M38 (open cluster in Auriga)
390: M38
M38 is the last of the three open clusters in Auriga, and was originally reported by Giovanni Batista Hodierna before 1654, then rediscovered independently by le Gentil in 1749, and finally logged by Messier on September 25, 1764, three weeks after he added M36 and M37 to his catalog. At 21 arcminutes in diameter, M38 is slightly smaller than M37, and about 2/3 the diameter of the full moon. It is noticeably dimmer than M36, and more than a full magnitude dimmer than M37.
M38 has about 100 member stars, ranging from magnitude 7.9 down, and is located within less than 2.5 degrees from M36. Its distance is variously estimated from 2,750 to 4,200 light years, with the latter being the currently accepted value. M38 is about 220 million years old, which makes it middle-aged for an open cluster. The brightest stars near the center of the cluster form what has been variously described as a Greek letter Pi or an oblique cross.
400: M38 with the Telrad
Getting M38 into your scope’s field of view is a bit harder than getting M36 or M37, but still pretty easy. Use the imaginary line joining the two Auriga pentagram stars 37 Theta Auriga and Alnath. Place your Telrad as shown, with the bulk of its field inside the pentagram with the 4-degree circle just touching the line between 37 Theta Auriga and Alnath at about the halfway point. Continue on that perpendicular for about half a Telrad field, bearing very slightly toward 37 Theta Auriga. M38 should appear in your low-power eyepiece, although you can verify its location with your optical finder and center it if necessary. Like other open clusters, M38 is a low- to medium-power object. Its extent is a circle of about 21 arcminutes, or 2/3 the diameter of the full moon. That means an eyepiece with a true field of view of 45 arcminutes or so allows you to view the details of the cluster in context with its surroundings.