DRAGGIN' THE DRAGON
Long streams of stars attract the eye. Among the most famed constellations made of them are Eridanus, the River, which ends far down in the
southern hemisphere in blue Achernar,
and the longest, Hydra, the Water
Serpent, which wraps its slithery body a third of the way around
the sky with only its head sticking up into the northern celestial hemisphere as if from
a deep southern ocean. To see a whole such figure in a single
glance, turn your eyes to the far north, where shy Draco the Dragon, hides between the Bears and Hercules, waiting to
be dragged out. It could almost be called the English
Constellation, so rich is its historical connection with the land,
and is among the central figures in the story of the Earth's
motions.
Every skygazer is familiar with the celestial poles, which lie above the
terrestrial rotation poles and about which the sky seems to turn.
The northern one is beautifully marked by Polaris, which serves as the ends of both
the Little Dipper's handle and the tail of Ursa Minor, the Smaller Bear. But there are other
poles of equal importance. As the Earth rounds the Sun, it -- and its apparent solar motion
against the stars -- carves out the great circle of the Ecliptic, which defines the
dozen constellations of the Zodiac. The perpendicular to it
goes to the north and south ecliptic poles. Since the Earth's
rotational axis is tilted by 23.4 degrees to the orbital axis, the
ecliptic poles lie 23.4 degrees from the celestial ones. Out of
sight for northerners, the southern one (the SEP) falls in Dorado not far from the Large Magellanic Cloud. The North Ecliptic Pole (NEP),
however, circumpolar from everywhere north of the Tropic of Cancer, is eminently visible. Admire it in
Draco wrapped within the Dragon's eastern
curl midway between Zeta and Delta Draconis.
Why should we care? Because the ecliptic poles are central to the
location of the celestial ones. As the Earth spins, it throws
itself slightly outward at its equator, the equatorial diameter
about 40 kilometers greater than the polar. The Sun and Moon,
riding at and near the tilted ecliptic path, pull on the bulge,
which because of the Earth's spin, causes the rotational axis to
wobble, or precess, like a top.
As a result, the celestial poles slowly move in a circle around the
ecliptic poles with a radius of 23.4 degrees, taking 25,800 years
to make the journey. The motion has been known for more than 2000
years since Hipparchus of Nicaea discovered it in the second
century BC.
But from our perspective, it's the sky that seems to be moving.
Polaris, now about 3/4 of a degree off the pole, will keep getting better until around the year
2100, when it will be just a quarter of a degree away. Over the
long haul, though, Polaris is a temporary marker. Thirteen
thousand years ago, it was 47 degrees away from the pole, passing
nearly overhead at the latitude of New York! Looking back into the
past along the circle of precession, among the most famed of ex-
pole stars is Thuban, Alpha Draconis,
which almost perfectly marked the rotation pole during the time of
ancient Egypt of 2800 BC. Just fourth magnitude (3.65), this white
class A0 giant, 300 light years away, is easily found between Ursa
Major's Mizar/Alcor pair and Gamma
Ursae Minoris in the Little Dipper's bowl.
Thuban's Alpha status comes not from brightness, but from location.
Draco's luminary is actually Eltanin,
Gamma Draconis, a second magnitude (2.23) orange K5 giant that shines within the
Dragon's lopsided head. The star's position 51.5 degrees north of
the celestial equator took its daily path directly overhead as seen
from London, and gave it the popular name "Zenith Star." To the English astronomer
James Bradley, it was the perfect star for the first measure of parallax (hence the star's
distance), since when it slid perfectly above, it was unaffected by
atmospheric refraction, which
raises stars above their true positions.
As the Dragon dragged itself across the Zenith, Eltanin then became
something of a paradigm for how science often works: you go out to
find one thing and accidentally stumble over another. Instead of
measuring parallax, in 1728 Bradley announced the discovery of a
much larger shift caused by the "aberration of light." Driving
into the rain on a windless day makes the drops seem to come from
a point in front of you; the faster you go, the lower the point
gets. Light behaves the same way. Gamma Draconis's position was
constantly being shifted 20.5 seconds of arc in front of the zenith
in the direction of the Earth's motion. The aberrational shift was
not only the first actual proof of Copernican theory, but allowed
for a measure of the speed of light! The phenomenon must be taken
into account in any measure of star positions.
But Bradley was not yet done. Further observation revealed a
smaller 17 X 9 second of arc wobble with an 18.6 year period called
the "nutation." The lunar orbit, tilted by 5 degrees to the
ecliptic plane, precesses as well, which causes the orbital "nodes"
(the points where the Moon crosses the ecliptic) to go around
through the Zodiac, taking the 18.6 years for a full circuit.
(Nodal position is crucial in the calculation of eclipses, which
can take place only when the Moon is near one of them.) Nutation
is the result of a slight variation in the sum of the gravitational
torques induced by the Moon and Sun on the bulge. Over 100 more
subtle nutational terms, all caused by irregularities in the lunar
and terrestrial motion, are known.
While the Dragon is not famed for many celestial showpieces, a few
do stand out. Try, for example, fourth magnitude (4.13) Nu Draconis, the faintest of the stars of
Draco's head, a striking double made of nearly identical white
class A dwarfs just over a minute of arc apart and that can be
split with binoculars. Not quite 100 light years away, the eastern
member is a spectroscopic binary,
making Nu Dra a triple system. Well away from the dust of the Milky Way, Draco also breathes fire into several
fine telescopic galaxies that include the lovely spiral NGC 5985
(among a small group some 100 million light years away) and, at
about half that distance, edge-on NGC 5907, the "Knife-Edge
Galaxy."
But for the best of Draco's sights, go local. In 1785, William
Herschel announced the discovery of the first of his "planetary nebulae," NGC
7009 (the Saturn Nebula), the generic term coming from its
disk-like appearance. (The Ring Nebula in
Lyra was already known, but was not put
into that category until later.) Among his planetaries was Draco's
spectacular northern nebula, NGC 6543.
Roughly 3000 light years away, and better known now as the "Cat's
Eye," it has a profound place in nebular history. In 1786,
Herschel wrote that he noticed a bright spot in the center. Four
years later he reported it as a central star, making it the first
known planetary nebula nucleus, which was "involved in a shining
fluid, of a nature unknown to us." Many decades later, in 1864,
Sir William Huggins solved the riddle. Looking through his
spectroscope at the nebula, he found not the absorption lines that appear in
the spectra of stars, but emissions, which through the work
of such scientific giants as Gustav Kirchhoff proved that these
nebulae were made not of stars, but of radiant gases.
We know now that the planetaries are the ejecta of giant stars that are lit by the
glowing coals of the remaining stellar cores. Too far north to be
observable by the restrictions of the 100-inch telescope at Mt.
Wilson, in later years the Cat's Eye was among the first known to
be surrounded by a giant halo of earlier leavings. In our own
time, it was the first of its kind to be observed by the Hubble
Space Telescope. Paraphrasing his words, one observer noted that
if he'd known they were this complex, he would have gone into
another line of work. We still do not know the origins of the
nebular structures. Given this marvelous jewel, maybe we can still
(if you will excuse one last time) drag it out of the Dragon. To
complete the circle, the Cat's Eye is almost dead on the North
Ecliptic Pole, a mere 10 minutes of arc away, making it the only
"Pole Nebula," which allows you to admire both of them at the same
time!
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 August/December 2010 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.