HAEDUS
The Kid goat of the Charioteer — a blue star barely forty million years old, burning five and a half suns at once, riding on the shoulder of Auriga.
Haedus: the Kid
Haedus is Latin for “kid” — a young goat — and the name is exactly as literal as it sounds. In the sky, the Charioteer Auriga carries a she-goat across his shoulder; that goat is Capella, the sixth-brightest star in the night sky. Clustered just beneath her are two fainter companions, her offspring: the Haedi, the Kids. Eta Aurigae is one of them, and on 30 June 2017 the International Astronomical Union's Working Group on Star Names formalised the ancient label, adopting Haedus for η Aurigae and Saclateni for ζ Aurigae.
Earlier catalogues called it Haedus II or Hoedus II, since Zeta Aurigae held the primary "Kid" position. The star also carried the rarer name Mahasim — from the Arabic al-miʽşam, “the wrist” — a leftover from an age when the pair of stars marked the charioteer's rein-hand rather than a goat's young. In Chinese astronomy it was 柱三, Zhǔ sān, the Third Star of Pillars, a ladder-like grouping within the Net mansion of the White Tiger of the West.
A furnace in the winter sky
Haedus is a single main-sequence star of class B3 V: a blue-white hydrogen burner with a temperament far fiercer than the Sun's, and a life spent at full gallop.
Bars show Haedus relative to the Sun on a compressed scale. Its projected rotational velocity of 95 km/s turns the whole star once every 1.8 days — the Sun takes about 25.
A standard candle of its own trade
Since 1943, the spectrum of Eta Aurigae has served as one of the stable anchor points of the MK system — the reference library against which astronomers decide what any other B star looks like. When you read that a star is “B3 V,” you are really reading “it looks like Haedus.”
It is also a slowly pulsating B star and chemically peculiar: its surface breathes in and out with a primary period of 1.29 days, detected cleanly in TESS photometry, while magnetic fields sort its atmosphere into patches of unusual abundance. That same 1.29-day rhythm is close to, but distinct from, the 1.8-day spin of the star's body — a beating and a turning happening at once.
Coordinates & motion
Right ascension 05h 06m 30.9s, declination +41° 14′ 04″ (J2000). Hipparcos parallax of 13.40 ± 0.20 mas places it 243 ± 4 light-years away — at least ten times closer than its neighbouring star Almaaz, which only looks like a companion. It is drifting away from us at +7.3 km/s, and its proper motion carries it across the celestial sphere by about 75 milliarcseconds each year.
The hexagon, the goat, and her two kids
Auriga is a five-sided chariot wheel of first- and second-magnitude stars, with Capella blazing at its apex. Haedus and Saclateni form a tiny pair just below Almaaz — the Kids suckling at the goat's side. Hover the stars.
What the ancients saw
Amalthea
Capella is the she-goat Amalthea, who nursed the infant Zeus in a Cretan cave and hid him from his father Cronus. In gratitude he set her among the stars — and her two young went with her. Haedus is one of that litter.
The Charioteer
Auriga is variously Erichthonius, the legendary king who first harnessed four horses to a car, or Myrtilus, or a humble goatherd. Whoever he is, he is drawn carrying the goat overhead — and the Kids dangle from the crook of his arm like a second, smaller constellation.
Zhǔ sān
In the Chinese system the star is the Third of Pillars, a mast-like grouping inside the Net asterism of the White Tiger of the West — a wholly separate taxonomy laid over the same handful of photons.
Al-miʽşam
Arabic star lore read this patch of sky as the charioteer's wrist rather than a nursery. The derivative name Mahasim lingered in European catalogues for centuries before the goat won.
A constellation of its own
The Goat and the Kids were once counted as a separate constellation from the Charioteer. Ptolemy folded them into Auriga in the 2nd century, where they have remained — a small family pressed into a larger figure.
A winter sign
Capella and her Kids rise high by February, anchoring the Winter Hexagon alongside Rigel, Sirius, Procyon, Pollux and Aldebaran — the brightest ring of stars in the northern year.
Forty million years, and counting fast
Ignition
Haedus condenses from a collapsing molecular cloud, its core reaching the densities needed to fuse hydrogen. For a star this hot, the process is not a gentle glow but a torrent: it fuses roughly 1,450 times the Sun's output and burns through its fuel in a geological instant.
Main sequence, class B3 V
Stable hydrogen burning at 18,660 K gives the star its blue-white cast. It spins once every 1.8 days, slightly oblate, while its surface pulses on a 1.29-day cycle. It is a spectral standard — the reference image of what a B3 V star is.
Red giant phase
Hydrogen in the core runs out. The star swells and cools toward the red, engulfing whatever close companions may orbit it, and begins fusing helium in a hotter core.
Planetary nebula, then a white dwarf
Haedus is not massive enough to die as a supernova. Instead it sheds its outer layers into a glowing shell of gas and leaves behind an Earth-sized white dwarf — a fading ember that will cool, in darkness, for longer than the present age of the universe.
How to find Haedus
- Face north-east in the northern winter. Auriga culminates high in the evening sky around February, and the whole constellation is visible from latitudes north of 40° S.
- Spot Capella. Nothing nearby competes with it: magnitude 0.08, golden-white, the brightest star for a huge stretch of the northern sky.
- Follow the hexagon. Capella, Menkalinan, Mahasim, Elnath, Hassaleh and Almaaz close a great irregular wheel. Haedus sits inside it, near the Almaaz corner.
- Look for the little triangle. Almaaz with the two fainter stars below it — Saclateni and Haedus — make a tiny three-pointed figure. The brighter of the two lower stars is Haedus.
- Binoculars help. At magnitude 3.18 it is plainly naked-eye from a dark site, but ten times its colour contrast improves sharply in 10×50 glasses. No resolved companion awaits: it is a single star.
Observing notes
February, high in the evening
North of 40° S latitude
Yes — 3.18 mag, easy
Blue-white, clearly blue in a small scope
Messier 36, 37 and 38 open clusters; the Flaming Star Nebula IC 405