In Caroline Herschel's Footsteps

Caroline Herschel (1750–1848) was the first famous female astronomer. Though celebrated in her day for discovering eight comets, she started as a deep-sky observer. In fact, it was Caroline's initial deep-sky discoveries that inspired her brother William Herschel to undertake the monumental sky survey that ultimately netted more than 2,000 new galaxies, star clusters, and nebulae.

In the past, lists of Caroline's deep-sky discoveries were based on annotations in the printed version of William's catalogs. But in Sky & Telescope August 2007, page 59, leading Herschel expert Michael Hoskin describes some revisions to this list based on inspection of the manuscript sources. These include the Herschels' original observing notes and a catalog of Caroline's discoveries that she drafted but never published.

Several of the identifications are still somewhat tentative, and this presents a rare opportunity for amateur astronomers to make a real contribution to astronomical scholarship. If you own a small telescope and have access to dark skies, try to observe the objects listed here and compare your notes to Caroline's. If there's a major discrepancy, or if you see something that matches Caroline's description better, please submit a comment at the bottom of this article or e-mail a report.

Almost all the deep-sky objects mentioned here are easy to locate with standard charts such as Sky & Telescope's Pocket Sky Atlas. But some of the faint clusters in Cassiopeia are shown only in the largest atlases. To aid your search, we've made a special detailed chart of Cassiopeia that you can download and print.

Caroline's Observations

Caroline's main telescope was a 4.2-inch richest-field reflecting telescope that her brother built for her. Contrary to widespread claims, its speculum mirrors, when freshly polished, were only moderately less reflective than modern coatings. And as for optical quality, William Herschel was indisputably one of the greatest telescope makers of all time. So her telescope was probably roughly comparable to a modern 4-inch reflector or 80-mm refractor.

Caroline identified her targets using an eyepiece that delivered a 2° 15' field of view either at 15x (according to Caroline) or 24x (according to William). Then she would examine them in more detail at 30x. This is a pretty good match to using 20-mm and 16-mm Plossls on an 80-mm f/6 refractor — just to mention one fairly common observing setup. If you find that you consistently see more than Caroline's notes describe, consider masking your telescope's aperture to reduce its light grasp.

The table below lists all of Caroline's deep-sky discoveries, and tentatively identifies them with modern designations. "CH No." and "WH No." refer to the numbers given in Caroline's handwritten catalog and William Herschel's published catalog, respectively. Click on an object's modern designation to see explanatory comments and a selection from Caroline's notes.

For full details, refer to Hoskin's articles in the November 2005 and August 2006 issues of the Journal for the History of Astronomy. You may also be interested in Hoskin's books on the Herschels. And William Herschel's catalog can be downloaded free from the NGC/IC Project website.

Title
Discovered Modern Name	CH No. WH No.	Type Const.	Mag. Size	RA	Dec.
Feb. 26, 1783 NGC 2360	2 VII.12	Cluster Cma	7.2 14'	7 h 17.8 m	-15° 38'
Mar. 8, 1783 M48	5 VI.22	Cluster Hya	5.8 30'	8 h 13.7 m	-5° 45'
July 23, 1783 NGC 6866	7 VII.59	Cluster Cyg	7.6 15'	20 h 03.9 m	+44° 09'
July 31, 1783 NGC 6633	8 VIII.72	Cluster Oph	4.6 20'	18 h 27.2 m	+6° 30'
July 31, 1783 IC 4665	— —	Cluster Oph	4.2 70'	17 h 46.2 m	+5° 43'
Aug. 27, 1783 M110	9 V.18	Galaxy And	8.1 22'×11'	0 h 40.4 m	+41° 41'
Sep. 23,1783 NGC 253	10 V.1	Galaxy Scl	7.6 28'×7'	0 h 47.6 m	-25° 17'
Sep. 27,1783 NGC 225	11 VIII.78	Cluster Cas	7.0 15'	0 h 43.6 m	+61° 46'
Sep. 27,1783 NGC 189	12 —	Cluster Cas	8.8 5'	0 h 39.6 m	+61° 06'
Sep. 27,1783 NGC 659	20 VIII.65	Cluster Cas	7.9 6'	1 h 44.4 m	+60° 40'
Sep. 29,1783 NGC 752	13 VII.32	Cluster And	5.7 75'	1 h 57.6 m	+37° 50'
Oct. 30, 1783 NGC 7789	— VI.30	Cluster Cas	6.7 25'	23 h 57.5 m	+56° 43'
May 12, 1784 NGC 6819	16 —	Cluster Cyg	7.3 5'	19 h 41.3 m	+40° 11'
Aug. 7, 1787 NGC 7380	19 VIII.77	Cluster Cep	7.2 20'	22 h 47.3 m	+58° 08'

NGC 2360
was Caroline's first deep-sky discovery, made with a tiny refractor. Her notes say "Following γ Canis majoris, a very faint Nebula," and then go on to describe her brother's observations of this object, presumably made through a much larger instrument.

M48
is the last object that Caroline "discovered" with her small refractor. Her notes say "At an equal distance from 29 & 30 Monocerotis, making an equilateral triangle with those stars is a nebulous spot. By the telescope it appears to be a cluster of scattered stars." The stars 29 and 30 Monocerotis are now usually called ζ Monocerotis and C Hydrae, respectively, and there's no doubt that the cluster in question is the one now called Messier 48. Messier's original catalog gave incorrect coordinates for this object, so she and William assumed that this was an original discovery. As I interpret Caroline's description, she located M48 with her unaided eyes before turning her telescope to it. Can you?

Caroline discovered NGC 6866 and the remaining objects with her 4.2-inch comet sweeper. Her notes include coordinates that lie just a half degree south-southwest of NGC 6866 after accounting for precession. Then they say: "Some small stars, or perhaps a Nebula. My brother put, I believe, a power of 70 to the Sweeper, then what is call'd some small stars are about a hundred or more." Apparently she found resolving the cluster into individual stars to be difficult at low magnification but easy at 70X. This fact should help you calibrate what you see through your telescope against Caroline's observations.

Caroline found NGC 6633 "about halfway from S Serpentarii [71 Ophiuchi] towards θ Serpentis." She saw "a Cluster of large stars. I counted around 80." It's intriguing that she didn't notice neighboring IC 4756. But the IC cluster was discovered much later, by examining a photographic plate. IC 4756 is quite obvious to modern observers using small instruments — and even our unaided eyes. But presumably 18th- and 19th-century astronomers weren't expecting star clusters to be so large and loose. William could easily have missed this cluster because his scope's field of view was so small, but Caroline's comet sweeper would have framed IC 4756 perfectly at low power.

IC 4665 is pinpointed precisely by
Caroline's notes. They go on to say: "A cluster of stars. I counted about 50 in the field; rather more than less." William observed this cluster through a larger telescope that night and reported: "It consists of about 14 or 16 large ones with several very small ones between. ... Lina found it."
In fact, this cluster and NGC 663 had been seen earlier by Philippe Loys de Chéseaux, but those discoveries were not published.

Charles Messier drew M110 on a sketch of the Andromeda Galaxy, but he didn't include it in his published catalog, so Caroline was unaware of his prior discovery. Her notes read: "About ½ deg preceding & a little north of Mess 31^st a nebula."

NGC 253 is one of the brightest galaxies in the sky, and a splendid sight from southerly latitudes. But it was a tough find for Caroline Herschel, who couldn't have seen it more than 12° above the southern horizon. She described it as "a faint nebula below the 2^d Triangle under β Ceti . . . Mess. has it not."

NGC 225 was the first of three star clusters in Cassiopeia that Caroline observed on September 27, 1783 and reobserved on October 30th. William didn't see any of these at the time, so any identifications that he made with clusters from his own catalog must have been based on his sister's notes or memory.

Caroline's notes from the October session pin the location of the first cluster quite precisely: "1 ½ deg. from γ toward κ Cass. (by the finder) the first cluster of Sept^r 27^th." NGC 225 is almost directly between these two stars, and it's easy to see in small telescopes, so there's no real doubt about this cluster's identity. True, the distance from γ Cas is actually 1.9° rather than 1.5°, but it's easy to be off by 30% in a distance estimate.

The September notes describe the location somewhat differently: "About 2 degrees from γ Cassiopeia making an Isosceles triangle with γ & κ, a small cluster of stars, seemingly intermixed with nebulosity." NGC 225 is indeed nearly equidistant from these two stars, but it's odd to describe what is in effect a straight line as the legs of an isosceles triangle. It's true that Caroline could fit her cluster in the same field of view with either star individually, but couldn't fit all three together. So she might indeed have determined that the distances were equal without noticing that they were also in a straight line. But it also seems conceivable that Caroline observed some cluster or asterism other than NGC 225 on September 27th.

NGC 189 is even more mysterious. William identified Caroline's second Cassiopeia cluster as NGC 381, but this is nowhere near the location that she specified. What Caroline actually wrote on September 27th was: "About 1° south of the above cluster a faint nebula surrounded with a great number of both large and small stars. there are more large stars in the field than are marked here but I took particular notice of the two between which the nebula is situated."

If Caroline's first cluster was NGC 225, then NGC 189 is a pretty good match for the location. But NGC 189 is not between two bright stars, and at magnitude 8.8 it would be difficult to see through her modest telescope. Caroline's sketch is shown at right. Can you find a star grouping in this area that resembles it, with another cluster or asterism 1° to the north that might be confused with NGC 225? It's interesting that in the catalog that Caroline drafted but never published, she annotated this one as "not to be found."

NGC 659 is credited to Caroline in William Herschel's published catalog, but I have some doubts about the identification. Her notes from September 27th say: "δ and ε Cassiopeiae & χ Persei making a trefoil. A cluster of stars in the middle." And on October 30th: "I saw the cluster which is placed between &delta & ε Cassiopeiae and χ Persei (a crouded place)."

The star that the Herschels mistakenly called χ Persei is in fact 7 Persei (S&T: Feb. 2003, page 116). This star forms an extended triangle with δ and ε. Cassiopeiae. NGC 659 is indeed inside this triangle, but it's far off to one side.
Moreover, I find NGC 659 very hard to resolve in a small telescope at low magnification. And I can't imagine viewing NGC 659 without noticing its much bigger and brighter neighbor NGC 663 just {1/2}° away.

There is indeed a cataloged cluster near the center of the triangle formed by δ Cas, ε Cas, and 7 Per, namely NGC 743. But this cluster was first sighted by John Herschel, William's son, long after William had identified Caroline's cluster as NGC 659. Perhaps William came to this conclusion because of the three clusters that he did observe in this area (NGC 654, 659, and 663), this is the one that's closest to the triangle's center.

What do you think is the best candidate for a star grouping between δ Cas, ε Cas, and 7 Per that's prominent in a small telescope? It might be an asterism rather than a true cluster. As Caroline said, central Cassiopeia is "a very crouded place." Do I detect a hint of frustration in that phrase? Dazzled as we are by the Herschel siblings' accomplishments, it's easy to forget that they were novice deep-sky observers at this point — inventing the entire discipline as they went. All they had to guide them was the thin catalog that Charles Messier had just published. Yet even the most experienced observer, armed with modern charts and catalogs, can be confused by the swarms of stars in this part of the sky.

There's no such doubt about NGC 752, which Caroline noted as: "About 3° south of γ Andromedae, a fine Cluster of Stars." The actual distance is 4.5°, but otherwise NGC 752 matches this description perfectly.

NGC 7789, perhaps the loveliest of Caroline's discoveries, features in her notes as follows: "Between σ and ρ Cassiopeiae a fine nebula, very strong." It's intriguing that she described it as a nebula, since it must have been very nearly resolvable in her telescope. Too bad her brother didn't stop by with a higher-power eyepiece, as he did for NGC 6866!

NGC 6819 is located "halfway between δ Cyg & η & θ Lyrae making an isosceles triangle downward when in that situation," just as Caroline noted. She described it as a star cluster but gave no details.

NGC 7380 is noted in William's catalog as one of Caroline's discoveries, but it's somewhat off the position described in her notes: "I saw a nebulous patch in a line from ε Cephei continued through δ towards 1^st and 2^nd Fl. Cassiopeiae." A line from ε through δ Cephei has to take a pretty sharp turn to reach NGC 7380. Perhaps the usually careful Caroline really meant ζ rather than ε Cephei.