Messier 40 - Winnecke 4

Messier 40 is one of the three unusual objects in Messier's catalog. It is a double star, which was also cataloged later as Winnecke 4 (WNC 4). Most probably, it is an optical double star, i.e. a chance alignment of two independent stars at different distances.

Messier 40 is a wide, 50 arc second pair of 9th magnitude stars. While Messier was searching for a nebula at the position specified by Johann Hevelius in 1660, he saw this pair of stars. He stated that it was difficult to separate the double, and saw no nebula, but included the stars in his catalogue anyhow.

The double, with components of visual magnitude 9.0 and 9.3, was re-observed by Friedrich August Theodor Winnecke at Pulkovo Observatory in 1863, at the same position as Messier's description. Just 16' from this double star is the faint galaxy NGC 4290, which Hevelius could not have seen with his small telescope. Thus, M 40 is associated with the double star Winnecke 4, and not the galaxy NGC 4290.

At the time of Winnecke's discovery, the angular separation between the double star's components was 49.2"; it gradually increased to 52.8" in 1991, as measured by the Hipparcos satellite. Their observed relative proper motion is consistent with straight-line, independent motion; i.e. one star is crossing between us and the other.

Spectroscopic evidence gathered by Brian Skiff in 2001 also supports the hypothesis of an optical double star. The primary component (HD 238107) has a spectral type of K0 III; the secondary (HD 238108) is of type G0 V. From these spectral types, the stars have masses of 1.1 and 1.2 Suns, absolute magnitudes of +0.88 and +4.0, and implied distances of 1900 and 550 light-years. Thus the secondary component may be much closer to us than the brighter primary (but with great uncertainty due to observational inaccuracies). Additional investigations would be welcome to confirm these preliminary results.