by Jay Reynolds Freeman
Several people have privately expressed admiration or disbelief of my observation of the entire Herschel-400 list in a 55 mm refractor, reported on p. 114 of the May, 1999 issue of Sky & Telescope. I welcome skepticism, but I do not think praise is warranted. I paid little attention to published magnitudes of the objects during my survey, for the target was a specific list, not a magnitude limit. Yet I did not and do not believe that the faintest objects on the Herschel-400 list are as difficult as commonly perceived.
I have informally known for a long time that many older cataloged magnitudes of deep-sky objects systematically underestimate their brightness to the visual observer -- the fact that many magnitudes were obtained in blue wavelengths guarantees that result, for many kinds of deep-sky objects are intrinsically brighter in the visual than in the blue, and reddening by interstellar dust creates an additional brightness difference, favoring visual wavelengths over blue, for objects near the plane of the Milky Way. To quantify these effects, I decided to look up recent visual magnitudes for at least the fainter objects on the Herschel-400 list.
I downloaded the official Herschel-400 list from the web site of the Astronomical League (AL) on April 3, 1999, from the URL
The magnitudes given by the Astronomical League are asserted to be visual ones, though rather old. I investigated objects for which a magnitude 12.5 or fainter was given, as well as those for which no magnitude was provided. (The latter are indicated by "0.0" for the Astronomical League's magnitude.) That gave me 72 objects in all, 18 percent of the list. I looked up their visual magnitudes in more recent catalogs (see footnotes), and have tabulated the results in Table 1.
I found visual magnitudes for 57 of the 72 objects. The faintest was 12.32, for NGC 4085, and only one other object, NGC 3395, had a recent visual magnitude fainter than 12.0. Some of the objects for which I found no visual magnitude are galaxies with blue magnitudes faint enough to suggest that their visual magnitudes might also exceed 12.0. The faintest such blue magnitude is 13.06, for NGC 3912.
The faintest Herschel-400 object, according to the Astronomical League's data, is NGC 6540, with visual magnitude given as 14.5. Other catalogs suggest this value was obtained in the blue, as further indicated in the tabulation and footnotes. Brian Skiff (private communication, April, 1999) suggested that there had been no good measurement of the visual magnitude of NGC 6540 so far. Given the position of NGC 6540 in the Sagittarius Milky Way, it would not be surprising if it were highly reddened, with visual magnitude significantly brighter than blue, but I shall not speculate on how much.
The second-faintest Herschel-400 object, according to the Astronomical League's data, is NGC 6369, with a visual magnitude of 14.0 given. A more recent visual magnitude is 11.4. The third- and fourth-faintest objects, according to the Astronomical League's data, are the planetary nebulae NGC 1501 and NGC 7008, both with magnitudes given as 13.5, but with recent visual magnitudes of 11.5 and 10.7 respectively.
I don't think the general nature of these differences is as well-known as it ought to be. Perhaps because the Astronomical League's tabulation is commonly used as a reference for magnitudes of the Herschel 400, that list is often believed to be a difficult challenge, containing objects beyond magnitude 14. Yet in fact, its faintest visual magnitudes seem two magnitudes brighter. Other things being equal, that correction corresponds to a reduction by 60 percent -- a factor of 2.5 -- in the clear aperture (diameter) required to see all the objects on the list. Thus visual observers should take heart, for the Herschel-400 list seems much more accessible to them than is widely believed.
| NGC# | Type (1) | Magnitudes (2) | Comments | |
|---|---|---|---|---|
| AL | recent | |||
| 246 | PlN | ---- | 10.9 | |
| 278 | Gal | 12.5 | 10.85 | |
| 596 | Gal | 12.5 | 10.87 | |
| 615 | Gal | 12.5 | 11.5 | |
| 1022 | Gal | 12.5 | 11.44 | |
| 1501 | PlN | 13.5 | 11.5 | |
| 1535 | PlN | ---- | 9.6 | |
| 1788 | DfN | ---- | ----- | |
| 1931 | C/N | 13.0 | 11.3 | |
| 1999 | DfN | ---- | ----- | |
| 2022 | PlN | 13.0 | 11.6 | |
| 2024 | DfN | ---- | ----- | "Flame Nebula" or "Tank Tracks" |
| 2185 | DfN | ---- | ----- | |
| 2371 | PlN | 13.0 | 11.2 | for NGC 2371 and 2372 combined |
| 2372 | PlN | 13.0 | see NGC 2371 | |
| 2392 | PlN | ---- | 9.1 | |
| 2479 | OCl | ---- | ----- | 9.6 (4), 10 (5) |
| 2742 | Gal | 12.5 | 11.7 | |
| 2782 | Gal | 12.5 | 11.49 | |
| 2811 | Gal | 13.0 | 11.27 | |
| 2950 | Gal | 12.5 | 10.95 | |
| 2964 | Gal | 12.5 | 11.34 | |
| 2974 | Gal | 12.5 | 10.78 | |
| 3193 | Gal | 12.5 | 10.92 | |
| 3226 | Gal | 12.5 | 11.4 | |
| 3242 | PlN | ---- | 7.7 | |
| 3277 | Gal | 13.0 | 11.74 | |
| 3395 | Gal | 12.5 | 12.1 | |
| 3608 | Gal | 12.5 | 10.98 | |
| 3619 | Gal | 12.5 | ----- | blue magnitude 12.6 (3) |
| 3655 | Gal | 13.0 | 11.64 | |
| 3665 | Gal | 12.5 | 10.76 | |
| 3729 | Gal | 13.0 | 11.38 | |
| 3813 | Gal | 13.0 | 11.72 | |
| 3900 | Gal | 12.5 | 11.38 | |
| 3912 | Gal | 13.0 | ----- | blue magnitude 13.06 (3) |
| 3962 | Gal | 12.5 | 10.59 | |
| 3982 | Gal | 12.5 | ----- | blue magnitude 11.74 (3) |
| 4085 | Gal | 13.0 | 12.32 | |
| 4102 | Gal | 12.5 | ----- | blue magnitude 12.3 (3) |
| 4143 | Gal | 12.5 | ----- | blue magnitude 12.1 (3) |
| 4150 | Gal | 12.5 | 11.66 | |
| 4245 | Gal | 12.5 | 11.36 | |
| 4273 | Gal | 12.5 | 11.92 | |
| 4281 | Gal | 12.5 | 11.32 | |
| 4346 | Gal | 12.5 | ----- | blue magnitude 12.18 (3) |
| 4419 | Gal | 12.5 | 11.13 | |
| 4478 | Gal | 12.5 | 11.23 | |
| 4485 | Gal | 13.0 | 11.96 | |
| 4550 | Gal | 12.5 | 11.59 | |
| 4660 | Gal | 12.5 | 10.99 | |
| 4800 | Gal | 13.0 | ----- | blue magnitude 12.3 (3) |
| 4845 | Gal | 12.5 | ----- | blue magnitude 12.10 (3) |
| 5273 | Gal | 12.5 | 11.57 | |
| 5473 | Gal | 13.0 | 11.4 | |
| 5557 | Gal | 13.0 | 11.10 | |
| 5631 | Gal | 12.5 | ----- | blue magnitude 12.50 (3) |
| 5689 | Gal | 12.5 | 11.9 | |
| 5982 | Gal | 12.5 | 11.13 | |
| 6207 | Gal | 12.5 | 11.62 | |
| 6217 | Gal | 12.5 | 11.22 | |
| 6369 | PlN | 14.0 | 11.4 | |
| 6401 | GCl | ---- | 9.5 | |
| 6426 | GCl | 12.5 | 11.20 | |
| 6445 | PlN | 13.0 | 11.2 | |
| 6517 | GCl | 13.0 | 10.3 | |
| 6540 | OCl | 14.5 | ----- | 14.6 (4), 15 (5) |
| 6544 | GCl | ---- | 8.25 | |
| 6781 | PlN | 12.5 | 11.4 | |
| 7000 | DfN | ---- | ----- | North American Nebula |
| 7008 | PlN | 13.5 | 10.7 | |
| 7448 | Gal | 12.5 | 11.65 | |
Footnotes:
The exact nature of a few of these objects possibly differs from the types listed, which are as the Astronomical League provided; however, I shall not address those issues here.
m2: Visual magnitude from more recent compilations, generally from
Sky Catalog 2000.0, volume 2, 1985. Alan Hirshfeld and Roger W. Sinnott, eds. Sky Publishing.
Sky Catalog 2000.0 did not give visual magnitudes for planetary nebulae, so I obtained those from
Planetary Nebulae, 1991. Steven J. Hynes. Willmann-Bell.