Good day to you all! This is my first post here but I am not a n00b to forums or science. I hope you all find this an interesting topic and we can find some answers. For the last few weeks I've been searching the net for definitions to the acronyms we use in classifying extremly deep space galaxies and quasars.

Here is what I have found so far:

A - Abell - a class of galaxy clusters or groups of galaxies (also ABCG, AC and ACO)

Arp - Halton Arp Atlas Peculiar galaxies - odd shaped glaxies.

M - Messier - A smaller collection of ~110 galactic object farthest of which is 50 million light years

NGC - New General Catalog - the best known system of tracking most galaxies within 75 million light years

PKS - Perek-Kohoutek System - more nebulae then galaxy

Q - Quasar - active galaxy emiting radio waves

QSO - Quasi Stellar Object - quasar without the radio

However I cannot find information on these and I do hope you can help.
Where possible I will give examples:

3C - Celestial Candidate Companion??? - can't seem to confirm 3C9, 3C68.1

OH - ??? - OH471

OQ - ??? - OQ172

PG - ??? - PG 1247+26

PHL - ??? - PHL957, PHL5200

So could you please help?

For one thing, NGC is not just galaxies. It is a list, ordered in RA, of non-stellar objects. It includes many nebulae, open clusters and globular clusters.

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3C = Third Cambridge Catalog (of radio soures that are bright at low freqeuncies). This got many of the local radio galaxies and quasars as well as galactic sources, so these names are still used a good bit. 3C 273 is the brightest quasar, 3C 405 is the radio galaxy Cygnus A, 3C 120 is well-known only by that name. 3C 75 is a binary pair of galaxies each with twin radio jets. (The first two catalogs in teh series were much less reliable and immediately dropped from usefulness). There are also 4C, 5C, 6C surveys which are much deeper cover progressively less of the sky.

OH, OQ... are radio designations from a survey done at Ohio State University almost 50 years ago. The second latter denotes declination bands around the sky.

PG - Palomar-Green, a survey of the sky for blue objects done by Richard Green for his dissertation. This captured a fair number of brightish QSOs that had been hitherto unknown (thus giving clues about how complete the surveys might be), as well as nearby white dwarfs and cataclysmic variable stars.

PHL = Palomar Haro-Luyten, a search for faint blue stars at high galactic latitudes which also netted some things which eventually turned out to be quasars (PHL 5200 with its spectacular absorption lines from an outflow may be the best known).

How does the NGC catalog number tie to the r.a. number, or is there a direct connection?

So there are Green stars, but they are blue. Stellarchromology is pleased to learn this.

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The good news is that the NGC numbers are in running order of right ascension. The bad news is that the right ascension was as estimated (often rather poorly by today's standards) and was expressed in the then-current epoch of 1855 (not so new anymore). By now, precession has changed the RAs by amounts which depend on location in the sky, so that NGC objects with very different declination can be substantially out of RA order. This is the largest catalog I can think of with serially numbered entries which is that old, but the problem also occurs with the UGC (epoch 1950). The Third Reference Catalog would list the NGC numbers of galaxies on each page at the bottom, evidently for those of us who would try to find galaxies by paging through looking at the NGC numbers and hoping the declination wouldn't have not them too far out of order. For example, NGC 3310 is about 40 degrees to the north of 3308,9,11,12,13... and has thus jumped line in contemporary coordinates.

Ok, so the NGC number itself is only a general r.a. indicator with about every increment of roughly 325 representing the next hour angle of r.a.

From my little Google work, there is quite a history to all this. Very interesting. Dreyer had his hands full, apparently.

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Very roughly. With the Virgo and Coma clusters, the single range from 12-13 hours in RA encompasses (pretty exactly) NGC numbers from 4000-5000 all by itself (at one time that was a handy thing for galaxy aficionados to know, as was having NGC 3000 at about 10 hours).

It is a bit ironic that at the time of the original catalog, there were no galaxies; these were all nebulae.

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OH - ??? - OH471

To expand on what Kaptain K already wrote, "OH" refers specifically to hydroxyl maser emission sources, which is just a fancy way of saying something gives off lots of microwave radiation.

See James Kaler's "The Hundred Greatest Stars" (Copernicus, 2002), p. 137.

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Except in the context of a cosmic radio source, in which it's one of the pieces of the Ohio State radio survey. NED lists sources starting with OA, OB, OC...all the way to OZ. As if that's weren't bad enough, just get me started on all the meanings of z,Z or k, K. But it's not very long before we just run out of letters and have to recycle.

Time to introduce new alphabets! Hebrew is a good one...

A mathematician friend of mine told me that someone was describing a theorem to him, but before they could start, they had to teach him two non-latin alphabets...

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Any AL classifications? Crimson dust concentrations perhaps?

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George...and we all thought you'd be green with envy... pete

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I'll let you inform the BA that there are green stars -- contrary to his recent blog.

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