Messier 40

Double Star M40 (WNC 4) in Ursa Major

Winnecke 4

Discovered by Charles Messier 1764.

Messier 40 (M40) is one of the three "curiosities," or 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.

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. According to his catalog description, Messier did not see any nebulosity associated with them. 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, of previously recorded "nebulae."

In comparing Messier's description with the sky, John Mallas noted the double star Winnecke 4 at the right position (Mallas 1966). It had been reobserved by Friedrich August Theodor Winnecke at Pulkovo Observatory in 1863. The two components are of visual magnitudes 9.0 and 9.3, and their separation on the sky is about 49 seconds of arc (from Mallas/Kreimer). F.A.T. Winnecke, in 1863, had reported a position angle of 88 degrees, which seems to have decreased to 80 degrees until 1966, and further to 77 deg in 1991. He published his "Doppelsternmessungen" (Double Star Measurements), including 7 "new" double stars (Winnecke 1869), and refers to his fourth "new" double as Groombridge 1878.

The image in this page was obtained by Evered Kreimer in 1966, the time when John Mallas identified it.

At the time of Winnecke's discovery, the angular separation of these stars has been determined as 49.2"; it has gradually increased to about 51.7" in 1966, when the Kreimer photograph was taken, and further to about 52.8" in 1991, as measued by the Hipparcos satellite. The Lick Observatory Index Catalog lists the spectrum of the primary as G0, while SIMBAD lists them as A=HD 238107, spectrum G0 and B=HD 238108, spectrum F8. Brian Skiff gives their spectra as K0III and G0V (Skiff 2001).

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.

In March 1998, Mike Feltz communicated to me his evaluation of the data obtained by ESA's astrometrical satellite Hipparcos for the components of the binary M40 or Winnecke 4. According to his analysis, the brighter component was measured at a distance of 510 light years (corresponding to a parallax of 6.4 milli arc seconds, and a "distance module, m-M", of roughly 6.0 - this is the difference between apparent and absolute magnitude). The fainter one had a nonsensial negative parallax, which frequently happens when two stars are close in Hipparcos data. At this distance, the brighter star is of absolute visual magnitude of 3.0, or about four times more luminous than our sun.

A fresh investigation of the nature of M40 was undertaken by Richard Nugent (2002); his results support the hypothesis of an optical double star, i.e. different distances of the two stars: The observed relative proper motion, as measured in separation and position angle, is consistent with a straight, independent motion of the two stars, one crossing between us and the other. From the spectral types provided by Skiff (2001), he estimates the absolute magnitudes as M_v=+0.88 and +4.0, masses as 1.1 and 1.2 solar masses for the primary (A) and secondary component (B), and thus derives spectroscopic distances of 1900 +/- 750 and 550 +/- 230 light-years, respectively, the great uncertainties coming from observational inacuracies. This indicates that perhaps the secondary component, B, may be much closer to us than the brighter primary, A. Additional investigations would be welcome to confirm or falsify these preliminary results.

The double lies 16' NE of the 5.7-mag star 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).

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