Caroline Herschel Objects
By Jane Houston Jones
from Nov 2002. Sky & Telescope Magazine, p. 107.
Twelve deep-sky sights make up the "other" Herschel catalog. Most are in view this season. Can you log them all?
WHEN I GAZE AT CANIS MAJOR sparkling on a cold night I sometimes time-travel back to 1783. I picture Caroline Herschel, age 33, at her small telescope near Windsor, England. The Sun has set, her older brother, William Herschel, is out of town, and she is about to make a discovery that now bears her name.
Two years earlier William had become famous by discovering Uranus. On quiet nights when he was away, Caroline swept the sky with her small reflector for discoveries of her own, particularly comets. Caroline did not have much free time. When William was at the eyepiece of his 20-foot-long reflector she was always there with pen and ledger, recording his observations of faint deep-sky objects. On cold winter nights the ink sometimes froze in her inkbottle. William kept his telescope aimed at the meridian; Caroline noted the elevation of each object he sighted and the sidereal time, as well as his spoken comments, and the next day she reduced the many readings into right ascensions and declinations. Then she would plan the next night's observing schedule. Nearly 2,500 of these "Herschel objects" would form the core, a century later, of the NGC, the New General Catalogue of Nebulae and Clusters of Stars.
"Do you know how to find Caroline's Cluster?" a friend asked me at a club star party on Mount Tamalpais near San Francisco early this year. Her coincidental question jolted me out of my 18th-century reverie. After collecting my thoughts, I obliged her by aiming my 12.5-inch f/5.75 reflector in the direction of Caroline's Cluster, NGC 2360 in Canis Major, the lovely object that Caroline discovered with her telescope that night in 1783.
In addition to hunting comets (she eventually discovered eight), Caroline Herschel cataloged 13 deep-sky objects of her own while not working for her brother. Four are in Cassiopeia, and two each are in Andromeda and Canis Major. Cepheus, Sculptor, Hydra, and Ophiuchus each hold one "C. Herschel" object. And a mystery object in Monoceros has never been identified.
Caroline Herschel taking notes as her brother William observes. This engraving by P. Fouché was meant to illustrate the pair on March 13, 1781, the night William discovered Uranus.
Unlike most other Herschel objects, hers are generally bright and easy. Many amateurs are familiar with the selected "Herschel 400" list; the Astronomical League grants its Herschel Award to observers who log all 400, a worthy goal for amateurs who have recorded the 110 or so Messier objects (see www.astroleague .org/al!obsclubs/herschel/hers400.html). The Caroline Herschels form a small and interesting subset of the Herschel 400. Why not plan to observe them this year? Rather than just ticking off these objects on a checklist, I hope you will stop and contemplate them for a while. Imagine the thrill of discovery Caroline must have felt on those long-ago nights.
Ten of the objects make good fall targets for Northern Hemisphere observers. Save two of them for spring and summer stargazes. And tackle the remaining mystery object in the armchair some rainy night.
NGC 7789 in Cassiopeia is a beautiful and well- known open cluster, a rich swarm with, however, no stars brighter than 11th magnitude. Compared to many clusters it's delicate and lacelike. I observed it several times before someone told me its modern nickname, the Magnificent Cluster. I first logged it and the other three C. Herschels in Cassiopeia on November 2, 1997, with Strider, my brand-new 12.5-inch Litebox Dobsonian reflector made by Barry Peckham in Hawaii. I like to observe clusters with a variety of eyepieces, and each view from 83x to 200x was different. The lower power yields more than 100 stars, with some pretty star chains and many dark lanes. At 200x there's a small, dark, starless spot in the middle.
NGC 7789 is easy to find 3° southeast of Beta Cassiopeiae, the easternmost star of Cassiopeia's W pattern. Caroline discovered it in the fall of 1783, and it is cataloged in the William Herschel list as H VI-30, the 30th object in his Classification VI: "very compressed and rich clusters of stars?"
NGC 225 is a loose cluster that looks great in every telescope. It's a cinch to locate halfway between Gamma and Kappa Cas. To me it looks like a triangle of triangles, with one little trail of stars dividing them. About 30 stars show in my 12.5-inch. Caroline discovered this object in 1784, and it became the 78th object in Herschel's Classification VIII: "coarsely scattered clusters of stars?"
NGC 381 is a fainter and smaller open cluster located just 1.6° northeast of Gamma Cassiopeiae. With a wide-field Panoptic eyepiece giving 114x in my 12.5inch, it shows a chain of stars leading toward the center, sort of like a candied apple on a stick. Unfortunately, it's not plotted on Sky Atlas 2000.0; you'll have to mark it there yourself using the position in the table on the facing page. It's between two 6th- magnitude stars 1° apart.
NGC 659, our last open cluster in Cassiopeia, looks like a patch of Milky Way anchored by a pretty triangle of stars including 44 Cassiopeiae. I've never seen more than about two dozen stars in this cluster through any telescope. It's northwest of Delta Cassiopeiae, quite near the brighter clusters M103 and NGC 663. The three-cluster grouping looks great even in binoculars.
NGC 7380 in Cepheus is a triangular cluster embedded in a larger, faint emission nebula, Sharpless 2-142. I first observed this cluster from my back deck in San Rafael, California, on December 26, 1997, using my 12.5-inch reflector at 1 14x. Without a filter I can barely see the nebula, but a narrowband 0 III or UHC filter significantly increases its contrast with the sky.
NGC 253 is a big, dusty, nearly edge-on galaxy in Sculptor famous from photographs. I've observed it in every telescope I've owned, and it's one of my all- time favorites. If it weren't so far south in such a dim, out-of-the-way constellation it would be almost as well known as the Andromeda Galaxy, M31. And it's not that far south. If you can see Antares or Fomalhaut, you can see NGC 253.
My most memorable observations of it have been at Dillingham Airfield on the island of Oahu with the Hawaiian Astronomical Society. In 12.5-inch Strider at high power I see dark dust lanes, mottled lumpiness, and an oval core. In my larger 17.5-inch f14.5 Litebox (which I've named Hagrid), it takes on more substance and greater length. Clumpy starforming regions brighten dramatically into visible knots, and foreground stars appear embedded in the galaxy south of the dust lane. Caroline discovered this glorious object on September 23, 1783, and it is the first object of Herschel's Classification V: "very large nebulae?"
Ml10 (NGC 205) is the dimmer, less condensed of the Andromeda Galaxy's two elliptical companions. My first observation of M110 was in October 1989 on my back deck with Stardust, my homemade 10-inch f/7.3 reflector. I sketched each new object I observed, and those early sketches are amusing to look at now. My sketch looks like a smudged oval, a little brighter in the middle. It was my 29th Messier object and my first Caroline Herschel. She discovered it and the next object, NGC 891, on the same night, August 27, 1783. What a night that must have been!
NGC 891 in Andromeda is a classic edge-on galaxy with a dust lane down the middle. It holds a special place in both my heart and logbook, being one of the first deep-sky objects I viewed through 12.5-inch Strider. Despite its fame it is dim the dimmest of the C. Herschel objects - and has a low surface brightness to boot. But this elongated spindle of light looks unusual in any telescope that shows it at all. You're not likely to detect the central dust lane, however, in less than a 12-inch.
Caroline's Cluster," NGC 2360 in Canis Major, is relatively small and compact. It lies 0.4" east of a 5.5-magnitude star. The frame is 0.90 wide. Photo from Digitized Sky Survey.
NGC 2360 is Caroline's Cluster in Canis Major and a winter favorite of mine. I love its rich Milky Way background. This irregular-triangular cluster displays about 40 to 60 stars, depending on aperture and magnification. In it are lovely star chains, star clumps, and inky black spots with no stars at all. My first observation of it was on January 17, 1998, through 10-inch Stardust on my back deck. At 58x it looked oblong with 20 or so stars. In 12.5-inch Strider at 83x the number climbs to about 50, and the cluster looks a little like an arrowhead, with more stars on its western side. One bright star, 9th- magnitude SAO 152691, highlights the cluster's eastern edge.
Caroline's Cluster is easy to find. Starhop from Sirius to Canis Major's eastern "dog ear," 4th- magnitude Gamma (y) Canis Majoris. Continue the same distance farther in the same direction and you'll have it in your eyepiece. It's only 0.4° east of a 5.5- magnitude star. It is incorporated in William Herschel's list as H VII-12, the 12th object of his Classification VII: "compressed clusters of small and large stars?"
NGC 2204 is the other C. Herschel open cluster in Canis Major; it's less than 2° west of the dog's front paw, Beta (f3) Canis Majoris. In my 10-inch it is distinct with a bright orange star just to its northwest. At l42x I see dozens of stars and several star chains. Caroline discovered this cluster too in 1783, and William numbered it just after NGC 2360, as H VII-13.
"NGC 2349," supposedly in Monoceros, is a mystery object that Caroline apparently cataloged incorrectly. It remains a puzzle to this day. William and Caroline recorded a nebula centered near 7h 10.8w, -8° 36' (in 2000.0 coordinates). They noted that it is easily identified by the "extending branch towards the south- preceding" (southwest). William's son John Herschel, however, later called it a "poor straggling cluster" and took its position as that of a double star some 50 seconds of right ascension (12.4') west of the object observed by his father and his aunt. He adopted this position in his General Catalogue of Nebulae, published in 1864, and John Dreyer perpetuated this position in his New General Catalogue, published in 1888.
M48 (NGC 2548) in Hydra is the one object in this list best viewed in late winter or springtime (for the Northern Hemisphere). The identification of this fine cluster was lost for nearly two centuries. Charles Messier discovered it in 1771, but due to an error in data analysis he gave the wrong position in his catalog so it was a "missing Messier" until 1959, when T. F. Morris at last identified it. Caroline Herschel discovered it independently in 1783, and William included it in his catalog.
I've observed M48 in every telescope and binocular I've ever owned. At 62x in Red Dwarf, my 6- inch f15 reflector made by Pierre Schwaar, I can see 40 stars including three yellow giants and two huge semicircles like eyes on either side of the central ridge. This ridge or chain of stars forms a nearly north-south nose. Larger apertures show more stars, including many pretty doubles. M48 can be seen with the naked eye in a very dark sky, south of the head of Hydra.
NGC 6633 is an open cluster in Ophiuchus as large as the Moon, displaying a W-shaped string of stars as well as other wispy star chains. Its brightest stars are elongated northeast-southwest. In my 12.5-inch at 82x I count more than 100 stars, including many colorful yellow, orange, and bluish ones. This cluster is naked-eye visible in a dark sky as a bit of detached Milky Way halfway between Alpha (a) Ophiuchi and Delta (6) Aquilae. (Larger, looser IC 4756 is 3° to its eastsoutheast.)
A sweep of the Caroline Herschels would not be complete without observing the lunar crater C. Herschel in Mare Imbrium near Sinus Iridum. Day 10 of the lunar month is just about the best night to observe Caroline's crater. The next night the great walled plain named for Caroline's nephew, John Herschel, becomes visible north of Mare Frigoris. It's more than 10 times as large, but that's a story for another time.
JANE HOUSTON JONES, president of the Astronomical Association of Northern California, also enjoys observing Abell, Barnard, Berkeley, Czernik, Dolidze, Hickson, Minkowski, Sharpless, Tombaugh, and Zwicky objects.
The author preparing Hagrid, her 17.5-inch Litebox reflector, for a night of viewing at Lake Sonoma, California, one of her favorite observing sites.