Roaming the Deep Sky #10
M38
By Don Clouse
This large open cluster, located in central Auriga, is one of by favorite deep sky objects. M38 has a very striking appearance in the eyepiece. It contains conspicuous, and quite beautiful, chains of stars. The largest runs for 20 in a long graceful north to northwest curve. A second, no less prominent, but shorter chain is located to east of the longest chain. This second chain curves off to the northeast in a 10 long, shallow, arc. A thick chain, or "bar", of stars connects the two slender chains. This thick bar can be seen as extending beyond the ends of the two slender chains. The whole arrangement looks very much like an inverted version of the Greek letter "Pi". Some describe these long arcs of stars as forming the letter "X". Much, of course, is in the eye/mind of the beholder. How do you perceive it? I perceive M38, mainly, as extremely beautiful bright and gaudy, but delicate as well. So much for waxing poetic, on to the "technical" details.
Most sources list M38s magnitude as 6.4 and its diameter on the sky as 21. The circle on the chart shows a field 21 in diameter. Note, however, that the long northwest curve extends well outside this somewhat arbitrary boundary. I judged it to be about 25 in extent. (Pretty much any star chart will show M38s location in Auriga.) M38 is listed as having 100 stars. Viewing it in my 8" Schmidt-Cassegrain telescope at 58 power, I estimated 60 stars in the cluster. At 85x the count jumped to 150 stars in the 36 field of view (which probably included a lot of non-members). I viewed the cluster as being moderately concentrated, especially in the area from which the larger chains seem to diverge. M38s Trumpler classification is given variously as "3 2 m" and "2 2 r". The classification of "2 2 r" (detached from the field with a weak concentration toward center, moderate range in brightness, and rich having more than 100 stars) seems most appropriate to me. Having dissected M38s visual appearance, what can be said of its nature?
M38 lies just north of the plane of the galaxy near galactic latitude 173 degrees basically in a direction nearly opposite that of the center of our galaxy in Sagittarius. In this area, the structure of the Perseus Spiral Arm (I particularly like the Star Trek view of the galaxy on this site) of our galaxy becomes rather patchy and may even merge with our Local Spiral Arm. M38 is part of the Auriga OB1 Association. Along with the other two bright Messier clusters in Auriga, M36 and M37, this loose association of type O and B stars spans a few hundred light years. The distance to M38 seems to be a bit in question. One source gives a distance of 2,800 light years while another puts the cluster at 4,200 light years. Similarly, M38 could be anywhere from about 16 light years up to 25 light years in extent. M38s age is given as 220 million years.
Take advantage of the mild, clear nights that we are going to have this spring. (One must always remain optimistic in this part of the country.) Auriga and M38 will be high in the southwest during early spring evenings well placed for early evening observing.
Sources:
- The Night Sky Observers Guide, George Kepple and Glen W. Sanner, Willmann-Bell, Inc., 1998.
- SkyAtlas 2000.0 Companion, 2nd edition, Robert A. Strong and Roger W. Sinnott, Sky Publishing Corporation, 2000.
- MegaStar 4.0, E.L.B. Software, Willmann-Bell, Inc., 1997.
- Observing Handbook and Catalogue of Deep-Sky Objects, Christian B. Luginbuhl and Brian A. Skiff, Cambridge University Press, 1990.
- Students for the Exploration and Development of Space, www.seds.org/messier.
- The Guide to the Galaxy, Nigel Henbest and Heather Couper, Cambridge University Press, 1994.
- The NGC IC Project, www.ngcic.org.
- The Cambridge Star Atlas, Second Edition, Wil Tirion, Cambridge University Press, 1994.
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