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Ancient planetaries.

YOUNG, DYNAMICALLY EVOLVING planetary nebulae are among the most visually appealing of all deep-sky objects. Ejected during the death throes of red-giant stars, these expanding shells of gas eventually dissipate over tens of thousands of years, leaving only a tiny white-dwarf star as a reminder of past glories. As they reach the end of their evolution, planetary nebulae become huge, faintly glowing shells that can be several light-years or more in diameter. Astronomer George Abell was one of the first to search for these faded giants on red-light photographs made with the 48-inch Oschin Schmidt telescope at California's Palomar Observatory. In 1966 he published a list of more than 80 discoveries. Known as the Abell planetaries, they include many of today's best-known ancient planetary nebulae. Now is a fine time of year to hunt down some of the Abell nebulae as well as a few other large planetaries.

Fun with Abells

Abell planetaries comprise a considerable fraction of the "largest planetaries" list, and quite a few are scattered in the northern spring skies (see Eric Honeycutt's article "The Best Abell Planetary Nebulae" in last month's issue, page 98). Many are relatively structureless, dimly glowing disks of gas, but a few retain hints of more interesting detail. Abell 31 is a typical member of the class, a huge, irregularly round shell of gas spanning about 1/4[degrees]-- roughly the angular size of the Helix Nebula, but not nearly as bright. Located about 2 1/2[degrees] south-southeast of the open cluster M67, Abell 31 is overlooked by most amateurs. Its surface brightness, however, is high enough to be glimpsed with moderate-aperture scopes under good sky conditions.

Farther to the south lurks Abell 33, the "smallest" giant on my list this month. A mere 4 1/2' wide, it doesn't make everyone's list of giant planetaries, but visually it's a nearly perfectly round shell of gas with a bright 7th-magnitude star on its southwest edge. The surface brightness is high enough to partially overcome the interference of the bright star, but moving the star just outside the edge of your eye-piece field will help in the observation of the nebula.

Abell 34 is similar in size and surface brightness to Abell 33. It is located about 2[degrees] northwest of the 4th-magnitude star [Nu.sup.1] ([[nu].sup.1]) Hydrae. In my 17 1/2-inch reflector this fairly challenging object showed only a hint of annularity.

The last two Abell planetaries on my list are more interesting. Abell 35 is about 1/4[degrees] in diameter, and its central star is a 13th-magnitude binary. Surrounding this star are a bow-shock structure best visible in O III emission and two parallel nebular structures, or "pipes," that cut across the object in an almost east-west direction. With my 17 1/2-inch scope I can easily see the central star, but there are only vague hints of structure within the nebulosity. Perhaps an observer armed with a larger scope under truly dark skies will discern these elusive features.

When I first observed Abell 36 at a regional star party several years ago I found it to be one of the most visually interesting members of its class. Instead of the usual dimly glowing shell, it has an elongated shape with lots of dark rifts and bright arcs and knots. It reminded me of a poor man's version of the Trifid Nebula, M20. Astronomers have described this planetary as having a helical structure much like NGC 6543 (the Cat's Eye or Helical Nebula), except on a much-expanded scale. The central star is easy to see at 12th magnitude.

The first non-Abell planetary on my list is perhaps the best-known giant in the northern spring skies. Designated Jones-Emberson 1 (JoEr 1 for short), it was discovered in 1939 by two astronomers at Harvard Observatory. But for more than 40 years its position and designation were sometimes confused with a pair of tiny galaxies (NGC 2474 and 2475) located about 1/2[degrees] south of the planetary. Some of the confusion arose from JoEr 1 having two prominent knots in its annulus with nearly the same separation as the galaxies. Eventually this cosmic identity crisis was resolved by astronomers Nancy and Ron Buta, who wrote about their finding in this magazine's April 1981 issue, page 368.

Buried near the center of the constellation Lynx, JoEr 1 is easier to see and structurally more interesting than most other ancient planetaries. An 8- or 10inch scope fitted with a Lumicon UHC or O III nebula filter should show JoEr 1's disk spanning more than 6'. But it may take a larger telescope to catch sight of the two bright knots.

Lynx is also home to one of the largest planetaries in the sky, the elusive Purgathofer-Weinberger 1, or PuWe 1. Discovered in 1980, it is a whopping 20' across--more than 30 percent larger than the famous Helix Nebula (NGC 7293). At a distance of nearly 1,000 light-years PuWe 1 is an impressive 6 light-years in diameter. Yet the 15th-magnitude central star barely illuminates this monster, making it difficult to observe.

On a very good night I was able to detect the brightest portions of PuWe 1's ring structure using the Atlanta Astronomy Club's 20-inch f/4.5 reflector in Georgia. Armed with a 16-millimeter Nagler eyepiece and an O III filter, I could barely glimpse two broad arcs, one stretching about 120[degrees] from north-northwest to northeast and a smaller segment in the southwest.

My last planetary this month is Longmore-Tritton 5 (LoTr 5), a dim giant located only 2[degrees] south of the Coma Galaxy Cluster. It has the distinction of having the highest galactic latitude of any planetary, being a mere 1 1/2[degrees] from the north galactic pole. The central star is part of a binary system that includes a primary that varies between 8.7 and 8.9 magnitude and is bright enough to hamper observation of the nebula. Nevertheless, highly skilled observers under ideal conditions have glimpsed the nebula with apertures as small as 6 inches. My best view was with Vic Menard's 22-inch Starmaster Dobsonian reflector at the April 2000 Peach State Star Party in Georgia. Using a 16-mm Nagler eyepiece and an O III filter, several of us glimpsed a huge, diffuse, nearly round glow surrounding the bright central star.

The challenge of hunting down giant planetaries has become quite popular during the last decade. Several Web sites have pages devoted to this facet of deep-sky observing. Two of the best ones covering Abell planetaries are maintained by Tom Polakis (www.psiaz.com/polakis/ abell/abell.html) and Eric Honeycutt (www.icplanetaries.com/abell.html). Both sites contain tables, images, and a wealth of detail on these fascinating objects. A more complete listing of the largest planetaries is available on Jens Bohle's site (http://home.teleos-web.de/jbohle). Although this site is in German, much of the information can be deciphered without knowing the language.
A Selected List of Ancient Planetary Nebulae

Name       P-K Number       R.A. (2000.0)

PuWe 1     PK158+17.1    [6.sup.h][19.6.sup.m]
JnEr 1     PK164+31.1    [7.sup.h][57.9.sup.m]
Abell 31   PK219+31.1    [8.sup.h][54.2.sup.m]
Abell 33   PK238+34.1    [9.sup.h][39.9.sup.m]
Abell 34   PK248+29.1    [9.sup.h][45.6.sup.m]
Abell 35   PK303+40.1   [12.sup.h][53.7.sup.m]
LoTr 5     PK339+88.1   [12.sup.h][55.6.sup.m]
Abell 36   PK318+41.1   [13.sup.h][40.7.sup.m]

Name             Dec.         Size    Constellation

PuWe 1     +55[degrees] 37'   20'          Lynx
JnEr 1     +53[degrees] 25'    6.3'        Lynx
Abell 31    +8[degrees] 54'   16'         Cancer
Abell 33    -2[degrees] 49'    4.5'       Hydra
Abell 34   -13[degrees] 10'    5'         Hydra
Abell 35   -22[degrees] 52'   13'         Hydra
LoTr 5     -25[degrees] 54'    9'     Coma Berenices
Abell 36   -19[degrees] 53'    6'         Virgo


An amateur who takes deep-sky observing seriously, RICHARD JAKIEL writes frequently in our pages on a wide variety of topics. He can be contacted at rjakiel@earthlink.com.
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Title Annotation:observing planetary nebulae
Author:Jakiel, Richard
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jun 1, 2002
Words:1366
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