Open Star Cluster Observations

One of the joys of being a visual  observer is in describing how an object appears. This description will vary according to the light (dark) conditions, the magnification, the field width and the dark adaption of the observer. It will also vary based on the amount of time an observer spends directly observing the object. It is pretty safe to say that no two observations will be exactly alike, but each are equally valuable as a description. So, this week, I want to talk a bit about open star cluster descriptions and how I go about recording the visual image

My first step in observing an open cluster is to inspect the cluster under very low magnification and a wide field of view. For me this is my 30mm wide field which yields 102 arc minutes at 40X magnification. With this eyepiece, I inspect how the cluster is situated within the star field. Is it highly concentrated with respect to the surrounding field, or is it fairly loose and almost indistinguishable against the field stars? Robert Trumpler developed a classification system for describing this. Trumpler assigned a Roman Numeral to describe the concentration and detachment of the cluster. Here is the classification that Trumpler used:

     I Detached with strong central concentration
    II Detached with little central concentration
   III Detached with no central concentration
   IV Not detached but apparently like a strong field concentration

At this point, I increase the magnification so that the cluster fills the majority of the field of view of the eyepiece. Using this higher magnification, I record the dimensions of the cluster. If the cluster is not circular then I describe the shape and which direction it is elongated in.

At this point, I estimate the number of stars that appear to be associated with the cluster. If the cluster is not detached from the background, or foreground stars then this can be difficult. Trumpler divided clusters according to the number of stars:
 
    p     less than 50 stars
    m    50 - 100 stars
    r      more than 100 stars

Trumpler used the Lick Observatory equipment for his survey, so don't expect to come up with the same number of stars that he did. Often if the cluster is very rich in faint stars, then it appears more as a mottled effect. If this is the case then I just record it as such.

The next aspect to observe is the variety of brightnesses and the colors. The higher proportion of red stars indicate that the cluster may be older than a cluster with more blue stars.  Observe the brightness ranges. Are they all the same magnitude, or is there a significant difference in magnitudes. Trumpler used a scale of 1 to 3 to classify brightness differences 1 being very little difference in brightness and 3 being a large difference of several magnitudes.

So now for some examples:
NGC 1647 is an open cluster in  Taurus. I have described this cluster as having many stars, barely concentrated with a density higher than the background field. The stars are mostly the same brightness.

Trumpler classified this cluster is II-2-r which translates to detached with little central concentration, minimal range in brightness, and more than 100  members.




NGC 2360 is an open cluster in Canis Major. I described this as " Highly concentrated open cluster, 2 dozen brighter stars, many,many fainter stars. density higher than surrounding area"

Trumpler classified this as II-2-m - Detached with little concentration, 50 - 100 stars, minimal range in brightness.






NGC 457 is a really nice OC in Cassiopia. My description goes like this: Nice irregular shaped open cluster. Elongation N-S direction with 34 Cas and its companion on South side. The double is a yellow and green pair. Many stars of different brightness, slightly concentrated with density higher than surrounding area.

Trumpler classification is I-3-r - detached with strong central concentration, large brightness differences with more than 100 stars.




The trick with the descriptions is to give as much detail as possible. You will find that the more descriptive you are, the more you will remember these little gems.

Next time I will talk about getting ready for a star party....

Clear skies,

rw

"Star Poachers"

Well, not everything goes as planned on a observation night. Let me explain....Friday night, our club, the Brazosport Astronomy Club, had scheduled an observation night at our dark sight. Our dark site is at the end of a cul-de-sac in an undeveloped area of a rural housing development. The sight offers horizon to horizon views and a relatively dark sky, well as dark as you can get living in the Houston - Galveston corridor. The area as I said is rural and it very common to see deer and hear coyotes after dark.

There were three of us out with our scopes that night. I had my 10" Dob, which has now come to be named "Zoe". Maggie had her 6" Meade LS and Jim had his 11" Celestron on its Losmandy mount.  My choice of the low-tech dob was once again re-affirmed while watching Maggie and Jim fight through the alignment process on their scopes. I probably had bagged a half dozen objects before they got their scopes aligned.  I am not sure if Maggie ever was successful in aligning her scope.

Well it was about half way through the observation session when we noticed a vehicle that seemed to be circling around us on the rural roads that ran around our site. That is not too uncommon, as there is always some traffic on these roads, however, this vehicle abruptly turned off the road and started to drive towards us across the pasture. Well this was new! At first we had thought this might be a club member unfamiliar with the access road into our site. Well, that theory was soon to be discarded as the vehicle pulled up and out stepped a Texas Park and Wildlife officer. "Howdy folks. What are y'all doing back here in the dark." It was evident that the officer had seen our red lights and assumed we were poaching deer. Well our telescopes soon betrayed our true purpose as "Star Poachers." The officer looked at Zoe and exclaimed " I bet you can see Jesus up there with that!" Well I couldn't locate the Jesus object, but nether could my two Goto companions. I did treat the officer to views of Jupiter and the Orion Nebula that he he probably will not forget.

Well the officer departed after enjoying a few views and we spent another hour or so "poaching" some of the best views of the Texas skies. Zoe and I bagged a few program variables and a couple of clusters in Canis Major...but more about that next time....

Clear skies;
rw

Galaxy Trio in Cetus

Every once in a while I will see something through the eyepiece that just begs for more investigation. This happened one night in November when I was observing the Herschel objects in Cetus. I came across a field that held three galaxies, NGC 1035, 1042 and 1052. While having three  galaxies in a 1 degree field in a 10" scope does not seem all that unusual, the fact that these three galaxies all looked very different did intrigue me a little and I made a mental note to follow up and learn a little about each of these objects.

My first question was this just a chance alignment or are these galaxies "neighbors." Well to answer this, I went to the  NASA Extragalactic IPAC Data Base. This is a great resource for details on galaxies. In looking at the distances of the three galaxies we see that NGC 1035 and 1052 are 62 and 64 million light years distance while NGC 1042 is a mere 34 million light years away. So it would appear that NGC 1035 and 1052 are somewhat related as their separation is only about 2 million light years. This is similar to the separation between our own Milky Way galaxy and our close neighbor the Andromeda Galaxy.

The database says that NGC 1035 is a SAc normal spiral galaxy. Through the eyepiece, the galaxy appears to us as more edge on than face on.  It has a visual magnitude of 12.2 and is approximately 2 by 4 arc minutes in size. It has an even brightness along its major axis.
You can see from the UKSchmidt photograph to the right that there are beads of higher density stars clusters distributed along the spiral arms. This structure was not visible through the eyepiece at 75X.

NGC 1042 is a face on SABc barred spiral galaxy. It appeared as a soft circular patch in the eyepiece with relatively even brightness. I estimated the dimensions to be 3 by 3 arc minutes. The visible magnitude is 11.0.
The MW_Pal photograph to the left clearly shows the spiral arms and the barred center. These structures were not readily visible through the eyepiece.

NGC 1052 is an elliptical galaxy. Through the eyepiece it is clearly elongated with a hint of a denser central core area.  This is the brightest of the three galaxies with a visual magnitude of 10.5 and I estimated the dimensions at 4 by 2 arc minutes.
The Palomar 200 photo at the right shows little detail of the galaxy. I suppose this somewhat normal of ellipticals as they are relatively homogeneous in stellar density except for the extreme edges where the density falls off rapidly. I believe this is what gives the galaxy the appearance of having an overly large core or halo.

I look forward to revisiting these galaxies next fall to see if any of the structures visible in the photographs are visible with a more concerted effort through the eyepiece.

Clear skies;
rw