|Rektascenzija||12 : 22.4 (u:m)
|Deklinacija||+58 : 05 (sto:m)
|Razdalja||0.3 (*1000 sv.l.)
|Vizual. magnituda||8.4 (mag)
|Zorni kot||0.8 (loc min)
This faint double star was found by Charles Messier when he was searching for a nebula which was - erroneously - reported by the 17th-century observer Johann Hevelius in this vicinity. As Messier had measured the position of these stars, he gave them a number in his catalog. This fact gives some suggestion on how this catalog was compiled: Messier collected positions while he was cataloging the star clusters and nebula which could be taken for comets. M40 was apparently the last one he recorded when he was busy in checking the reports available to him in 1764.
In comparing Messier's description with the sky, John Mallas noted the double star Winnecke 4 at the right position. It had been reobserved at Pulkovo Observatory in 1863. The two components are of visual magnitudes 9.0 and 9.3, and their separation on the sky is 49 seconds of arc (from Mallas/Kreimer). A. Winnecke, in 1863, had reported a position angle of 88 degrees, which seems to have decreased to 83 degrees since. The Lick Observatory Index Catalog lists the spectrum of the primary as G0.
The binary lies 16' NE of 6-th mag 70 UMa. It forms a rectangular triangle with the faint barred spiral (type SBb), NGC 4290 (12.5 mag, 2.5x1.9 arc minutes angular diameter, receding at 2885 km/s which corresponds to about 125 million light years distance; one of the faintest objects the present author has seen with a 4-inch).
Assuming the primary is a main sequence star, it should be roughly of Solar luminosity, so that one can give an estimate of the order of magnitude of its distance: It should be of the order of 100 parsecs, or 300 light years. If anyone knows a measured value, or other additional data on this binary, please email me.
Mallas and Kreimer point out that, although Messier 40 is without doubt
Winnecke 4, Hevelius had observed another star, 5th-mag 74 UMa, more than
a degree away.
Some printed versions of the Messier catalog omit M40 as "obscure" object,
despite its reality in the sky.