The galaxy Messier 33 is a beautiful spiral star-city that is relatively close and therefore huge from our viewpoint, but it has such low surface brightness that it can be hard to find by telescope. M33 is also unusual for other reasons, including that a New General Catalog object resides within it.
A quick explanation of the Messier (M) and New General Catalog (NGC) numbers:
Charles Messier, 1730-1810, who served as chief astronomer at the Marine Observatory, Paris, searched diligently for comets, eventually discovering more than a dozen, NASA notes. He was so successful that King Louis XV called him the "comet ferret." He thought he was onto one when he studied a fuzzy object, not a star, but then realized it was motionless and could not be a comet. So he began listing non-stars, that weren't comets, that searchers shouldn't bother with — NASA says these were termed "objects to avoid."
The first of these, Messier 1 or M1, was the Crab Nebula, the remains of a supernova in the constellation Taurus. In 1054, Chinese astronomers recorded a bright new star in that position, which eventually faded from view — the supernova explosion. Since Messier's first notation of an object to avoid, the Messier catalog has grown to number 110. M110 is a dwarf elliptical galaxy gravitationally tied to M31, the great Andromeda Galaxy. Far from avoidable nuisances, Messier objects are among the brightest and sometimes most interesting targets that astronomers can observe from the Northern Hemisphere. As you'd expect, with the limitations of 18th century telescopes, they're among the easiest deep-space objects to examine.
In 1786, Sir William Hershel and his sister Caroline Hershel began their own catalog, which eventually included 2,500 objects. John Hershel, William's son, expanded the project into the New General Catalog with 5,079 listings of star clusters, galaxies, nebulas and supernova remnants. As discoveries brought better information, the catalog was updated and corrected. The latest version is the "Revised New General Catalogue and Index Catalogue" by Wolfgang Steinicke, with 13,226 entries. It's not the only list, but NGC numbers are commonly used by astronomers.
The Messier objects are incorporated as NGC objects, with new numbers. To confuse matters further, several NGC-numbered features actually are inside other NGC objects.
That's the situation with M33 (also designated NGC 598). The galaxy is the only Messier object in the small Triangulum constellation, which is defined by a triangle of three moderately bright stars. The constellation has been recognized since the time of the ancient Greeks.
M33, the Andromeda Galaxy (M31) and our own Milky Way are members of the local group of galaxies, according to NASA. M33 may even be a satellite of Andromeda. In billions of years, the trio will "all undergo mutual close encounters and potentially mergers." Astronomers agree the Milky Way and Andromeda will merge; if all three mash up, the result would be a monster elliptical galaxy. M31 is 2.5 million light-years away. Estimates for M33's distance center around 2.7 million light-years.
After Andromeda, M33 is the closest spiral galaxy to the Milky Way. At 50,000 light-years across, about half the diameter of the Milky Way, it hosts only a tenth as many stars — 40 billion, as opposed to 400 billion for our galaxy. It is tilted largely toward us.
Although close enough that it appears four times the size of the moon, Messier 33 has low surface brightness, making it difficult to spot with a small telescope or pair of binoculars.
"Now for a word of warning: even if you're staring right at the Triangulum Galaxy, it's still possible to miss it," Bruce McClure and Deborah Bird posted last November on EarthSky.org. "You won't see the galaxy's stars at all. Sometimes, this galaxy looks almost transparent, like a water spot on a window. The small blob in your binocular field might resemble an unwashed spot on an otherwise clean window. If you've never seen this deep-sky object before, it's hard to know what to look for.
"Once you finally spot the Triangulum Galaxy, you may wonder how you overlooked it so many times before."
An astonishing feature of M33 is that, unlike most other large galaxies, it apparently doesn't host a supermassive black hole at the center. An article published in The Astronomical Journal, November 2001, tells the tale with its title, "M33: A Galaxy with No Supermassive Black Hole."