RIDING THE CHARIOT
Galloping across northern skies, Auriga
and his chariot fly through the darkness of the northern winter Milky Way. Mixing myths and metaphors, like
winter's St. Nicholas, he -- in the form of his constellation -- is
filled with marvelous gifts for the backyard observer. Though his
story, as a charioteer or wagoner, is mixed (and which are not?),
he is often identified with Erichthonius, the son of Hephaestus
(Vulcan) and Athena.
He's usually portrayed as carrying a she-goat in the form of the
most northerly of the generic "first magnitude" stars, Capella, Alpha Aurigae. The sixth
brightest star in the sky (number three in the northern hemisphere
after Arcturus and Vega), it's actually of magnitude zero (0.08)
and so far north, 46 degrees above the celestial equator, that it's
circumpolar above latitude 44 degrees north, which includes all of
the UK and the vast majority of Canada. Clearly stronger than the
proverbial ox, Auriga not only carries Capella, but her kids that are classically represented by Zeta, and Eta
Aurigae (Haedus I and II), the two southern stars of an
isosceles triangle to the south-southwest of Capella, to which in
more modern times has been added Epsilon
(Almaaz), the northerly one. The trio includes one of the weirdest
objects in the sky.
Along the eastern side of the constellation, Theta, Beta,
Pi, and Delta
Aurigae lie in a line almost exactly along the "solstitial
colure," the great circle that connects the solstices with the celestial poles, the quartet thereby
pointing the way to the North Star much
as do the Pointers of North America's Big
Dipper (the two front stars of the Plough). To the south, the
line leads to the Summer Solstice itself.
Traditionally in Gemini, as a result of
precession (the 26,000-year wobble
of the Earth;'s axis), the Solstice now lies immediately across the
border into Taurus.
On a personal note, as one of the first sky-figures I found as a
child, Auriga epitomizes the propensity of the human mind and eye
to create constellations. To me he was not the Charioteer, but a
springtime gigantic "arrow" pointing to the northwest made of (as
I know now) Capella, Epsilon, and Eta (of the Kids) at the front,
followed by Theta and Beta Aurigae at the rear. The Arrow still
flies nearly 60 years after I "found" it. (My inventions seem to
focus on Auriga. Just to the northwest of Theta is another,
fainter, triangle that is almost the exact reverse of the Kids,
Tau, Nu, and Upsilon making my "Little
Kids.")
Only a bit to the northwest of the Solstice is the Anticenter of the Galaxy, which falls barely
on the Auriga side of the Bull's formal celestial fence. The
Anticenter, as the name implies, is directly opposite the sgr-
t.html">Galactic Center in Sagittarius. While everyone knows about the famed
central three-million-solar-mass Galactic black hole (which is
deeply involved with the year-2012 nonsense), few seem to
appreciate the opposition. Looking outward, away from the Galactic
Center through the thinnest part of the Galaxy, the Milky Way
becomes faint and hard to discern. The effect is exaggerated by
the "Taurus-Auriga dark clouds," among the nearest star-forming regions, which lie a
mere 460 light years away.
On to specifics, beginning with Auriga's luminary. Among the
reasons for Capella's brightness is its small distance of 43 light
years and that it is not one star, but two. Typically separated by
just 0.05 or so seconds of arc, the members of the pair are
therefore very difficult to split apart. Orbiting each other every
104 days at an average distance of 0.7 Astronomical Units -- the
distance between the Sun and Venus -- are magnificent evolved class
G giants with masses of 3.1
and 2.6 times that of the Sun, radiating at 93 and 64 times solar.
The lesser (fainter and hotter) G0 component has just given up core
hydrogen fusion and is cooling and growing, while the other more
massive G8 star has probably already begun fusing helium to carbon.
For serious strangeness, return to the Kids. Two of them (Epsilon and Zeta)
are eclipsing binaries.
More or less normal Zeta (as far as stellar "normalcy" goes)
consists of a coolish class K bright giant (or supergiant) of around six solar
masses in mutual orbit with a much hotter and smaller five solar
mass B8 dwarf. Every 2.66 years the hot star hides behind the
larger, causing the light to dip by about 15 percent. The event
allows astronomers to explore the K star's deep chromosphere as the
B-star's light penetrates it, acting like a celestial
probe.
Nothing, however, quite prepares one for unique Epsilon. Every 27
years, a class F supergiant roughly 2000 light years away is
partially eclipsed by a mysterious even-larger dusty cloud of
unknown origin that seems to contain a pair of orbiting hot class
B dwarfs. Try to imagine something that can eclipse a supergiant!
The system is so large that the eclipse, also with a depth of about
15 percent, lasts for an amazing two years. The last one began in
2009! If you don't see this one, it's a long wait until the next.
The supergiant carries a mass of between 15 and 20 times that of
the Sun, and in all likelihood will someday explode as a supernova.
Vastly different, near the other end of the scale, Beta Aur (Menkalinan) is also an
eclipser with a tenth of a magnitude drop, but a much shorter
period of 3.96 days, one star only partially eclipsing the light of
the other. Just 81 light years away, its outstanding
characteristic is that the two class A subgiants (in which core
hydrogen fusion has ceased, or at least nearly so) tidally distort
one another into out-of-round shapes that cause the star to vary
even when not in eclipse. Some 300 or more AU away hovers a dim
red dwarf.
After admiring the individual stars, it's time to scan the
constellation for its three famed open clusters, from west to
east, Messier 38, 36, and 37, all
easily accessible with the smallest of instruments. Lying between
3600 and 4900 light years away, the trio cannot help but charm the
eye. M 38 and 37 are of modest age, 300 to 400 million years old,
but M 36 -- in the middle -- comes in much younger, at a mere 30
million, the group still containing numerous hot blue stars. All
are dimmed by about a magnitude by the Milky Way's interstellar
dust.
The youthfulness of M 36 then leads us to the constellation's true
youngsters. Among the more famed of Auriga's stars is AE Aurigae, an O9 dwarf 2000 or so light
years away that lies a bit toward the southern end of the
constellation near a complex line of stars. It's notorious as one
of the first known "runaway" stars, speeding at over 200 kilometers
per second in a direction opposite its counterpart Mu Columbae, which lies nearly 70 degrees to
the south. The two have been traced back to the odd double star Iota Orionis, which lies near the Trapezium and Orion Nebula. Some two million years ago, a binary
apparently passed too close to Iota. The four stars mingled, two
were retained as present-day Iota, and the other two were ejected
in opposite directions.
Even younger is seventh magnitude AB Aur,
which lies near the Taurus border. About 450 light years away,
it's a "Herbig Ae/Be star," one still in the act of forming itself and accompanied by
a surrounding disk. Which brings us back to the Taurus-Auriga
star-forming dark clouds and the Anticenter, against which the
speeding Charioteer rides.
Copyright © James B. Kaler, all rights reserved.
These contents are the property of the author and may not be
reproduced in whole or in part without the author's consent
except in fair use for educational purposes. First published in
the January/July 2010 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.