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

Features:

1. Messier, who was the observing assistant at the Marine Observatory in Paris (which may or may NOT be the same as the Paris Observatory), made a determined effort to discover all the brightest nebulae (No-403--404).

2. From his discoveries, he compiled a what we call the Messier catalog of the brightest and largest nebulae. The word nebula is used here in the historical sense: i.e., to mean cloudy-looking astronomical object. In modern times, we know that historical nebulae 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 in the Milky Way.
   3. other galaxies outside of the Milky Way.

3. The final published version of the Messier catalog contained 101 nebulae (No-403--404), but several others were added in Messier's notes, and so today there are conventionally 110 Messier objects: 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. Many astronomical objects have special proper names and they probably all have NGC New General Catalogue) numbers: e.g., M31 is the Andromeda Galaxy (AKA Andromeda Nebula) and NGC 224.

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

5. Messier was actually interested in discovering comets and he discovered more than a dozen.

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

   To explicate: Comets when they don't have obvious tails, strongly resemble nebulae---both kind of astronomical objects are fuzzy little clouds if NOT looked at too closely. So a nebula can give a false positive for a comet if you do NOT have a catalog of bright nebulae 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-inch (45.72 cm) reflector telescope which William Herschel (1738--1822) probably used in finding most of the ∼ 2000 nebulae he had found by 1800 (see No-404; Linda Hall Library: Scientist of the Day - William Herschel, 2018nov15; 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 ∼ 20.25 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.