800 Bright Deep Sky Objects
By Paul Markov, January 14, 2002

In this day and age having the right information is everything, and this rule seems to apply to deep sky observing as well.  More specifically, having the correct deep sky lists and catalogues will open up a window of deep sky observing possibilities that you were probably not aware of.  Did you know that there are almost 800 deep sky objects brighter than 10^th magnitude that are most likely visible with your telescope from your light polluted backyard?  This is hard to believe, but if you obtain a good deep sky catalogue, you can prove it to yourself.

I chose the Saguaro Astronomy Club (SAC) deep sky database (available as a free download at www.saguaroastro.org) which contains a total of 10593 deep sky objects from a collection of deep sky catalogues. All the information in this article is derived from version 7.2, the latest SAC database revision.

All of you are familiar with the Messier and New General Catalogues, and most have also heard of the Index Catalogue.  However, many observers do not know about the multitude of smaller catalogues like Trumpler, Collinder, Stock, Harvard, Basel, Berk, Melotte, etc. These lesser known catalogues make up about 20% of the entire SAC database and many of these are well within the reach of an average amateur telescope, even from light polluted skies.

The database includes objects visible from both northern and southern hemisphere, therefore before determining the number of objects visible from Toronto we must eliminate the ones that are not visible from our latitude. In theory we should be able to locate objects to about declination minus 45 degrees, however, due to atmospheric extinction (see RASC 2002 Observer’s Handbook pg. 57), I chose to only include objects with declination minus 35 or higher. In doing so the number of available deep sky objects decreased from 10593 to 8728 (the balance of the objects either never rise above the horizon from Toronto or are too close to the horizon to be seen).

Because the purpose of this article is to discuss brighter objects, I chose to eliminate objects fainter than magnitude 10.0, which further reduces the number of objects to 556. It is very important to note that many deep sky objects (especially the ones listed in the more obscure catalogues) do not have measured magnitudes and are not included in the 556 count noted above. There are 703 of these in the SAC database with a declination of minus 35 or higher.  From the 703 objects I estimated that approximately 237 are magnitude 10.0 or brighter (based on my own observations, I find that the majority of deep sky objects that are stellar in nature, such as open clusters and asterisms, are relatively bright, thus I estimated that 50% of such objects are brighter than magnitude 10.0. Galaxies and all types of nebulae were not included at all because these are most certainly fainter than magnitude 10.0).

The sum of 556 and 237 results in a total of 793 deep sky objects that are waiting to be observed, even from your urban backyard!  The only caveat is that many of these obscure catalogue objects are not terribly impressive, but then again, neither are barely detectable 13^th magnitude galaxies!

Let’s take a closer look at these 793 “brighter” deep sky objects and try to categorize them. Table 1 outlines deep sky object types of magnitude 10 or brighter, while Table 2 outlines deep sky objects with unknown magnitude, but estimated to be at least magnitude 10 or brighter.

TABLE 1 – Magnitude is known to be 10 or brighter

TABLE 2 – Unknown magnitude, but estimated to be mag. 10 or brighter

Objects categorized as “1 star and 2 star” are typically NGC objects that were thought to be actual deep sky objects but were later found out to be just individual stars. There are hundreds of these objects but I suspect the majority are quite faint, therefore none were included in Table 2. Certainly these will be the most “boring” deep sky objects you will have ever seen, but you can use them to practice your star hopping skills! And although these are not true deep sky objects they are still part of the NGC catalogue which can be “checked off” when found. Asterisms are star groupings that to us amateur astronomers are just as nice to look at as “real” open clusters, except that scientifically speaking, asterisms are not considered to be true deep sky objects because they are just chance alignment of stars which are not gravitationally related. The 44 asterism listed in Table 1 and Table 2 all carry NGC designations, thus these are also worthwhile observing and checking off your observing list.

The objects listed as Bright Nebula, Cluster + Nebula, Galaxy, Planetary Nebula, and Supernova Remnant in Table 1 may be a little more difficult to see depending on your telescope size and amount of light pollution. However, I think that given a telescope of about 8-inches and moderate light pollution these non-stellar objects should be visible.

That leaves just the Open Cluster objects listed in both Table 1 and Table 2 – a total of 524 objects.  Coincidentally open clusters are the most abundant given the magnitude 10 limit, and they also happen to be the least affected by light pollution. The sheer number of objects should be enough to keep you busy for years, but do not expect any of these clusters to be very impressive.  The majority will be just one or two dozen scattered stars, which can be easily mistaken for background stars.  Still, there are a handful of open clusters that are ridiculously bright and many observers do not realize these are actual deep sky objects that belong to one catalogue or another. Table 3 lists some these objects to a limiting magnitude of 4 (does not include obvious objects such as the Pleiades, the Beehive, or Hyades).

TABLE 3 – List of bright deep sky objects

(Cr = Collinder, Mel = Melotte, Steph = Stephenson)

Finally, if you intend to track down these hundreds of objects, you will definitely need a good star atlas, such as Uranometria 2000 as a minimum.  Most will not be plotted in Sky Atlas 2000, and you cannot rely on a GOTO telescope to show you the correct object because distinguishing the target from background stars may be difficult in many cases. The only way to be absolutely certain you have located an object is to compare what you are seeing against a good star atlas. Another alternative is to get a good star charting software package, such as Earth-Centered Universe, which actually uses the SAC database as its deep sky engine and can plot stars to 16^th magnitude.

Copyright (C) 2002 by Paul Markov

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