Meet Almaaz, the Billy Goat Star – Sky & Telescope

Physical Characteristics

Physical Characteristics

The leftover debris fields of planet formation accompanies some stars, surrounding them in loosely organized collections of dust fragments and asteroids of shattered protoplanets. Vega is famous for its large debris field, and even the Sun has two minor fields, including the main asteroid belt between the orbits of Mars and Jupiter as well as the Kuiper Belt beyond Neptune’s orbit. But for sheer grandeur, the debris field orbiting the companion of Epsilon Aurigae — also named Almaaz — is certainly among the most impressive.

Almaaz is an eclipsing binary — a type of star pair in which one member periodically passes in front of the other, temporarily causing a change in the system’s brightness as viewed from Earth. The star Algol, which predictably dips in brightness every three days, is probably the most well-known example of an eclipsing binary.

But the story is different with Almaaz, in which an evolved (large but diffuse) supergiant star and a smaller but heavier B-class companion mutually orbit each other. In this case, the two stars are separated far enough to make their orbital period — and thus the time between eclipses — take 27 years. But when the eclipse does occur, it does so in spectacular form, taking more than 700 days to complete!

For decades, astronomers puzzled over the nature of Almaaz’s dimming and tried to envision a scenario that would explain a two-year eclipse. They suspected that the supergiant is eclipsed not so much by its stellar companion, but by the field of rocks and dust surrounding the secondary star.

The time to test that theory came during the 2009–2011 eclipse. The CHARA telescope array was able to image the dark material crossing the face of Almaaz during the event. (The next opportunity for such imagery won’t occur until 2036.) One study placed the dust cloud’s diameter at 7.6 astronomical units (1 au is the average Earth-Sun distance), with a thickness of about ¾ au. This is a debris field of truly magnificent proportions.

In that study, the masses of the stars were also found to be lower than expected, with the supergiantstar only about twice as massive as the Sun, and its companion at about 6 solar masses. However, a later study made clear that these estimates are still uncertain.

Origin of Almaaz / Mythology

Almaaz is the brightest member of a small triangle of stars, an asterism known fondly as “the Kids” — the offspring of nearby Capella, the she-goat. Even though we recognize three stars in the asterism, it may have originally been thought that only the two fainter members were actually Capella’s kids, and not Almaaz. However,the name Almaaz is Arabic for ­he-goat, or billy goat, so we’re left to wonder about the connection between the etymologies. Is Almaaz the father of Capella’s kids? Or is this little buck one of the litter?

The constellation Auriga as a whole is intended to depict Erichthonius, a legendary Greek king who invented the four-horse chariot in myth. But the goat (“caprine”) associations with the star Capella were also quite strong prior to the formation of the constellation. As a result, most artistic interpretations of Auriga combine the two ideas into a peculiar amalgamation where the charioteer is holding his driving equipment in one arm and carrying the doe Capella and her baby goats — including the baby buck Almaaz — in the other.

You’ll find the concept of Auriga and chariots in ancient China as well. Those astronomers took the brightest stars in that area of the sky to form the Five Chariots. Almaaz and the neighboring stars of the Kids were known as “the Pillars,” which offered a place to tether the chariot horses when not in use.

How to See Almaaz

The star is an easy find, partly because its constellation Auriga is in that northern section of the sky where there are lots of landmarks. Capella is a bright star near Almaaz, so it makes a great beacon, twinkling rapidly when it’s near the horizon. But if you’re unsure which star is Capella, go find the Big Dipper instead. The two stars that make up the top of the Dipper’s bowl (these are Dubhe and Megrez) can be used as pointers to Capella (though it’s quite a jump). Then if you keep going in the same straight line for another brief distance, the next bright star in the line is Almaaz.

Almaaz isn’t circumpolar for all of North America, so it does sink below the horizon for some locations, but in general if you wait long enough you should be able to see it at some point during the night most times of the year — unlike zodiacal constellations that are hidden in the Sun’s light for months at a time. And if you patiently wait until 2036, you’ll be able to watch Almaaz dim as the companion’s debris field blocks the star’s light.

Daniel Johnson is a Wisconsin-based freelance writer and professional photographer and the co-author of over a dozen books. He’s a longtime amateur astronomer and fortunate enough to live in a rural region with excellent seeing conditions. You can view some of Dan’s photography (he does a lot of animals!) at www.foxhillphoto.comMythology