R Aquilae

Lore & Background

R Aquilae was discovered by Friedrich Wilhelm Argelander, who announced it on May 20, 1856, after observations at Bonn Observatory since 1854. Initially unnamed, by October 1856 it was referred to as R Aquilae in the astronomical literature. The star is an aging red giant on the asymptotic giant branch, with a stellar classification that varies between M5e and M9e, the 'e' suffix indicating emission features. Cooler spectral types occur near minimum visual magnitude, and hotter near maximum. The star may have recently undergone a helium flash. It is oxygen-rich, with the same mass as the Sun but expanded to times the Sun's radius, radiating 3,470 times the Sun's luminosity from a photosphere at about 2,800 K. It is losing mass at a rate of (6–35) × 10⁻⁷ solar masses per year, forming a dusty silicate shell.

Reader's Guide

R Aquilae's significance lies in its well-documented secular changes. When first observed, its period was over 300 days, but it has declined steadily from 320 days in 1915 to 264 days in 2010, an average rate of 0.4 days per year. The amplitude of its brightness variation has also decreased by about a magnitude since discovery. These trends make it a key object for studying the evolution of Mira variables, particularly the effects of thermal pulses and mass loss on the asymptotic giant branch. Its peak magnitude of 5.3 is bright enough for naked-eye visibility as a dim, red-hued star. The star's discovery by Argelander in 1856 and its subsequent monitoring provide a long baseline for understanding period changes in Mira stars. The presence of a dusty silicate shell and its oxygen-rich composition further inform models of dust formation and mass loss in evolved stars.

Did You Know?

• R Aquilae's period has decreased from 320 days in 1915 to 264 days in 2010, at an average rate of 0.4 days per year.
• The star's spectral type varies between M5e and M9e, with cooler types near minimum brightness.