Quote:
Originally Posted by shelltree
Hmmm  do you think maybe I was looking at something else then? Seemed to be in the right spot and had the slightest amount of fuzz. No detail at all, just seemed like a star that was smudged so I knew it wasn't "just a star". The picture you've provided seems spot on the money too, so I'm really confused now haha. And I've found it a few times now in the same spot. |
Shelley (and Suzy),
I shall try to explain a little more clearly how an Elliptical Galaxy or an S0 galaxy looks to the visual observer in many and various viewing conditions, so as to get a better handle on what we can actually hope to see in this quite bright - but
large and extremely diffuse - object;
(I used to come back to this galaxy every time I observed, when I was regularly using a Celestron C8 and also a 10 inch Newt, so I have seen it under many different sky qualities)
The picture of NGC 5102 which I previously attached in this thread is displayed to faint levels (fainter than we normally see with the eye!). So nearly all of the extent of this galaxy that is shown in this picture is
not visible to the eye, except in a very good dark sky.......However, I am quite sure that Shelley has seen at least the very central part of this galaxy in her recent observation;
The light which is diffused throughout a Moderately Light Polluted sky actually overwhelms
all but the fuzzy/star-like centre of this galaxy, so
what Shelley is probably seeing visually is
just the fuzzy starlike area which can be seen at
the very centre of the galaxy image that I provided. (call this the "core" of NGC 5102, if you like)
Shelley, I have little doubt that you
are seeing this galaxy(!), , because there is nothing much else that is diffuse (looking like an extended object) near to its coordinates in our sky. A lot of galaxies have a
small central part that is intense, somewhat star-like, or "fuzzy ball" in appearance......so if the sky is not good enough, then
that is all that the visual observer sees.
If we consider the observed intensity of the light coming from a galaxy (any galaxy), the observed
surface brightness (at a particular distance from the centre of a galaxy)
decreases with progressively increasing radius from the centre of that galaxy
[
surface brightness is similar in a way to the
observed intensity of a stellar object, except the concept of surface brightness applies to an
extended (non-stellar) object;
in other words, the surface brightness of a particular area of a nebula or galaxy is the
total observed brightness of that area divided by the actual area over which that brightness is spread)
The better the sky, the fainter the surface brightness that your eye can detect;
- so if the sky is real bad, then maybe you only see the point-like centre of a galaxy
- but if the sky is good, you can see out to a significant radius.
- If the sky is superb and you can see to a really faint surface brightness, then a galaxy can look very very extended.
Over the actual image of any galaxy, it is possible to draw
lines of equal surface brightness, which are usually shaped like circles or ovals. (these lines are called
isophotes).
The innermost ovals are all that can be seen in a poor sky, whereas, we see more and more of the fainter and outer ovals in better and better skies.
To illustrate this, here are the isophotes (ovals of equal brightness) in an image of NGC 5102:
The isophotes (ovals of equal surface brightness) get successively fainter and fainter, with increased distance from the centre of the galaxy, so it is easy to see how the galaxy looks progressively larger with better and better observations.''
cheers, Rob
P.S.
Suzy, I do agree that this galaxy can look somewhat rectangular at times.
Hey there folks, I just found some isophotes for NGC 5102 which are actually published ones (unlike my above crude attempt at displaying lines of equal brightness). Here are the isophotes of NGC 5102 from a Korean paper by Sohn, Chun, and Byun:
