Obj is the object's name. In cases where the
object was observed with a narrowband filter, there are two
entries, one under the normal name and one with an appended f,
The first entry gives my ratings for M97 as seen (or not seen)
without a filter. The entry under M97f gives my ratings for
the object as seen through a Lumicon UHC filter.
U178, and U70,
are my ratings for each object in under suburban and urban
skies, using my 178mm and 70mm telescopes. The number
indicates the difficulty of seeing the object, as follows:
and the letter indicates how interesting or beautiful
the object is:
- 1 - very easy, obvious even to a beginner
- 2 - easy, immediately obvious to an experienced observer
- 3 - moderate, may take a little looking
- 4 - hard, need to know where in the field to look
- 5 - very hard, borderline observation, intermittently visible
- 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
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
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
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
The average surface brightness was computed mechanically
from the listed magnitude and size. The computation of
peak surface brightness is explained in my
Surface Brightness page.