| Diffuse
Nebula?
aka PNG 173.7-05.8, PK 173-05.1, ARO 175 Integrated Visual Magnitude: 13.8 Apparent Diameter: 2.2' Magnitude of central star: 18.2 Mean Surface Brightness: 24.1 mag/arc-sec2 Distance: 3700 ly Actual Diameter: 2.3 ly Minimum requirements to view: 16-inch scope, OIII or UHC filter and very dark skies |
This
object is both a challenge and an enigma. A 12.6 magnitude irregular
galaxy (PGC 16765 = UCGA 100) is listed at this same position, and it appears
to have been included as a galaxy in research papers as recently as 1997.
It has also been variously classified as a planetary nebula, HII region,
and reflection nebula. That just about covers it! Even today
the SIMBAD database lists it as a reflection nebula, while the NED database
lists it as a planetary.
In photographs it looks like an irregular, yet diffuse, broken ring, not unlike a fat horseshoe. The OIII filter really brings it out, so it seems apparent that it isn't a galaxy. Galaxies aren't enhanced by this filter.
Observing this object will probably require at least a 16-inch scope and an OIII filter. Don't use too much magnification on this one. I recommend starting with your lowest power eyepiece. It was too large for my 12.5mm Super Plossl (166x) in my 18-inch f/4.5. It was invisible in that eyepiece even with the filter. Yet, at 94x it was readily apparent (although by no means bright). I noted that it appeared as a nice round haze with brighter outer edges on opposing sides.
The field in an 18-inch
at 94x. North is down and east is to the right.
| Millennium
Star Atlas Vol I Chart 136
Sky Atlas 2000 Chart 5 Uranometria 2000 Vol I Chart 97 |
