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An M and a number naming a deep-sky object.
Also known as: M number, Messier object
A Messier number identifies one of about a hundred and ten deep-sky objects. The catalogue was compiled in the 1770s as a list of things that were not comets, so it is a record of what a comet hunter kept mistaking rather than a survey of the sky.
Identifies a deep-sky object from Charles Messier's catalogue, written as a capital M and a number.
Contexts: Technical drawing, Maps
Was compiled as a list of objects to be avoided by comet hunters rather than as a survey, which is why it is neither complete nor systematic.
Contexts: Technical drawing, Maps
Remains in use because its objects are the brightest and most observable, so the accidental list became the practical one.
Contexts: Technical drawing, Maps
A capital letter M followed immediately by one to three numerals.