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Star associations in NGC 2403

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#1 IVM

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Posted 08 February 2013 - 06:01 PM

Earlier this week I went to Cherry Springs with my 16" and under fine but unexceptional conditions detected a dozen star associations in NGC 2403, a galaxy in Camelopardalis. I had Steve Gottlieb's latest S&T article with me and easily found the 5 objects labeled on the excellent photo there. I then made a sketch and upon return used the paper by Paul Hodge ("Stellar associations in the galaxy NGC 2403", Publications of the Astronomical Society of the Pacific, 1985, 97:1065) to identify the catalogued star associations on my sketch.

Associations Hodge 33, 29, 18, and 27 are unresolved individually but together form the most prominent spiral arm segment that was described already by Herschel as a "branch". Hodge 67 was, in essence, discovered by Bigourdan in late 19th century visually with a 12" refractor and catalogued as NGC 2404. I had seen these features previously with my 12" SCT along with the stellar-looking Hodge 41 (http://ivm-deep-sky....1/ngc-2403.html).

Associations 45 and 36 are compact and round and probably dominate visually without a filter the objects identified in Steve Gottlieb's article as HII regions VS 38 and VS 24. Similarly, associations 14 and 81 coincide with VS 3 and VS 52.

Association 59 contains the small bright knot on the edge of a star cloud that involves also Hodge 48 on the other end. As such, these two and a couple of fainter associations in that cloud or arm segment remained unresolved.

Along with the already mentioned 41, associations 49, 62, and 23 looked starlike. The last one, I believe, is mentioned as a "star" next to VS 3 in the S&T article. The other star in the pair pointing at VS 3 is indeed a Milky Way star. The foreground stars are labeled on my sketch with "*".

The peripheral associations 22, 24, 80, and 73 lay outside the visible body of the galaxy and had considerable visual extent and irregular shapes. The spiral structure in this galaxy is very loose, but these peripheral associations do seem to trace a certain spiral arrangement of luminous matter.

The galaxy is in the M81 group and about 8 Mly away. This may be the most detailed view I have had to date of a galaxy beyond the Local Group as far as the number of individually identified objects - or it is a tie with M101.

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#2 azure1961p

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Posted 08 February 2013 - 10:03 PM

Well there's testament to my big aperture query. Superb!!

Pete

#3 IVM

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Posted 09 February 2013 - 04:18 PM

Thank you, Pete, and I hope you will get that big-telescope project going soon. There is so much to see in these nearby galaxies, and aperture in reasonable doses helps tremendously. I should note on this example though that the chief difference between my last observation of NGC 2403 with the 12" and this much more detailed one with the 16" was not the aperture but preparation. A better photo to use at the scope, awareness of the Hodge associations catalog, and some determination to advance beyond the last view. I may be wrong but I feel that I would have detected most or all of these new features with the 12" also at this new stage in my study of this galaxy.

#4 azure1961p

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Posted 09 February 2013 - 06:27 PM

Clearly the 12 is nothing to sneeze at and I have a lot of respect for it as a viable working aperture on any number of things. It's interesting and maybe I'm wrong due to lack of personal owner-use but my experiences with a 12 or 13" reflector was to have it show steadily what the 8" barely flirted with. The difference though from 13 to 18 however did t seem as marked. Again I didn't own these scopes so my experiences are limited but the 18 seemed to show brighter 13" views while the 13 showed new details the 8" didn't see at all .

It's a perception issue I know as the numbers say its virtually the same jump in light grasp but those are my star party impressions.

Pete

#5 hbanich

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Posted 09 February 2013 - 08:27 PM

[quote...I should note on this example though that the chief difference between my last observation of NGC 2403 with the 12" and this much more detailed one with the 16" was not the aperture but preparation. A better photo to use at the scope, awareness of the Hodge associations catalog, and some determination to advance beyond the last view. I may be wrong but I feel that I would have detected most or all of these new features with the 12" also at this new stage in my study of this galaxy. [/quote]

I've found this to be true as well - preparation before observing can help quite a bit in seeing subtle but extremely interesting details. Knowing exactly where to look for a faint something always helps me detect it with certainty whereas I might have missed it altogether. Being able to see HII regions and star associations from millions of light years away is quite a treat!
Of course, knowing exactly where to look for a faint detail doesn't always mean it’s detectable, but it does help concentrate one’s gaze and greatly increases the chances of success, regardless of the size of telescope.

#6 IVM

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Posted 09 February 2013 - 08:34 PM

Pete, these are my impressions also. On nebulous objects and detail I like to think not so much in terms of light grasp (i.e. aperture area) as magnification at a given surface brightness (i.e. linear aperture itself). But no matter; the relative increase from 8 to 12 in these terms is indeed exactly the same as from 12 to 18. So why is it then? It may be in part because larger amateur scopes tend to suffer from poorer baffling. Probably more important is how magnification tends to be selected, especially during less than all-out dedicated viewing. I'd say the magnifications most commonly used with larger scopes for a variety of practical as well as subjective reasons (lower focal ratios as such, effective optical quality under the field conditions, tracking) tend to progressively fall behind the theoretically ideal proportionality to the aperture.

#7 IVM

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Posted 09 February 2013 - 08:46 PM

Howard, indeed I often think that our (mankind's) ability to visually discern detail in cosmic objects was probably aided recently more by the accessibility of information such as - in this case - a quality print of a photo from an 8-meter telescope and a chart from an "obscure" research paper than by the availability of larger amateur-class telescopes.

#8 GlennLeDrew

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Posted 10 February 2013 - 07:12 AM

If certain of the same objects are used for the 'comparisons of comparisons' ( 8 to 12 vs the same ratio of 12 to 18), the seeming lack of expected gain for the larger-pair comparo could be due to the test object itself.

To be valid, such comparisons must employ suitable test objects, where in this case the 8-12 object would be an *exact* replica in form *and* surface brightness, but of size 50% larger in diameter (and hence of area and integrated brightness 2.25X) than the 12-18 object. In such case I fully expect the gains would track exactly as expected.

What I'm trying to say is this. When the test involves an uncontrolled variable (here, the lack of suitably scaled targets), results will be unreliable.

#9 IVM

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Posted 10 February 2013 - 10:28 AM

Indeed, Glenn. If there are no appropriately scaled finer features in the galaxy that the larger aperture could resolve, there will be no improvement.

One special case is the starlike associations; to these, the usual rule of magnitude and mirror area probably applies that was worked out for stars.

Whole galaxies are generally of the same intrinsic size and brightness, so their apparent surface brightness is the same but their apparent size varies according to their inverse distance. I tend to think that the most commonly resolved features within galaxies - arms, star clouds, large associations - also tend to have that characteristic "galactic" surface brightness. This not coincidentally is also the average surface brightness of the Milky Way - or so it seems to me. These features in galaxies of course vary in size, being, again on average, tied to the apparent size of the galaxy they belong to. So the natural scaling of things, on average, tends to provide objects for comparison according to the principle you are pointing out.

I realize that this scaling is a very rough guide at best, especially at small intergalactic distances that are of interest to visual observes, where the sample is small.

#10 sgottlieb

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Posted 14 February 2013 - 02:07 AM

Excellent job, Ivan, dissecting NGC 2403! Several of the associations/HII regions you logged I've seen in larger scopes (including Lowrey's 48"), but hadn't yet confirmed them in my 18", so they weren't included in the S&T article.

> Along with the already mentioned 41, associations 49, 62, and 23 looked
> starlike. The last one, I believe, is mentioned as a "star" next to VS 3
> in the S&T article. The other star in the pair pointing at VS 3 is indeed
> a Milky Way star. The foreground stars are labeled on my sketch with "*".

I believe Hodge Association 23 (or at least the "core"), which you logged as "starlike" is a Milky Way star. That's the way it appeared in the 48" as well as this image (northwest of VS 3).

#11 IVM

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Posted 14 February 2013 - 03:21 PM

Thank you, Steve. It appears that there may indeed be a superimposed star that dominates association 23 visually. But I am not entirely certain the bright object is not a starlike HII region within the association. That would not fit with the color, but the color in the image you linked is the same for the (core of?) association 61. If both coincide with stars of roughly the same magnitude, that's too many coincidences.

On the photographic chart in the cited Hodge article I see only a couple faint starlike objects next to the bright one within the circle denoting association 23. The is no individual discussion of this association in the paper. In the table, it is said to contain HII regions 507 and 508 from the Hodge and Kennicutt 1983 (cited by Hodge mistakenly as 1982) AJ paper and 6 resolved stars to mag. 21.9 B. The size of the association is given as 22x18".

On the H-alpha photograph in the Hodge and Kennicutt 1983 paper I see in that position a highly unequal pair or starlike sources. Unfortunately the regions are not labeled on the photo and I did not attempt to apply the XY coordinates from the table. The brighter object of the pair (presumably HK 508) seems to be the star (or "star") in question.

On the chart in Sivan et al. (Astronomy and Astrophysics 1990 237:23) I see a lone prominent HII region in this location, remarkable also for a dashed outline attached to it. This is their #70. For it in the table they give H-alpha flux 144*10^-16 erg/sq cm/s (pretty large) and half-flux radius 35". They label superimposed Milky Way stars with "star" symbols, but I am not sure if in this part of the chart that is crowded by labels these symbols might be omitted. If there is no such omission, then maybe #70 of Sivan et al. (probably identical with HK 508) is the "star" that we see.

#12 george golitzin

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Posted 14 February 2013 - 10:03 PM

Wow, that's a fine observation of 2403! After I finished with the Perseus cluster (posted on another thread), I went over to 2403...but conditions at Lake Sonoma had deteriorated a bit, and frankly, I had a hard time even discerning which way the spiral arms opened. So I'm really impressed with the detailed view you give above. Inspiring! I'll have to tackle it again under better conditions.

-george

#13 IVM

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Posted 15 February 2013 - 12:48 PM

Thank you, George. Discerning which way the spiral arms open in this galaxy, I think, is extremely difficult. Herschel's branch is not much of a spiral arm segment, and the other star clouds near the core are not much better. Associations 14 and 24 on one end and 73 and 80 on the other are remarkably suggestive of a spiral organization on a sketch, but these four can hardly be held at the same time visually, because they are small, faint, and far apart. So there is much more "spirality" seen by daylight on the sketch than I actually perceived at any given time when looking through the eyepiece.

(Should it be said also that the sudden decision to fill the galaxy halo with the half-frozen pen in this spiral manner instead of the usual cross-hatching was not my best artistic move? I hope that nobody thought this swirl was an actual structure. The sketch was only meant to place the associations in reasonably correct positions with reasonably correct sizes and outlines.)






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