No Mistaking Astronomical Objects

On Starts With a Bang, Ethan Siegel makes headway on his tour of "110 spectacular deep-sky objects" first cataloged by Charles Messier in 1758.  Before powerful telescopes were developed, the heavens consisted of the sun, moon, stars, a few bright planets, and the rare passing comet.  Comets were actively sought by men like Messier, who one night saw a bright smudge—too ill-defined to be a star—that "neither brightened nor changed position nor altered in appearance over the subsequent nights."  He had spotted the beautiful Crab Nebula, an expanding lacework of stardust blown out by a supernova within our own galaxy.  Unknown to Messier, some of his nebulae were entirely different galaxies, millions of light years distant, a structure scarcely conceived of in the 18th century (and not proven, on the basis of redshift, until 1912).  Other Messier objects turn out to be spectacular star clusters, such as M13, which contains about 300,000 stars "from Sun-like ones down to red dwarfs and white dwarfs, a few blue stragglers (common to globulars), and a few red giant stars" within a diameter of 145 light years.  But all these wonders of the universe looked about the same to Messier: things not to confuse with comets, ice orbiting the sun.