Caption: Charles Messier (1730--1817) circa 1770---looking très Ancien Regime.

Features:

1. Messier, who was the observing assistant at the Hotel de Cluny Observatory (AKA Marine Observatory) in Paris (No-403--404) (which is NOT same thing as the Paris Observatory), made a determined effort to discover all the brightest nebulae (historical usage) (No-403--404).

2. From his discoveries, he compiled a what we call the Messier catalog of the brightest and largest nebulae (historical usage). The word nebulae (historical usage) used here in the historical sense: i.e., to mean cloudy-looking astronomical object. In modern times, we know that nebulae (historical usage) are 3 different things:

   1. actual clouds of interstellar medium (ISM): i.e., clouds of interstellar gas and/or interstellar dust.
   2. clusters of stars inside the Milky Way.
   3. other galaxies outside of the Milky Way.

3. The final published version of the Messier catalog contained 103 nebulae (historical usage), but several others were added in Messier's notes, and so today there are conventionally 110 Messier objects (Wikipedia: Messier object): M1 (Crab nebula, supernova remnant NGC 1952), M2 (globular cluster, NGC 7089), M3 (globular cluster, NGC 5272), ... , M31 (Andromeda Galaxy, barred spiral galaxy, NGC 224), M32 (dwarf elliptical galaxy, NGC 221), M33 (Triagulum Galaxy, unbarred spiral galaxy, NGC 598), ... , M87 (supergiant elliptical galaxy, NGC 4486), ... , M101 (Pinwheel Galaxy, unbarred spiral galaxy, NGC 5457), ... , M110 (dwarf elliptical galaxy, NGC 205).

   Note that like many astronomical objects the Messier objects often have alternative names. Some of them have special proper names (e.g, M1 is also the Crab nebula) and they probably all have NGC (New General Catalogue) numbers: (e.g., M31 is also NGC 224).

4. There are, of course, a quasi-infinity of nebulae (historical usage) in the observable universe. But Messier catalog contains just about all the brightest and largest nebulae (historical usage) as seen from the Earth, and so are especially important astronomical objects for both amateur astronomy and professional astronomy.

5. Messier was actually interested in discovering comets and he discovered 14 of them (Wikipedia: Charles Messier: Biography).

   He compiled the Messier catalog to eliminate false positives in comet discovery.

   To explicate: Comets when they do NOT have obvious comet tails, strongly resemble nebulae (historical usage): both kind of astronomical objects are fuzzy little clouds if NOT looked at too closely. So a nebula (historical usage) can give a false positive for a comet if you do NOT have a catalog of bright nebulae (historical usage) handy to eliminate it as potential comet.

6. In fact, Messier probably could NOT detect relatively dim comets NOR nebulae since he used a 10-centimeter (∼ 4-inch) refractor telescope which had very low light-gathering power (see Wikipedia: Charles Messier: Messier catalogue) compared to the 18.5-inch (47.0 cm) reflector telescope which William Herschel (1738--1822) (aided by his sister Caroline Herschel (1750--1848)) probably used in finding most of the ∼ 2400 nebulae (historical usage) he had found by the end of his career (Wikipedia: William Herschel: Deep sky surveys; Wikipedia: William Herschel: Works with his sister Caroline Herschel; Wikipedia: William Herschel: Herschel's telescopes). Recall light-gathering power scales as the square of the objective (primary lens/primary mirror). So Herschel's reflector telescope had ∼ 21.4 times more light-gathering power than Messier's refractor telescope.

7. Messier's ultimate fame is NOT for what he thought it would be---who cares who discovered 18th century comets---other than who was the eponymous discoverer of Halley's comet---who was you know who---and, of course, who was Caroline Herschel (1750--1848):
   And when I have finished all my other work, I sweep the sky.
   ---Caroline Herschel (1750--1848) quote from somewhere. Perhaps, apocryphal.

   Messier's ultimate fame is the Messier catalog.