S Arae

Lore & Background

The variability of S Arae was first suggested in 1896 by David Gill and Jacobus Kapteyn, who found it 'all but proved' from Cape Carte du Ciel photographic plates. In 1900, Robert T. A. Innes confirmed the star, then known as CPD-49 10361, as a variable. It received its modern variable star designation, S Arae, in Annie Jump Cannon's 1907 Second Catalogue of Variable Stars.

Initially thought to be an eclipsing binary, S Arae was reclassified as a Cepheid variable by Harlow Shapley in 1918. By 1939 it had been recognized as an RR Lyrae variable. It is an RRab type, meaning it pulsates in the fundamental mode and has an asymmetric light curve that rises rapidly to maximum brightness then fades more slowly.

S Arae's large negative declination makes it circumpolar in Antarctica, allowing continuous monitoring during much of the southern hemisphere's winter without gaps from daylight. It was the first star monitored that way at Dome C. Besides the main 10.85-hour pulsation, the Blazhko effect modulates the light curve with a period of 47.264 days, and three other periodicities have been detected.

Reader's Guide

S Arae holds significance as a well-studied RR Lyrae variable that exhibits the Blazhko effect, a phenomenon still not fully understood. Its classification history—from suspected eclipsing binary to Cepheid to RR Lyrae—reflects the evolving understanding of variable star types in the early 20th century. The star's circumpolar nature in Antarctica enabled a pioneering continuous monitoring campaign at Dome C, demonstrating the advantage of polar sites for long, uninterrupted photometric observations. The detection of multiple periodicities beyond the main pulsation and the Blazhko modulation adds to its value as a target for studying the complex dynamics of RR Lyrae stars. Its inclusion in Annie Jump Cannon's 1907 catalogue and its early identification from photographic plates link it to the foundational era of variable star astronomy.

Did You Know?

• S Arae was the first star monitored continuously from Dome C in Antarctica.
• Its Blazhko effect modulation period is about 105 times longer than its main pulsation period.
• The star was originally thought to be an eclipsing binary before being classified as an RR Lyrae variable.
• Three additional periodicities have been detected in its light curve beyond the main pulsation and Blazhko effect.