Reference Material For Distances And Sizes and How I Use It
(enlarged and roughly edited Mar 21, 2015)
Wherever possible, I have indicated the approximate distance and size of the objects included in these pages. For galactic objects the values are taken from whatever papers are willing to make an educated (or not) guess about the distance. For extragalactic objects, older entries estimate distances from recessional velocities by assuming a Hubble expansion velocity of 73 km/sec/Mpc, but I am gradually changing all such estimates to a value based on H = 70 km/sec/Mpc (not only is this closer to the currently accepted value, but happens to be the value I used during the 42 years I taught astronomy, so it has a certain nostalgia for me, aside from being more accurate). Such distances are subject to substantial error due to peculiar (non-Hubble expansion) velocities for nearby objects, so wherever possible I have compared the results to redshift-independent distance estimates. Sizes of objects are then obtained by comparing their apparent size to their distance.
If the recessional velocities of the objects are a substantial fraction of the speed of light, relativistic corrections are required to obtain accurate results. I find that the errors become noticeable at about 7000 km/sec, or a little over 2% of the speed of light, so eventually all entries with recessional velocities of 7000+ km/sec will include relativistic corrections. The calculations involved require certain assumptions, for which I have chosen (1) a "flat" geometry for the Universe, (2) a mass density of 27% of the "critical" mass density, and (3) a "vacuum density" of 73% (since the total for a "flat" Universe has to be 100%, this directly follows from the assumption that the mass of the Universe is only 23% of the critical density). For such entries I start with "a straightforward calculation indicates", meaning that if you just use the Hubble constant of 70 km/sec/Mpc, you get a particular distance. I then add "However", and a brief note about how the expansion of the Universe during the time it took the light to reach us alters the results. This not only allows posting more accurate results, but shows that for objects at large distances, the time it takes for their light to reach us is longer than the original distance in light years, because the intervening space expanded during the light-travel time. The times and distances involved are rounded to the nearest 5 million years or light-years, so the difference in light-travel time due to the Universal expansion is not exact, but merely an example of how things work.
No matter how the distance is determined, the size of the object is calculated by taking the distance in millions of light years, multiplying by the apparent size in seconds of arc, then dividing by 206000 (the number of seconds of arc in a radian). Recessional velocities are taken from the NASA Extragalactic Database (NED) when available, and if relativistic corrections are required, their value for z, the ratio of the recessional velocity to the speed of light, is used for the calculations. In some cases NED does not list a recessional velocity, but LEDA does; but since LEDA does not list z, I calculate that myself. (In very rare cases neither database lists recessional velocities, but I happened to find a value in a research paper.) In the case of most relatively nearby galaxies NED lists some redshift-independent distance measurements, and in such cases I include the range of such estimates in the discussion of the distance
When starting an entry, most of the physical information is taken from Dr. Wolfgang Steinicke's database (which is used by permission, per a Creative Commons license), and this includes the apparent size of the object (in rare cases he does not list a value, and I used NED or LEDA for one, if available). However, for fully completed entries, the listed size is determined by direct measurements of the close-up images posted with such entries. Image Sources and Copyright Information
(INCOMPLETE DISCUSSION; for a fascinating and very thorough discussion of the history and contents of the NGC, refer to Wolfgang Steinicke's book on the subject.)
When the New General Catalog was done, in the late 1800's and early 1900's, NGC numbers were assigned in order of right ascension, so NGC 1 had the smallest right ascension, NGC 2 the next smallest, and so on. Since then, precession has altered the positions of the Poles and the Celestial Equator by more than a degree, changing the right ascensions in different ways in different regions. As a result, objects at very different declinations may have their right ascensions slightly out of order. Objects in the same area, however, such as NGC 1 and NGC 2, still have increasing right ascension for increasing NGC numbers.
Epoch 2000.0 positions and basic object descriptions are based on the NASA/IPAC extragalactic database (NED), or where no description is available there, on Wolfgang Steinicke's databases. Where reasonably reliable, independent physical data are not available, estimates of size and distance are also based on that database. Where high-quality public-domain images are posted, credits are listed and links are provided to the original source material. Objects for which no other images are available are taken from, as indicated by their "mouse-over" descriptions, Wikisky cutouts.
Discovery data are taken from Wolfgang Steinicke's Historic NGC/IC databases, for which links are provided on the Discoverers page. Each observer's name is linked to an entry on that page, where additional links and biographical data links are provided.
I should note that the descriptions in Dreyer's catalogs are brief combinations of letters, which allowed for a lot of information in a very brief format. For instance, "F, S, dif, Epf" meant "faint, small, diffuse, extended east and west". To eliminate the need for readers of these pages to learn the abbreviations, I have expanded the abbreviations to full-word descriptions. However, there is an interesting historical aspect preserved in the abbreviation. Strictly speaking, Epf means extended preceding and following, not extended east and west; but historically, observers who cataloged hundreds or thousands of objects would measure their positions by letting them slowly drift across the field of view of their telescopes, as the sky rotated. As a result of the westward rotation of the sky, objects to the west would cross the field of view at a time preceding, and objects to the east would cross the field of view at a time following, the time at which the object in question did so. This relationship between the time that objects cross the sky and their position is in fact the reason that east-west positions (right ascension) are measured in time units. The Catalog of Principal Galaxies (PGC)
The PGC was first published as an extragalactic database in 1989 by Paturel et al. It contained coordinates and cross-identifications for 73,197 galaxies, with data taken from various sources for between twenty and sixty-seven thousand entries (more for data easily obtained, fewer for data requiring more effort). Due to errors in the references used to compile the catalog there are numerous duplications and non-galaxian entries, but overall it is a reliable reference, covering far more objects than earlier catalogs. Over the years additional data were added to the catalog, and in 2003 a completely new version of the catalog was published (primarilyl as an online database at HYPERLEDA), which was stated as "restricted to confirmed galaxies, i.e. about one million galaxies, brighter than ~18 B-mag", but the online database actually contained more than a million and a half entries, and as in the case of the original many are duplicate entries or nonexistent or misidentified non-galaxian objects. Over 50 catalogs were used as references for the 2003 version of the PGC, and between the numerous differing ways of listing objects and the inevitable errors in those references, PGC entry numbers actually run into the 4-millions plus (though searches of the database for high numeric entries generally fail, save as the result of a search for a non-PGC designation).
Despite the aforementioned problems, the PGC is the most reliable and consistent database of extragalactic objects, and where a PGC number (or numbers) can be unambiguously assigned to a NGC or IC object, I prefer using the PGC designation to any other method of identifying the object. Linking to Individual Objects
To link to individual Messier, NGC, IC or PGC objects, use the following format, where XX is the number of the object in question:
Messier Objects: http://cseligman.com/text/stars/messierpageaddress.htm#XX
NGC objects: http://cseligman.com/text/atlas/ngcpageaddress.htm#XX
IC objects: http://cseligman.com/text/atlas/icpageaddress.htm#icXX
PGC objects: http://cseligman.com/text/atlas/pgcpageaddress.htm#pgcXX