodis in a 1907 variable star catalogue despite being only a suspected variable at the time. Observations from a 1952 field star survey revealed that R Apodis was not variable at all. Keenan & Pitts (1980) found it varied between magnitudes 5.5 and 6.1, but this was never confirmed. Hipparcos photometric data showed it had constant brightness, and it has since been listed as class CST: in the General Catalog of Variable Stars.

R Apodis has a stellar classification of K4 III:, indicating an evolved K-type giant that has ceased hydrogen fusion at its core and left the main sequence, though there is uncertainty about the luminosity class. It has a comparable mass to the Sun at 1.1 solar masses but, at an age of 5.68 billion years, has expanded to 23 times the Sun's radius. It radiates 293 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,318 K. The star is metal deficient, with an iron abundance roughly half the Sun's, and spins slowly with a projected rotational velocity lower than 1.3 km/s. Parallax measurements imply a distance of 413 light-years, and it is drifting closer with a heliocentric radial velocity of −31.2 km/s. At its current distance, its brightness is diminished by interstellar extinction of 0.26 magnitudes, giving it an absolute magnitude of −0.22.

Reader's Guide

R Apodis holds a unique place in variable star history as the first star observed to be variable in the constellation Apus, discovered in 1873 by Benjamin Apthorp Gould. Its designation as R Apodis was assigned hastily in a 1907 catalogue despite only suspected variability. The star's variable status has been repeatedly questioned: a 1952 survey found no variability, Keenan & Pitts (1980) reported variation between magnitudes 5.5 and 6.1 but this was never confirmed, and Hipparcos photometric data ultimately showed constant brightness. Consequently, it is now listed as class CST: in the General Catalog of Variable Stars, indicating a suspected but unconfirmed variable. This history illustrates the challenges of early variable star classification and the importance of long-term photometric monitoring. Physically, R Apodis is an evolved K-type giant with uncertain luminosity class, having expanded to 23 solar radii after leaving the main sequence. Its metal deficiency and slow rotation are consistent with an older stellar population. The star's proximity (413 light-years) and measurable radial velocity (−31.2 km/s) make it a useful object for studies of galactic kinematics and stellar evolution.

Did You Know?

• Despite its variable star designation, Hipparcos photometric data revealed that R Apodis has constant brightness.
• R Apodis has expanded to 23 times the Sun's radius despite having only 1.1 solar masses.