Star cluster in Messier 101..?

[Clear Skies]

Hi all,

Working out observations I tracked down the little "stellar dot" I noticed just north of the center of the galaxy Messier 101. Scope used was my 12" SCT with a 17mm Nagler.

NED lists it as a cluster NGC 5457:[DBT2008] 0711 http://ned.ipac.caltech.edu/cgi-bin/objsearch?search_type=Obj_id&objid=175108494&objname=1&img_stamp=YES&hconst=73.0&omegam=0.27&omegav=0.73&corr_z=1, SIMBAD says it's a star AG+54 939 http://simbad.u-strasbg.fr/simbad/sim-id?Ident=%40526438&Name=AG%2b54%20%20939&submit=submit. As the object appeared stellar to me, but not starlike (subtle difference, I know) I am tempted to go with "cluster".

Any thoughts, data, observations?


Victor

[Marko]

Ngc5457 is the NGC number for M101 itself is my understanding. If the bright dot was 1.2' north of the core of M101 and is fairly apparent then that is likely to be the forground star GSC 3852:1194. Can you give a distance as to how far this dot was perhaps as a fraction of your full field of view in the 12" SCT using the 17mm Nagler? That would better pinpoint this object. The 2" 17mm would be 180x and the field would then in total be (82deg/180) or 27min (if I got all those assumptions right that is but that sounds close from what I recall in my 11" SCT with my 17mm)

[Clear Skies]

Hi Marko,

The position of the dot is that of the dot in the NED link (http://ned.ipac.caltech.edu/cgi-bin/objsearch?search_type=Obj_id&objid=175108494&objname=1&img_stamp=YES&hconst=73.0&omegam=0.27&omegav=0.73&corr_z=1), the image on the page shows it nicely. Below is a screenshot from AstroPlanner:

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This image shows a FoV of 30', actual FoV was 27' for the scope/eyepiece combination. The tiny gray arrow points from the center of the galaxy towards the dot, distance is approximately 75".
NGC5457 is indeed M101's designation, NED names the dot "NGC 5457:[DBT2008] 0711". I am sure about the position, just not about the classification. NED says cluster, SIMBAD says star.

[Marko]

Ok so it is the mag 13.6 dot that I usually use to locate north in quick views of M101 is what the gray arrow seems to point to. MegaStar then agrees with SIMBAD in that MegaStar calls it out as the mag 13.6 star I had mentioned. It's possible that there is a star cluster that the star happens to sit in front or right next to it thus making the star appear non stellar. Mag 13.6 in such a distant galaxy I don't think could be a star cluster all on it's own. The star clusters in M31 which is a lot closer are very very dim to us. There are experts on this list that know quite a bit about M101 so maybe they will comment and shed some authoritative light on the subject. Clear Skies, Marko

[Steve Gottlieb]

I agree with you, Mark, the clusters in M101 would be much fainter than this star. The NED designation NGC 5457:[DBT2008] 0711 is from this 2008 paper (http://iopscience.iop.org/1538-3881/135/3/823/). The position for the cluster does not precisely match the star. In fact, if you download the paper and look at the image for the "candidate" clusters in M101 on page 827, you'll see the mag 13.6 "star" does not appear to be coincident with one of the candidate clusters (north to the right).

[Clear Skies]

Thank you for the insight, Marko and Steve, and for the link to the paper. A star it is then!

[Marko]

Steve is one of the experts. Howard Banich, also on this board, has an active thread now here on DSF on his current article in S&T that subscribers would have just gotten in the new June issue. I did see a grouping/association/cluster on a high quality image that is true north of the core but it would likely appear to be just be part of the glow near the core in a 12" range instrument and not so distinct of a point. Much of my info comes from Steve's S&T article which is described in a cloudy nights thread quite nicely as 'An excellent reference for observing M101 is Steve Gottlieb's June 2004 Sky & Telescope article about M101'. Looking for the NGC objects in M101 is a great project. Thanks Steve for your inputs.

A word of caution is if you look at M101 in Jimi's scope, be sure to wear your sunglasses ... :)

[Howard B]

Although not mentioned in my M101 S&T article, I identified this star as AG+54 939 in the article's online supplement (http://www.skyandtelescope.com/skytel/beyondthepage/Guide-to-Messier-101-195550111.html). It's definitely stellar in my 28 inch at all magnifications, while several of the smaller NGC regions in M101 that appear stellar at low powers, like NGC 5455, give up their non-stellar nature at higher powers.

In the supplement I list an approximation of the number of objects that make up each NGC region within M101 as well as other, non-NGC groupings I identified in my drawing. That was a daunting task as there's no clear boundary to these regions so my count is very approximate, but it does show that the HII regions are more than just HII regions. Pretty cool!

AG+54 939 is a foreground star in the Milky Way and is described as a "high proper motion star" in SIMBAD, but its actual proper motion is much too small for this to be true. I did get pretty excited about it when I first read this though, as I imagined trying to detect its motion over 10 or 20 years. More like 10 or 20 decades!

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