…continuedExploring the Hubble Sequence by Eye
Amateurs and the Hubble Sequence
Of course, amateurs with telescope apertures of 16 inches or larger will be able to observe structural features better and, in exceptional cases, to make a detailed visual classification. Anyone who has viewed M51 in Canes Venatici, or the southern galaxy NGC 1365, in a large telescope has readily seen some spiral structure. But there's more to classifying galaxies than spotting spiral arms. The key parameters are (1) the relative brightness of the bulge or core of the galaxy to that of the outer disk (if any) and (2) the rate at which the brightness decreases outward from the nucleus.
In the uncommon pure elliptical galaxies, of course, there is no disk. The lenticulars (type S0) and “early”-type spirals (type Sa) have relatively weak disks and pronounced bulges. As one progresses to “later” galaxy types, cores grow fainter and fainter, until one reaches type Sm, where the last vestige of a bulge remains as a tiny starlike nucleus. The Magellanic-type irregular galaxies are defined as having no nucleus at all: whereas ellipticals are “all bulge,” Magellanic irregulars are “all disk.”
Elliptical and disk galaxies are also distinguished by the way the galaxy light falls off from the center to the edge. Spheroidal bulges decline rapidly in brightness (as the inverse fourth power of the radius, for the mathematically inclined). When combined with the eye's peculiar perception of brightness (Sky & Telescope: March 1990, page 311), this makes elliptical galaxies appear to fade rather smoothly from the center outward, regardless of telescope aperture. A spiral galaxy's disk, by contrast, fades much more slowly, and visually it can seem a dull, uniform patch surrounding the concentrated core.
Although galaxies of all types are scattered across the sky, it is convenient as an exercise in galaxy observing to have the Virgo Cluster close by (as these things go! the cluster's distance is roughly 60 million light-years). Most of the richest galaxy clusters contain only elliptical and lenticular galaxies no spirals. But Virgo, which barely makes the grade as a rich cluster by some definitions, contains every type of galaxy appearing in the standard classification schemes. It is convenient also to have a variety of galaxies in close proximity to one another on the sky so comparisons can be made quickly, sometimes in the same eyepiece field. Finally, a collection of galaxies all at roughly the same distance lets us compare the relative brightnesses of various Hubble types directly.