M40
Winnecke 4
Propriétés Physiques
Position et Identifiants
Visibilité
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À propos de M40
Description
M40 (Winnecke 4) is one of the most unusual Messier objects — it is simply a double star in Ursa Major, about 510 light-years from Earth. The two stars (magnitudes 9.0 and 9.3) are separated by about 49 arcseconds. Most astronomers believe the pair is an optical double (not gravitationally bound) rather than a true binary system.
Conseils d'Observation
Located about 1.5 degrees northeast of Megrez (Delta Ursae Majoris). At the eyepiece, M40 appears as two faint stars close together — nothing more. A telescope at 50-80x easily splits the pair. Don't expect a deep-sky spectacle — M40 is essentially just two stars. It is primarily of historical interest as one of Messier's mistakes.
Histoire
Cataloged by Charles Messier on October 24, 1764, while searching for a 'nebula' reported by Johannes Hevelius in this location in the late 1600s. Messier found no nebula but recorded the double star instead. The pair was independently cataloged by Friedrich August Theodor Winnecke in 1863 as Winnecke 4.
Faits Amusants
M40 is arguably the most disappointing Messier object for beginners expecting a spectacular nebula or cluster. Yet it reveals something important about the history of astronomy — Messier included everything he could find at reported nebula positions, even when the 'nebula' turned out to be nothing more than a pair of stars. Proper motion studies suggest the two stars are moving in different directions, confirming they are an optical pair.
Photos de la Communauté (1)
Credit: NOIRLab/NSF/AURA. License: CC BY 4.0. (Wikimedia Commons)
Skybred Feb 28, 2026