A catalogue of one hundred and nine deep sky objects created by the astronomer and broadcaster Sir Patrick Moore. The Caldwell Catalogue is designed to expand on the established Messier Catalogue to provide a comprehensive collection of the sky's most prominent clusters, nebulae and galaxies.
Origins and Purpose
Charles Messier first published his catalogue in 1774, and it has since been expanded to include a total of 110 objects, with Messier numbers still commonly used for some of the most important objects in the sky (such as M1 for the Crab Nebula, or M31 for the Andromeda Galaxy). It does, however, have a number of limitations. Notably, as Messier was based in Paris, he could not observe the far southern sky (the southernmost object in the catalogue is M7, Ptolemy's Cluster in Scorpius, with a declination of just -35°). Because Messier originally intended to confine his list to objects that could be mistaken for comets, certain important objects of the northern sky are also omitted, such as the Hyades in Taurus or the Double Cluster in Perseus.
The Caldwell Catalogue is an attempt to address these limitations by creating a complementary list to the Messier Catalogue. It contains an additional 109 objects, a number chosen to reflect the more traditional count of Messier's Catalogue (though to confuse matters, the galaxyM110 is now generally accepted as part of the Messier collection).
Messier objects have their numbers prefixed by 'M', so naming the new listing directly after its creator, as the 'Moore Catalogue', would have created some confusion. Instead, Moore (full name Sir Patrick Alfred Caldwell-Moore) used the 'Caldwell' element of his surname so that the alternative prefix 'C' could be used for Caldwell objects.
The Caldwell Objects
The Caldwell Catalogue lists 45 star clusters, 35 galaxies and 29 nebulae of various kinds. So, in combination with the Messier Catalogue, the two sources describe a total of 100 clusters, 75 galaxies and 41 nebulae. This gives a combined total of 216* of the most prominent and important deep sky objects.
Among the entries in the Caldwell Catalogue are some of the brightest or most recognisable objects in the entire sky. Some of the most important or familiar catalogue entries include:
The remnant of a supernova explosion that forms a wide ring-shaped structure in Cygnus. The two Caldwell numbers respectively refer to the eastern and western parts of the nebula.
A pair of galaxies in Corvus. These two galaxies are interacting with one another, sending long thin tendrils of matter (the 'antennae' of their name) streaming out into space.
A wide patch of dark material, mainly within the constellationCrux, that blocks out much of the light of the Milky Way behind it. Shining through this dark dust is another Caldwell object, number C98, also known as the Coalsack Cluster
This table just gives a brief selection of some of the best-known or most prominent of the 109 Caldwell objects. For a full list, refer to the complete Caldwell Index.
Objects were added to the original Messier Catalogue in a rather arbitrary sequence, broadly speaking (and with various exceptions) in the order they were discovered or described. Because of this, the numbered Messier objects are essentially distributed at random across the sky. Later catalogues used a more systematic approach, so (for example) Flamsteedstellar designations and deep sky objects in the New General Catalogue are numbered in increasing order of right ascension (that is, from west to east on the celestial sphere).
At least one pair of objects in the catalogue are, strictly speaking, out of order. C27 (the Crescent Nebula in Cygnus) is marginally northward of C26 (the Silver Needle Galaxy in Canes Venatici), and should theoretically precede it in order (though the difference is trivial, amounting to less than degree). It could be said that the same issue arises between C42 (a globular cluster in Delphinus) and C41 (the Hyades), though this is a less clear-cut case because the Hyades cover such a wide area of sky. The Hyades' northern edge is northward of C42, and by that measure the catalogue order holds, but its central point is southward of C42, and on that basis the order of the two objects should in principle be reversed.
Because of its somewhat idiosyncratic numbering system, the Caldwell Catalogue has a feature not shared by other catalogues. For any latitude on Earth, an observer will be able to see a continguous band of numbered Caldwell objects, but none above or below a limiting value. To illustrate, from Charles Messier's base in Paris, all the numbered Caldwell objects are visible southward to about -40°, so to a Parisian astronomer the extreme limit would probably be C74, the Eight-Burst Nebula in Vela. Beyond this, no higher-numbered Caldwell objects will ever rise above the horizon. The opposite is true for the southern hemisphere, though from that hemisphere all the higher-numbered Caldwell objects are visible, and it is the lower-numbered objects that are remain permanently hidden below the horizon.
* With 110 objects in the Messier Catalogue, and 109 in the Caldwell Catalogue, it would be reasonable to expect a total of 219 objects altogether. However, three of the objects originally classified by Messier as star clusters have later been found to be misclassifications. Two of these, M40 anf M73, are actually just groups of unrelated stars on a similar line of sight. The third, M24, is a star cloud, part of a wider starfield glimpsed through a gap in intervening dust and gas, so that it gives the appearance of a distinct cluster.
