Striking in photographs and picturesque in a telescope, the Sombrero galaxy offers a fine, if unusual, example of an edge-on spiral galaxy. This is a lovely object, with a huge and brilliant central galactic bulge likely caused by the machinations of a massive black hole, and an inky-dark dust lane that resembles the brim of the traditional Mexican hat that lends its name to this distant island universe.
Like many objects in the Messier list, M104 was discovered not by Charles Messier but by his contemporary Pierre Méchain in the early 1780s. Messier made a note of the galaxy and included it in a handwritten addendum to his famous list of 103 non-stellar objects. But the galaxy was not officially included in the famous “Messier list” until 1921.
It doesn’t take a lot of aperture to see M104. It appears nearly star-like in 7×50 binoculars in good sky, and a 3-inch telescope shows it clearly. The galaxy is located in relatively barren sky about 11.5° west of Spica and 5.5° northeast of η (eta) Corvi. It lies 20º south of the Virgo Cluster, a congregation of galaxies between the stars Denebola and Vinemiatrix, but the Sombrero is likely an outlier of this massive galaxy cluster.
In a 3-inch or 4-inch scope at low power, the galaxy presents itself as a small oval glow with a star-like core. Crank the magnification up to 75x to 100x to get a much better view and to glimpse the dark dust lane that runs across the middle. This striking lane splits the galaxy into two unequal sections, each with just vague hints of structure that can be seen with averted vision and patient observing. As with most galaxies, more aperture will get you a better view.
In the early years of the 20th century, no one understood the true nature of the Sombrero or any other galaxy. At the time, they were simply called ‘nebulae’. Astronomers speculated that face-on spiral ‘nebulae’ like the Whirlpool were either nearby nascent solar systems within our own galaxy, or enormously distant collections of stars in their own right, far outside what we now call the Milky Way Galaxy. Sliver-thin ‘nebulae’ like the Sombrero and the even more beautiful NGC 4565 in Coma Berenices were also a bit of a mystery.
In 1912, the hard-working American astronomer Vesto Slipher outfitted the big refractor at Lowell Observatory with a spectrograph and turned it towards the Sombrero and other needle-thin nebulae, as well as round spiral nebulae like the Whirlpool Galaxy. From the shape of the spectrum, specifically the deep absorption lines of hydrogen gas, he deduced in 1914 that the Sombrero was rotating. He further speculated, correctly, that M104 was a nearly edge-on version of a spiral galaxy like the Whirlpool Galaxy and that these face-on nebulae also rotated.
Rotation of these structures, and the fact that their spectra suggested they were made of the same stuff as stars, did not solve the mystery of the nature of these objects. But Slipher noted one more thing about the spectrum of the Sombrero. The spectrum suggested the assembly, whatever it was, was receding from us at the rather astonishing speed of 1000 km/s, faster than most any other object in the sky. This speed strongly suggested, but did not yet prove, that the Sombrero was far outside our own assembly of stars.
Within a decade, thanks in part to his additional painstaking observations, he was proven right.