Messier 38 — Open Cluster in Auriga

M38 is a large open star cluster in the constellation Auriga, located about 4,200 light-years from Earth. It spans roughly 25 light-years across and contains an estimated 100-150 stars. At magnitude 7.4 with an apparent diameter of about 21 arcminutes, it is invisible to the naked eye but an easy binocular target. The cluster is approximately 220 million years old. Its brightest star is a yellow giant of spectral type G0, shining at about magnitude 7.9. M38 is notable for its cross-shaped or oblique Greek letter pi pattern of brighter stars, which gives it a distinctive appearance at moderate magnifications.

Located in the rich Auriga cluster region, about 2.5 degrees northwest of M36. It forms part of a spectacular trio with M36 and M37, all easily fitting in a wide-field binocular view. Binoculars show a hazy patch of faint stars. A 4-inch telescope at 50-80x resolves the cluster into dozens of stars and reveals the cross-shaped pattern of brighter members. At higher magnifications, a rich scattering of fainter stars fills in. The small cluster NGC 1907 lies just 30 arcminutes to the south, making a nice companion in the same telescope field. Best observed from November through March when Auriga is high in the evening sky.

Discovered by Giovanni Battista Hodierna before 1654. Independently found by Charles Messier in 1764, who described it as 'a cluster of small stars in Auriga, near the star Sigma, a little below the preceding cluster (M36).' Its companion NGC 1907 was discovered by William Herschel in 1787. The cluster has been important for studies of stellar evolution in intermediate-age open clusters.