The Purpose Of These Pages, And Plans For Their Completion
The discussion of deep space objects in my online text requires only a few hundred images to adequately illustrate all the topics involving such objects. But there are thousands of images which could be presented, and more are being taken every year. What should be done with old images when new ones become available, which beg to be used as replacements? Or with new images which are truly spectacular, but don't provide any better illustration than the ones already in the online text?
The answer I have chosen is to create a set of pages listing various deep space objects according to one of the major catalogs of such objects compiled over the years -- Dreyer's New General Catalog (NGC) and Index Catalogs (IC), and Paturel's Catalog of Principal Galaxies (PGC). Those pages will be populated in stages, according to the following schedule:
(1) Pages have been created to provide a place for every NGC and IC object, and every page has some content. For the PGC pages, this stage is still under consideration, as there are so many PGC objects, that covering all of them is impossible.
(2) Every image already on the site which could be linked to an NGC/IC/PGC page will be linked to the appropriate page, and detailed information concerning those objects will be added to the linked pages, in addition to any information about them already on the site. This has been done for a small fraction of the images previously onsite, but should be completed by fall of 2010.
(3) A very long-term stage will be to provide entries for every NGC and IC object, and every PGC object which appears in one of the NGC/IC object images. Since this will involve 20 to 30 thousand objects, collecting and collating the data and images required will require a considerable amount of time. Given the rate of progress so far, I estimate that it will take the best part of 10 years to complete this stage. However, references are provided below which will allow interested individuals to look up the information that I will provide on this site on their own, if the objects they are interested in have not yet been posted on this site.
References Used For This Project
Needless to say, projects such as this collection of catalogs cannot be done by any individual, without relying on the work of others. The main references for my work are (1) The historic and modern NGC/IC data and list of NGC/IC discoverers compiled by Dr. Wolfgang Steinicke
, (2) Positional and identification information from the HyperLeda
database, (3) Extensive data listings from the NASA/IPAC NED
database, (4) Dr. Harold Corwin's
NGC/IC notes, (5) Malcolm Thomson's
Corrections to the IC, and (6) in circumstances where no other information was available, the WikiSky
visual database (unfortunately, WikiSky's correlation of images and data has many errors, but it does provide visual references which are not provided by the other sources). Much of the initial work on this project was based on the NGC/IC Project site, but most of the information of interest to me was obtained from the sources listed above, so I will be gradually replacing this second-hand information, and removing the links to it, as I revise individual pages.
It should be noted that although some of the information referred to here is available on Wikipedia and similar websites, much of what is present there is basically stolen from other sites. Wikipedia is doing a better job of requiring citations and references to the original work than it used to, but whether those citations actually received permission from the original authors is often ignored. Similarly, many of the images in Wikimedia Commons, though supposedly in the public domain, have incorrect attribution, and are actually copyrighted. I have tried as much as possible to find the original source material, and to receive permission to use whatever material is not actually in the public domain, and offer my sincerest apologies to anyone whose work I have inadvertently used without receiving proper permission; and would appreciate their letting me know if such is the case, so that I may ask for permission and provide them with credit where credit is due.
Use of Those References
(to be added)
Historical References for the NGC/IC Objects
Although I intend to eventually make this catalog sufficiently complete that casual users should have no need to refer to the original papers that the NGC/IC catalogs and their predecessors and successors were based on, there are bound to be users who would like to see the original historical records. There are hundreds of those, and finding them online (or anywhere else) can be next to impossible. To satisfy the needs of those readers, I am now adding Discovery Notes to many NGC/IC entries, and plan to add links from those notes to a page which contains links to all the online references
I am aware of. I created the page on Apr 20, 2017, so until that date is well in the past it will be somewhat incomplete, but I have listed most of the references, and am gradually adding links to online copies of the reference materials.
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 27% 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 usually 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 approximate 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 a few cases neither database lists recessional velocities, but I happened to find the value in a research paper.) In the case of most relatively nearby galaxies NED lists one or more 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 apparent size is determined by direct measurements of the images posted with such entries.
Image Sources and Copyright Information
All images on this site are either in the public domain, or are posted with the permission of the copyright holder; but as explained on my Copyright and Fair Use Page
, that permission does not extend to commercial use, and even noncommercial use requires credit to be given where credit is due, particularly where a caption reads "used by permission"; so read the Fair Use page before you use any image anywhere else. Many images have the original source credited, and a link to that source. The one exception (and a major one, because of the number of images for which it applies) is that for many objects the only images available are "cutouts" of DSS, SDSS or PanSTARRS-1 photomosaics. In general, these images may be used for non-commercial purposes, such as on this website, merely by giving credit where credit is due; but more information about copyright terms for DSS images can be found here
, and those for PanSTARRS-1 images are listed under "Credit Where It Is Due" on this page
. (Note: For images posted early on, the attribution for DSS and SDSS images is in the alt and/or title tags for the images; but the note "updating to current standard" found on many pages includes moving the attribution to the image captions, and for most pages that has already been done.) It should also be noted that I have digitally adjusted virtually every image in one way or another, to make it easier to see structures that are not as well displayed in the original. In addition, every entry represents an amalgation of material obtained from ten to fifteen references, and involved a considerable amount of work on my part to find, verify, edit and explain; so some kind of note crediting this site as the source of anything posted elsewhere would be greatly appreciated.
History of the New General and Index Catalogs
(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. Strictly speaking, PGC numbers greater than the original 73,197 objects are "deprecated" (that is, their use is discouraged); but unless a search of the LEDA database for a PGC number fails to return a result I plan to show the PGC designation in the title for its entry.
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: