• A - spectacular
• B - beautiful or unusual
• C - unspectacular but interesting
• D - detectable but nearly featureless

Type is one of the following:

• C/N - cluster with nebulosity
• GAL - galaxy
• GCL - globular cluster
• NEB - bright nebula
• OCL - open cluster
• OTH - other
• PLN - planetary nebula
• SNR - supernova remnant

The distinction between open cluster, nebula, and cluster with nebulosity is fairly arbitrary in many cases; all of the diffuse nebulae in the Messier list have embedded stars, and in some cases (like M8 and M16) it is not clear whether the Messier designation applies to the nebula alone or to the nebula and the contained cluster.

Con is the official abbreviation for the constellation containing the object.

RA and Dec are the celestial co-ordinates for the object, derived from a variety of sources.

Mag is the object's integrated visual magnitude (also known simply as visual magnitude or magnitude).

PBrt and SBrt are the object's peak surface brightness and average surface brightness respectively, both in units of magnitudes per square arcsecond. These data are described in more detail in my Surface Brightness page. This value is currently given only for galaxies and globular clusters, and is derived from Brian Rachford's computation of the brightness of the central 0.5 arcminutes.

Size is given in arcminutes.

The data are derived from numerous sources. For galaxies, size and magnitude are taken from Brian Rachford's web site. He, in turn, took size and magnitude from the RC3 catalog. The peak surface brightness is derived from Rachford's computation of the brightness of the central arcminute, which he describes in detail on his web site.

For globular clusters, peak surface brightness is again computed from Rachford's brightness of the central arcminute, and the magnitude is taken from Rachford, who took it from William Harris's compilation of Milky Way Globular Cluster Parameters. Note that peak surface brightness data is missing for M14. The sizes of the globular clusters are taken from the Observing Handbook and Catalog of Deep-Sky Objects by Christian B. Luginbuhl and Brian A. Skiff. The authors explain that the sizes represent the limits where the density of cluster members is equal to the background density.

For planetary nebulae, both size and magnitude are taken from Luginbuhl and Skiff. Magnitude is visual magnitude (taken through a V filter), which typically runs one or two magnitudes brighter than traditional blue magnitudes in the case of planetary nebulae.

The nebulae and clusters with nebulosity, the magnitudes are taken from Wolfgang Steinecke's Revised NGC and the NGC-IC Project web site. The sizes are derived by me based on measurements of photographs confirmed by my own observations under dark skies. Note that they are often much smaller than the accepted values; I believe that sizes like 1.5 by 1 degree for M42 grossly over-state what visual observers consider to constitute the Orion Nebula. There is no objective answer, because all of the diffuse nebulae in the Messier list are actually parts of much larger nebulous areas; for instance, M42 and M78 are both pieces of the same giant molecular cloud, which also includes the famous Flame Nebula and many other components. Magnitudes for nebulae are even less objective and reliable.

Size and magnitude for open clusters are taken from Steinecke's Revised NGC, except in the case of M26, M48, and M67, where I consider Steinecke's sizes to be implausible. In those cases, I took the sizes from The Messier Objects by Stephen James O'Meara, who credits them in turn to the book Star Clusters by Steven Hynes and Brent A. Archinal. Also, I took the size and magnitude for M45, which is not listed in the NGC, from the version 7.2 database from the Saguaro Astronomy Club.

I derived the sizes and magnitudes of M40 and M73 from data displayed by version 8 of Sky Map Pro, which takes its data in turn from the highly reliable Tycho-2 catalog. Note that widely published data for these objects are wildly innacurate. This is inexcusable for objects consisting of 2 and 4 stars respectively, where objective and reasonably accurate data has been available for many decades.

The size and magnitude of the star cloud M24 was taken from the Saguaro Astronomy Club database.

Note that the sizes of diffuse objects are generally quite generous, representing the efforts of the best observers under fully dark skies, or even photographic sizes which can never be observed by human eyes. They are usually much larger than anything that could possibly be seen under urban or suburban skies. Note also that sizes are fairly arbitrary, since most diffuse objects and star clusters fade out gradually instead of having sharp edges.