The image at left was cropped slightly and its contrast and brightness enhanced so as to emphasize the Crab Nebula's appearance, but in the course of doing that, the stars' colors were unfortunately washed out. This image, more authentic to the original, has better color rendition and therefore shows the deep orange color of the carbon star Y Tauri, discussed here in a Cloudy Nights thread.

This picture makes it clear how Messier and his contemporaries could have confused this object for a comet, more so than almost any other object on the list. This nebula would have presented differently in the telescope in the 1700's than it does today because it was created as a result of a supernova that occurred in the year 1054 AD. Therefore, it has only existed for 972 years and as such would have been about 74% of its age during the time of Messier as compared to today.

The first object in Charles Messier's catalog is the Crab Nebula in Taurus.  This object is the only supernova remnant in his list of 110 things that should not be mistaken for comets.  It is relatively easy to locate, since it lies just to the northwest of the star ζ Tauri, the tip of the southern horn of the bull.  This is a stack of six exposures for a total of 30 seconds, and as can be seen, it's still pretty small even though I used a 135 mm lens.  Unannotated image.