UU Aurigae

Lore & Background

John Birmingham observed the star from 1871 to 1875 and detected its variability. Thomas William Backhouse confirmed it as a variable star in 1905 based on observations from 1894 to 1904, and it received its variable star designation in 1912. UU Aurigae is a carbon-rich asymptotic giant branch star. Its spectral type in the General Catalogue of Variable Stars is C5,3-C7,4(N3), where the N3 refers to an older classification system for carbon stars, and the C5 to C7 indices correspond to temperatures approximately equivalent to early-M stars. The second numeric index (3 or 4) indicates the strength of Swan bands on a scale of 1 to 5. A more modern revised Morgan-Keenan scheme gives a spectral type of C-N5- C2 6-, with C-N5 indicating an N-type carbon star with a temperature index of 5- and a Swan band strength of 6- on a scale of 1 to 8.

Classified as a semiregular variable of type SRb, UU Aurigae has poorly defined variations. Its visual apparent magnitude varies from +4.9 to +7.0. The GCVS lists a period of 441 days, but a strong variation with a period of 235 days is also present. Using British Astronomical Association observations from 1971 to 1998, the periods are calculated as 439.4 and 233.1 days. The angular diameter has been measured at 12.07 ± 0.22 mas using very-long-baseline interferometry. Surrounding the star is a shell of dust composed largely of amorphous carbon and silicon carbide (SiC), with SiC appearing at three times the star's radius and amorphous carbon at nine times its radius. Further out lies a carbon-rich shell at 300 stellar radii and two oxygen-rich shells even farther away. UU Aurigae also possesses a bow shock 0.14 parsec wide, created by its motion through the interstellar medium.

Reader's Guide

UU Aurigae is significant as a carbon-rich asymptotic giant branch star, a type that provides insight into the late stages of stellar evolution and the production of carbon and other elements. Its variability, confirmed through early observations by Birmingham and Backhouse, places it among the semiregular variables of type SRb, a class of giant stars with poorly defined brightness changes. The star's dual periods—441 days (or 439.4 days from later data) and 235 days (or 233.1 days)—highlight the complex pulsation behavior common in such evolved stars. The detailed measurements of its angular diameter via very-long-baseline interferometry and the detection of multiple dust shells (amorphous carbon, silicon carbide, carbon-rich, and oxygen-rich) at various radii reveal a rich circumstellar environment shaped by mass loss. The presence of a bow shock 0.14 parsec wide further indicates the star's interaction with the interstellar medium. These features make UU Aurigae a valuable object for studying nucleosynthesis, dust formation, and the mass-loss processes that enrich the interstellar medium with carbonaceous material.

Did You Know?

• John Birmingham detected UU Aurigae's variability from 1871 to 1875.
• Its angular diameter is 12.07 ± 0.22 mas as measured by VLBI.
• The star has a bow shock 0.14 parsec wide from motion through the interstellar medium.

Ancient Roots and Shifting Identities

The stars we now recognize as Auriga carry some of the oldest astronomical records in human history. In ancient Mesopotamia, they were catalogued under the name GAM, a figure representing a scimitar or crook, sometimes called Gamlum or MUL.GAM in the MUL.APIN tablets. The crook symbolized a goat-herd or shepherd, and the grouping included nearly all the bright stars of the modern constellation, with only Elnath left out, as that star was traditionally shared with Taurus. When the tradition reached Greece, the identity of the figure shifted dramatically. The most common identification is Erichthonius, the chthonic son of Hephaestus raised by Athena, who supposedly invented the four-horse chariot and defeated the usurper Amphictyon to become king of Athens. Zeus honored his ingenuity by placing his Sun-like chariot among the stars. Yet other traditions saw Myrtilus, Hermes's son and charioteer for Oenomaus, or even Phaethon, the ill-fated son of Helios who lost control of the solar chariot. These layered narratives suggest the constellation served as a canvas onto which Greek culture projected its deep reverence for the chariot.

A Treasury of Stellar Phenomena

Auriga's position near the winter Milky Way makes it a region of exceptional richness for stargazers. Its brightest star, Capella, is not a single point of light but an unusual multiple star system that ranks among the brightest objects in the night sky. The constellation also hosts Beta Aurigae, described as an interesting variable star, and Epsilon Aurigae, a nearby eclipsing binary whose unusually long period has made it a subject of intensive study. Beyond individual stars, the region contains three celebrated open clusters—M36, M37, and M38—that are popular targets for amateur astronomers. A single prominent nebula, the Flaming Star Nebula, adds another dimension to the deep-sky catalog, its glow associated with the variable star AE Aurigae. Together, these objects give Auriga a remarkable diversity of targets, from multiple stars and variable systems to clustered star fields and glowing nebular gas, all concentrated within a single constellation's borders.

The Goat, the Kids, and a Merged Legacy

The story of Capella and its neighboring stars is one of astronomical consolidation. In earlier Greek tradition, Capella was sometimes treated as its own constellation, known by names like Capra, Caper, or Hircus—each reflecting its identity as the goat star. The two smaller stars Zeta and Eta Aurigae, first called the Kids by the ancient astronomer Cleostratus, formed a separate asterism. The mythological backdrop was the she-goat Amalthea, who nursed the infant Zeus, and in some tellings her Gorgon-like hide was skinned to become Zeus's aegis, a detail credited with contributing to the Titans' defeat. It was Ptolemy who, in his 2nd-century Almagest, merged the Charioteer and the Goats into a single constellation, a decision that shaped the modern sky. Meanwhile, in Chinese mythology, Auriga's brightest stars were woven into figures representing the celestial emperors' chariots, while Bedouin astronomers saw a herd of goats, each star a separate animal. The constellation also serves as the radiant for several meteor showers, including the Aurigids, Zeta Aurigids, and Delta Aurigids, plus the hypothesized Iota Aurigids.

Scale, Season, and the Art of Depiction

Auriga is a substantial presence in the northern celestial hemisphere, spanning 657 square degrees—roughly half the area of Hydra, the largest of all 88 modern constellations. Because of its northern declination, the full figure is visible only to observers as far south as 34 degrees south latitude; beyond that, parts of it sink below the horizon. Its seasonal prominence peaks during winter evenings in the northern Hemisphere, where it shares the sky with five other constellations whose stars form the Winter Hexagon asterism. The constellation's visual identity has long been captured in illustrations, though these have been inconsistent across centuries. The standard depiction shows a charioteer holding a goat over his left shoulder, two kids tucked under his left arm, and reins in his right hand—though some artists rendered those reins as a whip. Capella's position over the left shoulder and the Kids beneath the left arm have remained remarkably stable across these artistic traditions. The 1488 Hyginus atlas represents one early printed attempt to fix this imagery, and the constellation's inclusion in Ptolemy's list of 48 ancient figures cemented its place in the Western astronomical canon for nearly two millennia.