CHASING THE FOX
The ancient constellations form a
celestial picture-board for telling the myriad myths that wrap
around them. But old as the figures are, there is always room to
look at them in fresh ways that tell new tales. Bounding across an
open field surrounded by Lyra, Aquila, Pegasus, and Cygnus, Vulpecula the Fox (a modern constellation)
runs for his life. Dashing to the northwest, he's chased by Sagitta, the Arrow, which seems to have
been shot by a tiny hunter riding Equuleus, the Little Horse. The arrow has already sped
past Delphinus, the Dolphin, who has
decided to chase the Fox as well (anything being possible in myth).
The story ends well. Since all four of these charming
constellations actually fly across the sky at the same westerly
pace, the Fox is forever safe from harm.
Among the smallest constellations of the sky, the four are
dominated by their surroundings. Yet, modest as they are, they
collectively have a great deal to offer. The largest and most
westerly of the set, last in the alphabetical list, is modern Vulpecula (the inveterate Fox). Invented in
the seventeenth century by Hevelius, it is also the hardest to
find. (Perhaps that is why the arrow keeps missing....) The
brightest star, Anser (reflective of the
original name, Vulpecula cum Ansere, the Fox with Goose), is just
barely brighter than fifth magnitude, and is the only one with a Greek letter (Alpha). The telescope
reveals a ruddy red giant.
Almost 400 light years away, Anser may be in an advanced state of decay as it
prepares to lose its outer husk in the creation of a planetary nebula (a misnomer from
William Herschel), in which a revealed hot stellar core lights up
a fleeing dusty stellar wind that was once the star's outer
envelope.
Anser thus leads directly to Vulpecula's most famed guest, the
Dumbbell Nebula, Messier 27. Visible even in binoculars, this
mature planetary nebula is one of the great showpieces of the sky.
Through the telescope it appears to float in three-dimensions
against the sparkling background of the Milky Way. A mature expanding cloud 1250 light years
away and now two light years across, M 27 hosts a dying fourteenth
magnitude star with a temperature near 140,000 degrees Kelvin,
which, as the nebula dissipates into the mists of space, is
effectively a cooling white
dwarf.
Vulpecula's other prominent (and rather embarrassing) guest is the
"Coathanger Cluster," more formally
called Collinder 399. This glorious clump of stars, a wonderful
sight in binoculars, fools the eye. Not a cluster at all, its stars are
at quite different distances and are all moving separately, their
apparent proximity a mere line-of-sight coincidence.
Just to the southeast of Vulpecula flies the tiny Arrow, third smallest of all the constellations and one
of the few that actually resemble what they are named after (don't
look for the actual fox). While the Alpha star carries its only
proper name (Sham, derived from "arrow" in
Arabic), at fourth magnitude it's tied for the pattern's third
brightest star, and is beaten out by both Delta and third magnitude Gamma for luminary. Oddly, like Anser,
Gamma is also a red giant, though much closer, 275 light years
away.
The constellation hosts a pair of curiosities. First is Messier
71. Paling against rivals
M 13 and the southern hemisphere's Omega
Centauri and
47 Tucanae, M 71 has the distinction of being among
the lesser globular
clusters, revealing that they are hardly all the same. With a
mass of around 50,000 Suns and roughly 20
light years across, it reaches eighth magnitude only because (for
globulars) it is relatively close to us, a "mere" 12,000 light
years away. With an age estimated to be around 10 billion years,
it's also among the younger of its kind.
The prize for weirdest member of the constellation, however, goes
to FG Sagittae, a planetary nebula whose central star brightened
over the course of the twentieth century by four magnitudes. What
really happened was that it underwent a rare internal eruption (a
"helium flash" just outside its carbon-rich core) that caused it to
expand and cool, whence the light that was pouring out in the
ultraviolet rather suddenly began to appear in the visible. The
star is so odd that a whole scientific meeting was once devoted to
it.
Lying along a gentle curve a bit farther to the southeast swims
(or, if you like the story, flops after the Fox) the Dolphin. The fourth magnitude luminaries (Beta
slightly brighter than Alpha) carry some of the strangest of
stellar names, "Rotanev" and "Sualocin," which are "Nicolaus Venator"
spelled backwards, "Nick" the assistant to Giuseppe Piazzi, the
discoverer of Ceres, the first asteroid.
For such a small constellation (69th in order of size), Delphinus
is unusual in sporting not one, but two relatively bright stars
that hold planets. At sixth magnitude, 108
light years away, HR 7907 (the number
from the "Bright Star
Catalogue") is easily accessible in binoculars. The planet,
with a mass at least 1.8 times that of Jupiter, orbits in 386 days
at an average distance just a bit greater than the Earth is from
the Sun. The planet (or planets, we don't know) do not have much
time left to appreciate their host, which is a "subgiant" a bit
warmer than the Sun, a star that seems about ready to become a
swollen and much more luminous giant. Seventh magnitude HD 195019 (from the "Henry Draper Catalogue"), of a
more solar nature but also probably approaching its death throes,
has a "hot jupiter" (or at least a warm one) that carries a minimum
of 3.4 jovian masses and takes a mere 18 days to orbit at a
distance of 36 percent Mercury's from the Sun.
Like Sagitta, Delphinus holds a globular cluster with a difference.
This one, NGC 7006, is three or four times more massive than M 71,
but at a distance of 130,000 light years is one of the most remote
of its kind known in the Galaxy, eighth farthest away, which leaves
it at a dim 11th magnitude as seen from Earth.
Finally, at the southeastern end of the string of four, gallops Equuleus, the Little Horse that is forever
chasing the leaping Fox. It holds the rank of the smallest of all
the ancient constellations, and among the whole set of 88 is just
barely beaten out for the record by modern Crux, the Southern Cross (and then just by four
percent). Its brightest star, Kitalpha
(from Arabic for a "part of a horse") is -- like those of the
others of this group -- dim, just fourth magnitude (compliments of
a distance of 186 light years). It's an interesting very close
double made of a giant and an ordinary warmer dwarf (a star that,
like the Sun, is fueled by the core fusion of hydrogen into helium)
separated by only about Venus's distance from the Sun.
Of much more interest is the constellation's most famed member, Delta Equulei, which long held the record
for the shortest-period visual binary (one that can be seen as
double directly through the telescope). The two, never more than
0.35 seconds of arc apart (making it a very tough visual
observation), orbit eccentrically in just 5.7 years at an average
distance of 4.3 times Earth's distance from the Sun, both ordinary
dwarfs each a bit warmer and more massive than the Sun, the orbit
giving a total mass of 2.4 times solar. Greatest separation will
next take place near the end of the year 2009.
Continue the curve outlined by these four tiny figures toward the
southeast, and you run into the Water Jar of Aquarius, which with Delphinus, Pisces, and Aquarius help make up the sky's "Wet
Quarter." Continue it to the northwest, and there is another
exquisite small figure, Lyra (the Harp,
with brilliant Vega), that, within our new
myth, seems to be playing the background music for the unfolding
drama of the great Fox Chase.
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 2007 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.