By Jim Kaler

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.