January 14, 2008


Today, I'll talk about another northern constellation that is a part of the Winter Circle, Auriga the Charioteer. Allen strongly believes that this is one of the old constellations, like Taurus, that originated with the earliest civilizations of the fertile crescent. And like Taurus, the basic story hasn't changed much since then (though a few less common variations still exist).

Auriga is a chariot rider in the various myths (though in some he has lost his chariot, and he is not usually pictured in one now), holding a little she-goat and two goat kids. Even the Chinese mythology refers to the classic pentagon of stars (with one substitution, Alnath instead of Delta) as the "Five Chariots". In the Greek myths, the rider is the son of Hephaestus, and though he is lame, he is taught many skills by Athene, including how to tame horses and ride a chariot. Where the goats come from is not clear.

The brightest star is Capella, which translates to "little she-goat", and it is at the left shoulder of the rider, who is facing us. This is the 6th brightest star in the sky, and it is about 42 light years away. The main body of the figure is a slightly off-kilter pentagon much like the Ethiopian king, Cepheus. Capella is a very close non-eclipsing binary system with two giant-class yellow stars at a distance comparable to the Earth-Sun distance. These were once massive Sirius-like stars that are now evolving to become red giants. There is also a binary pair of cool dwarf stars orbiting a great distance from the central pair.

Burnham describes the scale of this system: If you shrink Capella A and B down to spheres about a foot in diameter, separated by 10 feet, then the two orbiting dwarf stars would each by about an inch in diameter, 420 feet apart, 21 miles away from the central pair. Beta Aurigae is Menkalinan, the right shoulder of the rider, another close binary system with two nearly identical blue main sequence stars in close orbit around one another, partially eclipsing each other about every two days. Also like Capella, a dim red dwarf star orbits the central binary at a great distance.

Gamma Aurigae is Alnath, shared with Taurus, which I've talked about previously. Delta is a fainter multiple star system representing the head of the rider. A famous asterism within this constellation is formed by Epsilon (Al Anz), Eta (Hoedus II) and Zeta (Hoedus I), the skinny triangle is a group of goats known collectively as "the kids". Epsilon (closest to Capella) and Zeta (the rightmost of the three if Cepheus is seen upright) are both eclipsing binaries, and Epsilon has long been a real puzzle to Astronomers.

This is a very widely separated system. Eclipses happen about every 27 years and last for 2 years. That's a LONG eclipse! What kind of gigantic companion could possibly cause such an eclipse? At first, we though the central star was gigantic, bigger than any other star known, but more data made us realize it can't be that big. The eclipsing companion must be somehow extended.

Perhaps the eclipsing star is surrounding by a large dusty disk, and it takes a long time for the disk to pass the central star. In fact, the central star gets slightly brighter near the midpoint of the eclipse, an indication that perhaps the dusty disk has cleared out a little bit close to the eclipsing star. The next eclipse of this system is scheduled to begin in August 2009, and it will be closely watched by professional and amateur Astronomers undoubtedly.

Rounding out the bottom of the pentagon are Theta (a strongly magnetic hot blue A star) and Iota (also known as Hassaleh). These two stars along with Alnath in Taurus form a triangle that contains all of the really neat looking deep sky objects in Auriga. The Milky Way also cuts through this region. AE Aurigae is a bright, hot main sequence O-type star and one of the "runaway stars" from the Orion Nebula region that I discussed when I was talking about the constellation Orion. Though intrinsically bright, this star is very far away (about 1500 light years) and obscured by intervening dust, so it is near 6th magnitude on average in the night sky, barely visible to the human eye under ideal conditions.

To find it, go in the direction opposite the skinny triangle of the kids until you're halfway to Alnath (the brighter of the two stars at the end of Taurus' horns). Extrapolating its motion back in time indicates that it left the Orion Nebula region about 2.7 million years ago, probably an ejected member of a multiple star system or a binary that was disturbed by another passing star. Surrounding this star is the beautiful Flaming Star Nebula, a cloud of gas and dust that AE Aurigae is passing through and lighting up much like the Pleiades system. As Burnham notes, you can actually see the wake of this star as it has moved from left to right through this field, pushing gas out of its way.

NGC 1931 and IC 417 form a nebula pair known as the Spider and the Fly. Both are regions where Hydrogen gas has been excited by nearby hot stars and some light from these stars is reflected in surrounding dust. There are three Messier objects clustered together in this region. M36 is also called the Pinwheel Cluster for its spiral pattern of bright stars. It is similar in age to the Pleiades but much further away (nearly 10 times further) and so much fainter. Nearby is M37, which Burnham considers the finest of the three for small telescopes. It is very rich in stars, with over 150 crowded into a small area and at least 500 in total, and it is somewhat older than M36 with a few bright red giants that stand out.

M38 is the third of the open clusters here. These types of clusters are fairly common in the galactic disk (as opposed to globular clusters, which are more common in the halo). Because they are immersed in so much surrounding material, these clusters are more likely to be spread out and ultimately disrupted by tidal forces over time. As such, any clusters that are still together are usually very young compared to our own Sun. Sometimes, groups of bright stars such as the Sco-Cen association or the Ursa Major moving group can have their motions traced back to the point where we realized they were once part of an open cluster as well.

Posted by Observer at January 14, 2008 03:10 PM

Comments on entries can only be made in pop-up windows while those entries are still on the main index page. Sorry for the inconvenience this causes, but this blocks about 99.99% of the spam the blog receives.

I thought arugula was a salad plant...

Posted by: Humbaba on January 14, 2008 04:04 PM

M37 is definitely well worth observing if folks get the chance to hunt it down, and is easy to find by starhopping. I spent a long time looking at this rich cluster last weekend and it's really spectacular.

Another great write-up!


Posted by: Phil on January 15, 2008 11:05 PM