THE BUNNY AND THE BIRD
When we leave the warmth of home for the chilly skies of northern
winter, we concentrate on the "Winter Six," on their luminous,
colorful first magnitude stars and other glories. Outlining a
sloppy circle, our eyes roam from Auriga (who holds Capella,
the most northerly of the top 20 stars),
to Gemini (with Pollux and sextuple Castor), Canis
Minor (Procyon), Canis Major (brilliant Sirius), mighty Orion (red Betelgeuse and
blue Rigel), and, closing back in on
Auriga, Taurus (orange Aldebaran plus the ever-popular Pleiades and Hyades).
Who then much bothers with more obscure figures, notably Orion's
prey, poor Lepus the Hare, on which the
Hunter stands, and yet farther down, Columba, the Dove. While about the same size and
brightness, their histories are very different. Lepus goes back to ancient Greece and before. To the
old Arabians, the keeper of the ancient constellations' flame, the
box-like figure was Orion's chair. Columba on the other hand, is
"modern," going back only to the early seventeenth century. And
while Lepus and Orion imply violence, Columba -- originally
referring to Noah's Dove -- evokes peace. Visibility comes into
play as well. Lepus can be seen by just about anyone south of the
Arctic. The exquisite flat triangle that marks Columba, however,
is a good 35 degrees south of the celestial equator. While not
hard to find in the continental US (and obviously Hawaii), it skims
the southern horizon from most of Britain, and is quite out of
sight from Alaska and Scotland.
Though the constellations' brightest stars seem similar (both third
magnitude), they nevertheless reflect the differences between the
two figures. Arneb (Alpha Leporis, the
Arabic proper name referring to the Hare) is a magnificent class F0
(temperature 7000 Kelvin) "yellow" supergiant that radiates at a
rate 13,000 times that of Sun from a
distance of roughly 1300 light years. Placed at the Sun, this 8-10
solar mass dying star would extend halfway to Mercury. Though its
constellation is of modern origin, Alpha
Columbae still carries an Arabic proper name, "Phact" (meaning
"ring dove"), but one also made up in modern times in reference to
Cygnus, the Swan. While Arneb shines with
a soft off-white glow, hot (12,500 K) class B7 Phact takes on a
harder blue-white cast. A subgiant 270 light years away, radiating
at a rate of 1000 Suns, it is a five solar mass star just starting
to evolve. Like many of its class, Phact is a rapidly rotating "B-
emission" star, one with a surrounding, radiating disk.
The constellations' two most famous stellar residents provide an
even greater contrast, indeed one about as great as possible. The
Dove shelters fifth magnitude blue class O9.5 Mu Columbae. Though a hydrogen-fusing dwarf,
with a temperature of nearly 34,000 Kelvin, a luminosity of 23,000
Suns, and a mass a dozen solar, it far outstrips Phact, its
apparent faintness coming from its distance of 1300 light years.
Mu is famous as one of the first known "runaway stars." It is
moving directly away from another class O star, AE Aurigae, at a separation velocity of some
200 kilometers per second. Tracing the stellar paths backwards
shows them crossing near the Trapezium
in Orion, very close to the weird hot double Iota Ori (at the bottom of Orion's
Sword), which consists of O9 and B1 stars in a tight, but unusually
eccentric, orbit. Apparently, a couple of million years ago, two
massive close binaries encountered each other in the crowded Trapezium region. In a member-exchange,
two of the stars became the current Iota, while the other two were
expelled at high speed. Somehow, even distant 53 Arietis got into
the act. We've since found a great many more such runaways, some
seemingly driven from interacting binaries, others shot out of
doubles when their massive companions exploded as
supernovae.
While Mu Columbae is indeed "blue," the color is at best subtle.
Such is true at the other end of the color wheel as well, most
"red" stars really appearing to the eye as a more washed-out
orange. Most, but not all. If you want real color, point your
telescope into northeastern Lepus to "Hind's
Crimson Star," discovered in 1845 by the English astronomer J.
R. Hind, and more commonly known as R Leporis, thus designating it
as the first variable star found in the constellation. And
variable it is. R Lep is a "long period"
Mira-type variable that roams over a range of about four
magnitudes during a 430 day pulsation period. Over longer
intervals, maximum brightness drifts from eighth to sixth, minimum
from twelfth to tenth.
Of greater significance, Hind's is a prime example of a "carbon
star." Highly evolved, Mira variables are all brightening giants with dead carbon-oxygen
cores as they prepare to eject their outer envelopes, whereupon the
cores are seen as dense white
dwarfs (like the companion to Sirius). In the outer layers of normal
stars, oxygen atoms are about twice as abundant as those of carbon.
A handful of ageing stars, however, have just the right structures
to send convection currents close enough to the old nuclear-burning
zones to dredge up freshly made carbon, and to more than reverse
the ratio. The carbon atoms link into molecules that fiercely
absorb blue light, which colors carbon stars a vivid red. R Lep is
losing about a millionth of a solar mass per year, condensing dust
most likely the culprit in causing the variations in minima and
maxima.
Eventually, the fleeing dusty gas cloud will be lit by the hot
stellar core within to create a planetary nebula (William Herschel,
the discoverer of the breed around 1790, responsible for the
misnomer). Planetaries are among the showpieces of the sky,
appearing as shining rings, spheres, dumbbells, their matter
ionized by the hot stars within. Getting hotter and hotter,
reaching to well over 100,000 Kelvin (turning as blue as they can
get), the central stars finally cool and dim as white dwarfs, the
nebulae continually expanding and then merging with the
interstellar gas. Who with a backyard telescope can ignore the
beauty of the Ring Nebula in Lyra, the
Owl of Ursa Major, or Vulpecula's Dumbbell?
Who then should ignore another classic example in Lupus, IC 418,
which from a spectacular image taken by Hubble was named the
"Spirograph Nebula." Only 12 seconds of arc across, the nebula
requires high telescopic power and a steady atmosphere to
appreciate. The readily-visible tenth magnitude central star,
which was once at the core of a Mira variable, has a temperature of
only about 35,000 Kelvin, and thus helps anchor the cool end of
planetary nebula evolution. Like most such nebulae in the Galaxy,
the distance is not at all well known, but probably lies between
2000 and 6000 light years. If at the nearer, the nebula is only a
couple tenths of a light year across, its expansion velocity of 12
kilometers per second giving it an age since ejection of only 1500
years. It too is carbon-rich, showing the nebula to have come from
a star that may have once been much like the Crimson Star. Much of
the Galaxy's carbon, including
that from which you are made, was created in stars such as
these.
Lepus then takes us back to a time when the Galaxy was much less
enriched by planetary nebulae and especially by exploding
supernovae. Before you get too cold, turn the telescope to the
globular cluster Messier 79. It and the rest of its rather rare
breed are among the first denizens of the Galaxy, with ages of some
12 billion years, born before there had been much chemical
enrichment. With a metal content of just three percent that of the
Sun, eighth-magnitude M 79 falls right in line. Though not
measuring up to the kingpins of globulardom (Omega Centauri, 47
Tucanae, M 13, M 5), M 79 is still
substantial, with a mass estimated at around 200,000 Suns, enough
to make it readily visible from its distance of 40,000 light years.
It distinguishes itself by having a mass of highly evolved blue
giants and by being quite concentrated from stars gravitationally
diving to its center. While seeming to have a diameter around a
quarter of a degree (60 light years), most of the mass is compacted
into a diameter only a quarter that.
So far, the Hare has escaped the Hunter's arrow, and still bounds
for us today, free as the Dove below it, the celestial figures
appreciated for what they have to tell us about the lives of stars
and of the Galaxy itself.
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/June 2007 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.