Quote:
Originally Posted by shelltree
Observation Report 20.7.12
The next target on my list was NGC5102. I had looked for this galaxy my last time observing and had come up empty. On my star maps, it seemed like the galaxy was almost on top of iota centauri but after checking it out in Stellarium, I realised it wasn't as close as I had first assumed. I made sure I placed iota centauri at the edge of the eyepiece and there it was! So very, very faint, I am really surprised I even found it. An average sized star, no bigger or brighter than the rest yet with the slightest of fuzzy smudges through the middle, I knew I had found it.
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NGC 5102 and NGC 5253 are two Dwarf S0 (pronounced "Ess-Zero") galaxies in the nearby Centaurus Group of Galaxies that includes NGC 4945, NGC 5128, & M83.
The innermost portions of NGC 5253 and NGC 5102 are of reasonable surface brightness, but over the years I have found that it takes a really excellent (dark and transparent) sky to see them as more extended objects.
Both objects contain a "spheroidal" component (a bulge) together with a flattened "disk" component, which makes them nominally galaxies of Hubble Type S0, in that S0 galaxies are disk+bulge systems having overwhelmingly old stellar populations.
What you may be seeing is just a small central part of the overall galaxy, in the case of NGC5102.
In the telescope, under perfect conditions, NGC5102 resembles a bright spheroid, together with a surrounding "envelope" with a lens-like outline.
The very extended envelope is what really makes this galaxy look like a galaxy, but it is totally invisible in conditions that are not perfect. (it is overwhelmed by too much sky brightness ; also there is not enough transparency to give a good signal to the eye from this vanishingly faint diffuse light that is composed of innumerable star images!).
Take another look at this galaxy in really excellent dark-sky conditions; you will be surprised how big it really is.
This said, both of the dwarf S0 galaxies in the Centaurus group have been thought to be unusual;
NGC 5102 has relatively blue colours (for an S0 galaxy) due to a recently ended ( though modest) episode of star formation, and NGC 5253 has a bright and compact centre of irregular appearance...... which is a bona fide ongoing (or very recent) burst of star formation!
[[ There has recently been an ongoing paradigm shift in humankind's knowledge of S0 galaxies; giant S0 galaxies are disk+bulge systems with overwhelmingly old stellar populations, as has been known for a long time, but it was recently discovered that dwarf S0 galaxies often do have some ongoing star formation ]
NGC 5102 looks "faint", but it ain't really! Think of this object as being comparable in brightness to a 9.6 magnitude star......except that the light of this galaxy is spread over a vast area of sky; the major (long) axis diameter of the diffuse envelope is probably some 9-10 arcminutes!!!