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M 40

Double Star M40 (WNC 4) in Ursa Major

Winnecke 4

Right Ascension 12 : 22.4 (h:m)
Declination +58 : 05 (deg:m)
Distance 0.51 (kly)
Visual Brightness 8.4 (mag)
Apparent Dimension 0.8 (arc min)

Discovered by Charles Messier 1764.

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 (as published in a letter to Sky and Telescope, August 1966, p. 83). 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. He published his "Doppelsternmessungen" (Double Star Measurements), including 7 "new" double stars, in Astronomische Nachrichten No. 1738, Vol 73, p. 145-160, Altona, 1869 Feb 8, and refers to his fourth "new" double as Groombridge 1878. 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.

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.

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.

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. Historically, various positions have been given from a number of measurements concerning M40:

These positions, besides that of Messier and perhaps that of Hevelius, are errors or point to different objects.

Some printed versions of the Messier catalog omit M40 as "obscure" object, despite its reality in the sky.

  • Historical Observations and Descriptions of M40
  • More images of M40
  • Amateur images of M40

  • SIMBAD Data of M40
  • NED Data of M40
  • Observing Reports for M40 (IAAC Netastrocatalog)

    Hartmut Frommert (spider@seds.org)
    Christine Kronberg (smil@lrz.uni-muenchen.de)

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    Last Modification: 8 Sep 2000, 22:00 MET