The galaxy we call M104 was discovered on May 11, 1781, by Charles Messier’s French colleague Pierre Méchain. Messier had just completed his now famous list of 103 deep-sky wonders—many of them found by Méchain and other observers — and Méchain’s additional find was too late to be included. It wasn’t until 1921 that Méchain’s object was designated M104 (and since then, astronomers have appended six more entries deemed historically worthy to be included in the Messier catalogue).
Pictures of M104 show an elliptical glow bisected by a thick dust lane. However, the edge-on system is not precisely edge-on. The galaxy is very slightly inclined: We see its nucleus and most of the central bulge above the equatorial dust lane. The spiral arms are so tightly wound, they form a solid-looking disc whose near-side edge is sharpened by the dark lane. It’s that hard edge and central hump — features discernible in a small telescope — that produce a shape reminiscent of the wide-brimmed Mexican sombrero.
M104 resides near the Virgo-Corvus border, 5½ degrees north and a bit east of 2.9-magnitude Algorab, delta (δ) Corvi. As shown in the chart below, not far from delta is a little asterism dubbed “the Arrow.” The galaxy is near the Arrow’s tip.
The Sombrero is 8.7 by 3.5 arc minutes in extent, glows at magnitude 8.0 and sports decent contrast. Too bad this southern wonder (declination –11.6 degrees) arcs low across my south sky, where the seeing is rarely steady and local light pollution is especially bad. Even so, my backyard 4¼-incher at 27× reveals a minuscule fuzz in the correct spot. A 93× eyepiece produces a diffuse patch just big enough, perhaps four by two arc minutes, to appear strongly elongated east-west. A 10th-magnitude star flickers a few arc minutes southwest of this stunted growth. (In reality, the galaxy’s western end stretches all the way to that star.)
In my 10-inch Newtonian at 155×, the galaxy’s tenuous tip still doesn’t reach the star, but my observing log records a satisfying Sombrero nonetheless: “Slender cloud E-W with tapered wings. Extremely faint extremities (averted vision), but large, bright core with starlike nucleus (direct vision). Central hub bulges northward. South flank is straight and sharply defined.” Away from town, I can glimpse additional subtleties: “Bit of bulge visible underneath straight edge. Dust lane cuts across core slightly south of nucleus.”
Take some time on the next clear evening to salute the Sombrero!