At the Ragged End of the Hubble Sequence

Late-type spirals bright enough to be readily visible in other than wonderful viewing conditions will necessarily be closer than Virgo. Our best bet is farther north among the galaxies in the Canes Venatici group, at roughly half to two-thirds the distance of Virgo. We've already mentioned M51, which you might compare with M100 in Virgo. Among the latest-type spirals that can be considered bright is NGC 4395, which has a type of Sm, the last stage before irregular.

Although it has a total magnitude of 10.2, making it one of the brightest galaxies in the whole sky, NGC 4395's average surface brightness is roughly 10 times lower than those of the Messier objects in the Virgo Cluster. This makes it a challenge even to find, though from a dark site in my 70-mm telescope it can be seen, albeit as a diaphanous "almost nothing." Telescopes in the 12- to 16-inch range will reveal numerous faint clumps in the weak spiral arms, three of which have NGC numbers of their own. In NGC 4395 we see essentially a pure disk galaxy, where the central bulge is all but invisible, though photographs show the galaxy's nucleus as a weak stellar point. (How large a telescope is required to spot this diminutive bulge by eye?)

Irregular galaxies exhibit a wide range of star-formation rates, and luckily for aficionados of galaxy morphology it is easy to find irregulars with plenty of star formation to make them bright. Two examples reside in the Canes Venatici group.

NGC 4214 and NGC 4449 are similar in size, shape, and brightness, showing numerous giant H II regions across their surfaces. Both are bright enough to be spotted in hand-held binoculars as tiny spots, but naturally they become more interesting with larger telescopes. With a 16-inch it should be easy to isolate some of the glowing H II regions by viewing the galaxies at moderate power (that is, by using an eyepiece that yields an exit pupil of about 2 mm). Use a nebula filter that favors both the hydrogen-beta and [O III] (doubly ionized oxygen) emission lines, which are conspicuous in the spectra of interstellar gases that have been energized by newborn stars. The H II regions are particularly easy to see across the face of NGC 4449.

While the various ordinary spirals I've mentioned share a common three-zone structure, these two irregulars seem somehow unlike them, even visually. Compare these objects to the bright spirals near them, such as M63 and M106, to see the distinction more clearly.