![]() ![]() The Large Cloud has a prominent bar that can easily be perceived with the aid of binoculars.Īlthough 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). The Large Magellanic Cloud (left) and the Small Magellanic Cloud (right) define the Magellanic class of irregular galaxies they are small satellite systems of our Milky Way. Southern Hemisphere observers can study the morphology of these galaxies with minimal optical aid. ![]() A spiral galaxy's disk, by contrast, fades much more slowly, and visually it can seem a dull, uniform patch surrounding the concentrated core. 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. Spheroidal bulges decline rapidly in brightness (as the inverse fourth power of the radius, for the mathematically inclined). The Magellanic-type irregular galaxies are defined as having no nucleus at all: whereas ellipticals are “all bulge,” Magellanic irregulars are “all disk.”Įlliptical and disk galaxies are also distinguished by the way the galaxy light falls off from the center to the edge. 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 lenticulars (type S0) and “early”-type spirals (type Sa) have relatively weak disks and pronounced bulges. In the uncommon pure elliptical galaxies, of course, there is no disk. North is up with east to the left.Ĭourtesy the Association of Universities for Research in Astronomy. The galaxies appear in the order mentioned in the text. The DSS enables anyone with Internet access to view small portions of wide-angle photographs taken at Palomar Observatory and elsewhere. These 15-arcminute-wide views from the Digitized Sky Survey (DSS) show 12 of the galaxies that Brian Skiff studied at the eyepiece in a quest to characterize their Hubble types visually. ![]() 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. ![]() But there's more to classifying galaxies than spotting spiral arms. Anyone who has viewed M51 in Canes Venatici, or the southern galaxy NGC 1365, in a large telescope has readily seen some spiral structure. 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. The origin and physical significance of galaxy types remains an area of active research, and they have yet to be completely accounted for in most professionals' eyes. Nevertheless, by noticing all you can about a galaxy's structure, you can make a fairly reliable guess about its classification, or Hubble type. Thus only the broadest categories of galaxies can be differentiated. The visual observer, however, is at the mercy of the eye's limited angular resolution (20 or 30 arcminutes) at low light levels. Indeed, photographs from Mount Wilson's great reflectors inspired Edwin Hubble to order galaxies along the lines of his now-famous “tuning-fork” sequence of galaxy types. Images taken with large telescopes show that no two galaxies are alike and that the variety of galactic forms is endlessly fascinating. Amateurs won't see anywhere near this much detail, but with patience and care they should be able to classify the galaxy on the basis of its appearance in the eyepiece. M100, a tightly wound spiral galaxy in the Virgo Cluster, was imaged by the Hubble Space Telescope to measure the universe's expansion rate. ![]()
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