Objects identified as normal galaxies in photographs often display spiral shapes much like our Milky Way or the neighboring Andromeda Galaxy. (Andromeda is the one and only distant galaxy that our naked eyes can see, perhaps best with averted vision, as a faintly glowing oval amid the constellation of the same name.) Each has a central bulge from which sport thin spiral regions, or “arms,” chock-full of stars. The apparent prevalence of spiral galaxies is just that—apparent—largely the result of whorled patterns easily noticed among the many other patches of light in the nighttime sky. In reality, galaxies have an array of morphologies and spirals are not the most common type of galaxy in the Universe.

Ellipticals After decades of observational campaigns in the mid-20^th century, inventories of extragalactic matter beyond our Milky Way are now nearly complete for that relatively local part of the Universe in which we live. At least for normal, baryonic matter, that is, for we are still unsure what to make of the “dark matter,” raised in the previous PARTICLE EPOCH as strongly inferred but not yet seen. Most abundant are galaxies shaped like footballs and officially called elliptical galaxies. Some are less elongated, akin more to very large beach balls; others resemble fat cigars and even thin pencils but that’s probably due to their tilted perspective; none of the ellipticals exhibit internal structure of any kind and are notably devoid of spiral arms. Regardless of their shape, most elliptical galaxies harbor the usual 10¹¹ or so stars, spread across a typical 10⁵ light-year domain. A minority of them measure up to 10 times that size and contain trillions of stars, but otherwise elliptical galaxies are undistinguished, albeit monumental, globs of stars.

Spiral arms are not the only trait that elliptical galaxies lack. Hardly any cool gas drifts among the stars of these galaxies; they have little or no interstellar matter. This implies that all the elliptical galaxies are old. Stars, which originate from interstellar matter, apparently did so in these galaxies long ago, leaving hardly any loose gas for the continued formation of future generations of stars. Analysis of the radiation emitted by individual stars within the ellipticals further proves that all those stars are also old. Evidently, nearly all their interstellar matter was used up eons ago, thus quenching the star formation process.

Spirals The infrequence of star formation in elliptical galaxies contrasts sharply with the abundance and activity of interstellar matter within the second galaxy class—spiral galaxies. Here, there are also a variety of shapes, though all the spirals are basically flattened disks resembling double sombreros clapped brim to brim. Some spiral galaxies have a large central bulge, mostly made of intact stars and diffuse gas, around which the spiral arms are tightly wrapped. Others have a more open pattern of arms emanating from an intermediate-sized central region. Still others have a rather small center from which long, stringy arms protrude, often making it hard to recognize these as spirals.

Spiral galaxies are known to contain lots of loose gas, and dust too, mixed throughout the vast spaces among their stars. The oldest stars extend up into the galaxies’ spherical halos, but their youngest stars are found exclusively in the thin disks. Furthermore, observations during the past few decades have shown that stars are still forming, most of them in the arms. Unlike the old elliptical galaxies, spiral galaxies have a good deal of vitality—which doesn’t necessarily mean that the spirals are young. Rather, they are simply still rich enough in interstellar gas to provide for ongoing stellar birth.

Some spiral galaxies sport a peculiar feature that has astronomers puzzled—namely, a linear “bar” of stellar and interstellar matter passing through their midsections. For these so-called barred spirals, as shown in Figure 2.1, the arms stem from near the ends of their bar rather than from the central bulge itself. The puzzle concerns the way the bars form, evolve, or maintain themselves. Even our own Milky Way is now judged, based on infrared observations of its central regions, to perhaps have a small bar passing through its galactic nucleus.

FIGURE 2.1 – Variation in shapes among barred-spiral galaxies. (NASA; AAT; ESO)

Irregulars A final, catchall type of galaxy class groups all those termed irregular galaxies, the type most widely seen in the Universe. These are oddly shaped structures of stars, gas, and dust, whose visual appearance prevents their placement into any of the other categories. The irregulars clearly have much interstellar matter yet no organized structure, spiral arms, or central bulge. By and large, irregulars tend to be a bit smaller than other types of galaxies, so some astronomers call them dwarf galaxies. They often do seem to be dominated by the larger spiral or elliptical galaxies near which they are usually found. In fact, irregular galaxies seldom exist alone in space; they are mostly allied with larger “parent” galaxies of the spiral or elliptical variety.

Their proximity to the big galaxies is probably telling us something: The irregulars might be severely distorted regular galaxies that have experienced close encounters, and thus great tidal disruptions, with their parent galaxies. Or, they might be leftover building blocks of the larger galaxies into which they have not yet fallen. Some observations do hint at possible bridges of hydrogen gas connecting parent and irregular galaxies, suggestive of interactions between them. We shall return to these issues when later discussing mergers and acquisitions among galaxies rife with evolutionary change.

Our Milky Way Galaxy has a few small companion irregular galaxies, most notably the Large and Small Magellanic Clouds, so named for the 16^th-century Portuguese voyager Ferdinand Magellan whose round-the-world expedition first brought word of these great fuzzy patches of light to Europeans living in the Northern Hemisphere. Resembling dimly luminous atmospheric clouds and seen easily with the naked eye, they can be viewed only from locations south of Earth’s equator, making them spectacular targets for first-time northerners traveling south, though they have undoubtedly served as celestial wonders to residents of the Southern Hemisphere since the dawn of civilization. Though one is slightly larger than the other, each is ~100 times smaller than our own Milky Way system—meaning that they house “only” ~10⁹ stars—and both reside not quite 200,000 light-years away. The Magellanic Clouds probably orbit our Galaxy, just as Earth orbits the Sun, or the Moon orbits the Earth. The periods of these irregular galaxies are long by human standards, however, and their orbital paths have not yet been firmly established.

Actually, these famous celestial objects (at least to residents “down under”) might not deserve the term galaxy at all, not even irregular galaxy. Though they do contain ~10⁹ solar masses and do measure ~10⁴ light-years across, they reside only ~50% farther away (namely, ~170,000 light-years) from the typical extent of our Galaxy’s disk. Thus, if our Milky Way system does harbor matter (dark or otherwise) in an extended halo—as many astronomers now suspect—then the Magellanic Clouds may well be merely rich regions of star formation in the halo of our own Galaxy. Perhaps all such dwarf structures now classified as irregular galaxies will turn out to be residents of the outer realms of larger, well-categorized galaxies—and therefore not genuine galaxies at all.