This is unique in Messier's list. It is a double star. Double stars are quite common and present their own challenges and opportunities for the observer, but it is odd that we would find such an object in Messier's list. After all, Messier's purpose was to compile a list of objects that might be confused with comets. How in the world can you confuse a double star with a comet?
Messier was searching the area for a nebula that an observer in the previous century had noted as being in this area. All Messier could find was this double star. Even though it has no nebulous appearance, it was assigned a number by Messier in 1764.
HOW TO FIND IT:
It will be easier to find the real M40 than to find its location on many star charts. Many atlases do not mention M40. Locate Delta Ursa Majoris. In the direction away from the dipper's cup, you will find a slightly curving row of three stars: Delta, 70 and 75 (with 74 near 75). You want to scan the area near 70 opposite the area of Delta.
WHAT TO LOOK FOR:
You'll see a double star, and that's about it.
Observe it at different nights with different conditions. I've been told that one can see an illusion of nebulosity in poorer viewing conditions, but I've never been able to do so. Perhaps I don't have enough averted imagination!