Open Cluster Observations

by Erika Rix

I seem to go in spurts of what I like to view at any given clump of time. At the moment open clusters have caught my eye and I am wondering out of my normal comfortable constellations, trying to expand into new studies.

I’m using “The Night Sky Observer’s Guide” for research in this report. I see that they in turn use “The Deep Sky Field Guide to Uranometria 2000.0.” The equipment used was an LXD75 German Equatorial mount with an Orion ED80 and Telrad in a very light polluted small town.

To begin the report, the focus is Perseus, the hero. Reading up on this constellation, I recall vividly “Clash of the Titans” and imagine Perseus and Medusa together among the pillars of the stone building. Knowing that death comes to anyone that looks at her directly, he uses her reflection in his shield during combat, eventually decapitating her. Holding her head up (marked by Beta, Pi, Rho, and Omega), her eye is the variable star Beta Persei, Algol. Perseus’ scimitar (curved sword) is the double cluster (NGC 869 and NGC 884).

M34 Credit: Erika Rix

M34, NGC1039

02h42.0m + 42deg47’

Star count: 60

Diameter: 35’

Apparent magnitude: 5.2 visual with the brightest star 7.33 visual

Approx. magnification: 30x

Open Cluster in Perseus discovered by Charles Messier in August of 1764. It is classified under the star cluster Trumpler system, which is a three part code that takes degree of concentration, range in brightness of the stars within the cluster, and degree of richness. M34 has been assigned Tr type II3m. Basically this means that according to Trumpler classification, M34 is detached and has weak concentration toward the center of the cluster, large range in brightness, and moderately rich with between 50-100 stars. It is thought that M34 is ~180 million years old and 1500 LY away.

For orientation, I used the stars 2-a and 4-b drawing an imaginary line to 33-a in Perseus. To complete a triangulation for ease of star hopping, I also grabbed 57-y from Andromeda. Aah, it seems only fitting that she is by Perseus‘ side, as he saved her from the sea monster Cetus. Cetus was set upon Andromeda because of the anger of Poseidon, the sea god. Andromeda‘s mother had told him that Andromeda was more beautiful than his sea nymphs.

M34 was easy to spot using a 20mm WO eyepiece. With open clusters, I find that you can’t go wrong with a lower magnification to spot them. At higher magnifications, pretty much everything looks like an open cluster to me when searching the field! What caught my eye initially while plotting the stars in this group, were all the doubles in the core. And the bright stars surrounding them resembled a bird in flight, frozen in time while flapping his wings. A meteor streaking the field of view is represented by the line in the sketch going to the preceding limb.

I moved on to Auriga, the Charioteer. This constellation is still a bit confusing to me on why it was name as such. Ancient Babylonians thought it could resemble a chariot. But I do not see how a person could be in command of the chariot to form the name charioteer. There is, however, a connection between Auriga and Erichthonius (a king of Athens). He was lame and that perhaps inspired him to create a chariot? Food for thought anyway, and gives me more to study up on.

I had four targets in mind, but not enough time to sketch them. I scanned the area with a 20mm and then increased magnification with a 12mm Burgess. The targets were, M36 (NGC 1960), M37 (NGC2099), M38 (NGC 1912) and it’s companion, NGC 1907. None of the three were distinguishable naked eye in our light polluted location. However, I may have detected a line of brightness in that area, similar to the double cluster, only finer.

M36, NGC1960

Open Cluster with approx 60 stars and diameter of 12’. It’s a Tr Type II3m, same as M34’s type, although smaller. It’s striking feature was that it looked like a wavy “X” to me because of the formation of the brightest stars. The apparent magnitude is 6.0v with the brightest star ~8.86v. It’s located at 05h36.1m + 34deg 08’.

It was discovered in 1749 by Le Gentil. Evidently M36 (~4100 LY away) is so bright that if it were the same distance away from Pleiades (M45), it would certainly outshine it with a luminosity of about 8000 suns. M45 is about only a third of that. I was so intent on plotting the stars correctly and recording proper magnitudes that I am embarrassed to admit that I didn’t notice color in the stars. It’s funny how you take things like that for granted, that if you really study something, features should automatically pop out for you. But it just goes to show you that a person can be so blind to their surroundings that they can’t see their own nose.

M37, NGC 2099

05h52.4m + 32deg 33’

Discovered by Messier in 1764, it is older than M36 and has a total luminosity of ~ 2500 suns. It also harbors red stars whereas M36 only has blue. Aha, red giants. It all fits into place more and more as my studies progress.

M37 has over 150 stars and is classified as Tr type II 1 r, which means detached with weak concentration toward the center, small range in brightness, and rich with more than 100 stars. Its diameter is 20’ with an apparent magnitude of 5.6v with the brightest star at 9.21v. I didn’t spend a lot of time with this cluster, as my eyepiece was fogging up and I wanted to observe M38 before I called it a night. I do have an eyepiece dew heater strip, but because Paul observes as well, he was using the dual heat adaptor for the LX200 and his guide scope. Needless to say, we have a spare on order.

Finally, this brings us to M38 (NGC 1912) and NGC 1907.

I had to go back to this one a few times to spot NGC 1907. I even had to stop to hold my headlamp nearly against the EP to dry up the dew…believe me, I was amazed that worked. Of course it could have just been the lack of my wet eyeball being so close to the glass.

M38, NGC 1912

05h28.7m + 35deg50’

Like M36, this open cluster was discovered by Le Gentil in 1749 as well. It has a number of A-type sequence stars and several G-type giants. It also has a new member being typed as a B5 with an absolute mag of -1.5.

As a refresher, type A stars have a temperature of 11,000-7600 Kelvin, blue-white to white color. The helium is absent but has strong hydrogen spectrum with weak Calcium II. Type G has a temp of 6000 to 4500 K with yellow color (like our Sun which is G2) and has a weak hydrogen spectrum with Calcium II at a maximum. B is 25000 to 10000 k, bluish color, and has no iodized He, strong neutral He, stronger hydrogen and few ionized metals. An example of B is Spica (B1) or Rigel (B8).

It’s categorized as TR type III 2 m (detached, no concentration toward center, moderate brightness range, and moderately rich of 50-100 stars). It has the upper end of that range with 100 stars and an apparent mag of 6.4v, with the brightest being 9.53. The diameter is 21’.

I didn’t have a lot of time to study this cluster. At first glance, it seemed rather condensed compared to M36 at the core. It also seemed elongated at either end giving it a misshaped oblong appearance. The almost elusive NGC 1907 was SW of it (if I’ve got my orientation correct).

NGC 1907

05h28.0m + 35deg19’

Open cluster of ~30 stars near M38. TR type II 1 m n (detached weak concentration toward center of cluster, small range in brightness, and moderately rich with 50-100 stars….HUH?…with nebulosity. ) Why do they list it has having approx 30 stars and then put it in a richer star field type? But I can certainly confirm the nebulosity portion of the description. That was actually how I found it after several attempts. There was a glow around what I though was a larger star at first glance. But the ‘star’ appeared oval. I checked my EP to make sure the dew hadn’t condensed on the lens and when it was clear, I checked again. Sure enough, this was a cluster of several stars that was creating the glowing effect.

It was tiny at 6’ compared to M38 at 21’. The apparent mag is 8.2v with the brightest at 1126v. There are 2 9.5 mag stars to the south of it, but they are recorded as not being part of this cluster.