Error on Abell 21 object name or coordinates?

General Questions
1

Hi,

Abell 21 object is considered in Telescopius as a planetary nebula (other name: Medusa nebula) located at 7h29 / 13°15 near Gemini constellation :

A search on google over Abell 21 confirm these information, considered as such everywhere (Wikipedia, astro amateur sites, etc.). Well, everywhere… except on the official catalogues !

A search on “Abell 21” in SIMBAD, for instance, gives a Galaxy cluster located at 20h30.8 / 28°37’39 near Pegasus constellation :

http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=abell+21&submit=SIMBAD+search

The same in NED:

https://ned.ipac.caltech.edu/cgi-bin/objsearch?extend=no&hconst=73&omegam=0.27&omegav=0.73&corr_z=1&out_csys=Equatorial&out_equinox=J2000.0&obj_sort=RA+or+Longitude&of=pre_text&zv_breaker=30000.0&list_limit=5&img_stamp=YES&objname=ACO%2021#ObjNo1

Stellarium locates it also as a galaxy cluster in Pegasus.

That’s quite strange, it seems there were an error made at some point and it has propagated everywhere. On what database is based Telescopius for the objects names and coordinates?

Regards,
Luis

2

Thanks for reporting this! You’ve actually stumbled upon one of the most difficult problems we face: correctly cross-reference different objects and all their designations.

About this specific issue, there are actually 2 Abell catalogs:

  • Abell Catalog of Planetary Nebula
  • Abell Catalog of Rich Clusters of Galaxies

That’s probably the reason why you find different objects for the same designation.

I’m planning a comprehensive review of catalogs later this year and I’ll hopefully address all issues like this one.

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