RT Aurigae

Lore & Background

The variability of RT Aurigae was discovered in 1905 by Thomas Hinsley Astbury, an English schoolmaster and amateur astronomer. It was quickly recognized as a member of the class of Cepheid variables, though the nature of these stars was not understood at that time. Radial velocity changes were detected corresponding to the brightness variations, but the idea that these were caused by stellar pulsations and temperature changes was largely dismissed in favor of orbital motions of a binary star. More accurate observations eventually proved beyond doubt that the brightness variations were caused by pulsations in the atmospheres of the stars, with the stars being smallest and hottest near maximum brightness.

RT Aurigae has been suspected to be a spectroscopic binary system, but this has not been confirmed. The strongest evidence was found in 2013 using CHARA array optical interferometry. The companion would be 6.7 magnitudes fainter than the supergiant primary, cooler and fainter than an F0 main sequence star. The two stars are separated by 2.1 milli-arc seconds.

Reader's Guide

RT Aurigae holds significance as a Classical Cepheid variable, a class of stars whose pulsations and period-luminosity relationship have been fundamental to measuring cosmic distances. Its discovery by an amateur astronomer in 1905 highlights the role of dedicated observers in early variable star research. The star's radial velocity changes, initially misinterpreted as binary motion, were later correctly attributed to atmospheric pulsations, marking a key step in understanding Cepheid variables. The unresolved suspicion of binarity, with evidence from 2013 CHARA interferometry suggesting a faint companion, adds ongoing interest. As a yellow supergiant visible to the naked eye, RT Aurigae remains a useful target for both professional studies of stellar pulsation and amateur monitoring of its regular 3.728309-day cycle.

Did You Know?

• RT Aurigae was discovered to be variable in 1905 by English schoolmaster and amateur astronomer Thomas Hinsley Astbury.
• Its brightness variations were initially thought to be caused by orbital motions of a binary star rather than pulsations.
• In 2013, CHARA array optical interferometry provided the strongest evidence for a possible companion star, 6.7 magnitudes fainter than the primary.

Ancient Roots and Cross-Cultural Identity

Long before European star charts took shape, the stars we now call Auriga were already being woven into meaning. In ancient Mesopotamia, a figure known as GAM—sometimes rendered as Gamlum or MUL.GAM in the MUL.APIN catalog—depicted a scimitar or crook, a symbol tied to shepherds and goat-herds. Whether this referred to the single brilliant star Capella or the broader group of stars is still debated. Centuries later, Bedouin astronomers reimagined the same stellar pattern as a herd of goats, an association that traveled into Greek astronomical tradition. By the time of the 2nd-century astronomer Ptolemy, the constellation had secured its place among the 48 he catalogued, and its identity had shifted toward a charioteer figure, though the shepherd motif lingered. Spanning 657 square degrees—roughly half the area of Hydra, the largest modern constellation—Auriga sits high in the northern sky. Its declination means observers can see it in full only down to about 34 degrees south latitude, beyond which it dips below the horizon. In Chinese star lore, its brightest stars were folded into figures representing the celestial emperors' chariots, showing yet another culture finding meaning in the same pattern.

A Constellation of Charioteers and Tragedies

Auriga's mythological identity is remarkably fluid, with at least four distinct figures claiming the same patch of sky. The most common identification is Erichthonius, the chthonic son of Hephaestus raised by Athena, who is credited with inventing the four-horse chariot and using it to defeat the usurper Amphictyon and claim the Athenian throne. Zeus, impressed by his ingenuity, placed him among the stars. Another candidate is Myrtilus, son of Hermes and charioteer to Oenomaus, whose chariot was destroyed during a suitor's race; after his murder by the victorious Pelops, Hermes elevated him to the heavens. Hippolytus, son of Theseus, also fits the chariot theme—he was killed when his chariot wrecked after refusing his stepmother Phaedra, though Asclepius later revived him. In late antiquity, the poets Claudian and Nonnus saw Phaethon, Helios's son, who lost control of the sun-chariot and was struck down by Zeus. A more macabre reading casts Auriga's stars as the dismembered limbs of Medea's brother, flung into the sea, represented by the Milky Way, during the Argonauts' voyage home. Scholars suggest the Greeks may have simply honored the chariot's central role in their own society.

Stellar Showcases and Deep-Sky Treasures

Because Auriga sits close to the winter band of the Milky Way, it has become a favorite hunting ground for both professional and amateur astronomers. Its brightest star, Capella, is not a single sun but an unusual multiple-star system and ranks among the most luminous points of light visible to the naked eye. Beta Aurigae draws interest as a variable star whose brightness shifts over time, while Epsilon Aurigae has been studied intensively for its status as a nearby eclipsing binary with an unusually long orbital period. Beyond individual stars, the constellation borders three celebrated open clusters—M36, M37, and M38—that are staple targets for backyard telescopes. The Flaming Star Nebula, linked to the variable star AE Aurigae, adds a colorful deep-sky object to the list. Auriga also serves as the radiant point for several meteor showers: the Aurigids, the Zeta Aurigids, the Delta Aurigids, and a hypothesized Iota Aurigids. Together with five other constellations, its stars form the Winter Hexagon asterism, making it a prominent fixture of northern winter evenings.

The Goat, the Kids, and the Charioteer's Pose

Capella's mythological backstory is inseparable from the she-goat Amalthea, who nursed the infant Zeus. Together with Epsilon, Zeta, and Eta Aurigae, Capella forms a small asterism; the latter two stars are traditionally called the Haedi, or Kids. Cleostratus, an ancient Greek astronomer, is credited with first naming them as such. Before Ptolemy's 2nd-century Almagest merged the Charioteer and the Goats into one constellation, Capella was sometimes treated as its own figure—Pliny the Elder and Manilius referred to it as Capra, Caper, or Hircus, all terms for a goat. In one telling, the goat's hideous, Gorgon-like face helped turn the tide against the Titans, and Zeus skinned the animal to wear as his aegis. Traditional star charts depict Auriga as a charioteer holding a goat over his left shoulder, cradling two kids under his left arm, and gripping reins in his right hand. Yet artists have never been entirely consistent: the reins have occasionally been rendered as a whip, and the 1488 Hyginus atlas represents just one of many varying interpretations. Despite these shifts, Capella almost always remains perched on the charioteer's shoulder.