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15.18 Overview of Galaxy Structures

No two galaxies are alike. Looking into the night sky, galaxies can be found in shapes that vary from bright irregular splatters of stars, to perfect pin wheels, to boring cotton balls. Part of what defines each type of galaxy is its mix of components. No single characteristic unifies the shapes of small galaxies, but larger systems, like our own spiral Milky Way and giant elliptical like M87, are characterized by the presence of a spheroid of stars (the galactic bulge for us, and the entire galaxy for M87), and a central super massive black hole. In addition to these unifying characteristics, they also may possess other common structures.

Such components of galaxies come in two basic forms: stellar and gas structures that are usually self-contained because of their internal gravity, and galaxy-wide structures. Self-contained structures include: globular clusters, open clusters, star forming regions, and molecular clouds. Galaxy-wide structures include spiral arms, bars, rings, and even tidal tails. The self-contained structures in some cases represent the same kind of object seen at a variety of different life stages. Molecular clouds can collapse into star forming regions that become open clusters as they complete the star forming process. The origin of globular clusters is still a matter for debate. The galaxy wide structures seem to typically be caused by gravitational perturbations that create what may be transitory structures. Both rings and bars may be caused by interactions with smaller galaxies, or develop over time from instabilities in the motions of a galaxy's stars. In our case, the Milky Way's bar may be caused by the Large and Small Magellanic clouds. Grand design spirals, such as the Pinwheel Galaxy (M101), often are the result of the influence of a near neighbor. Tidal tails are created through galactic mergers and may warn of complete destruction that is yet to come.

In classifying galaxies, astronomers typically only take into consideration a galaxy's overall shape (elliptical or disk-like), and for spirals they look to see if it there is a bar, and how tightly wound the arms may or may not be. Irregular galaxies lay entirely outside of the scheme. Large irregular galaxies typically arise through messy galaxy mergers, and remnants of the original galaxies' structures can often be seen. Small irregular galaxies may be nothing more then a lumpy, gravitationally bound blob of star forming regions, star clusters, and gas that has yet to settle into a spherical or spiral shape.

Several additional features can decorate a galaxy beyond its normal host of arms, open clusters, globular clusters, and nucleus or central star cluster. These typically rare features include gravitationally driven (and often) transitory structures such as rings and bars, and features of alignment, such as dust lanes. In their own ways, each of these often beautiful structures is temporary.

• Dust Lanes: these are often seen in spiral galaxies. They can be so dense at times that they block the starlight behind them. In the Milky Way, dust lanes in our galaxy form dark paths through the otherwise bright band of the Milky Way through the sky. These dust lanes contain material that may be used in future generations of star formation but until then they make it impossible to peer through the galactic disks the occupy in visible wavelengths of light. Only in very rare instances shortly after merger events are lanes like this seen in galaxies other than spiral galaxies.

• Galactic Bars: literally bars of stars extending from out from a galactic nucleus, they exist in approximately two-thirds of all spiral galaxies and are also found in lenticular galaxies. Bars are thought to arise due to a gravitational instability contorting a spiral galaxy's spiral density wave. Typically bars are found in systems with a nearby companion. The dynamics of bars serve to drive gas toward a galaxy's core as the bar grows longer and longer. This gas causes bars to often be rich in star formation. According to computer simulations over time bars grow too large and fall apartbut they are suspected to be able to reform again later. Some models predict that bars can form and decay away on cycles as brief as two billion years. Our own galaxy currently posses a central bar.

• Galactic Rings: one of the most beautiful and least understood galactic phenomena. Typically found wrapping the centers of barred spiral galaxies, rings are rich in star formation. They are likely caused by interactions between galaxies and only last a few billion years. In addition to these typically inner galactic rings, some galaxies — lenticulars in particular — have an outer ring circumscribing the main galactic structures, which may include a galaxy wide bar.

The final feature of every galaxy inspected closely so far is a super massive black hole. All galaxies have a gravity potential wells that leads to a very high density of stars at the center. This situation is conducive to forming and sustaining a black hole where the growth comes from rapid accretion of gas and stars in the central region, slower accretion of gas from intergalactic space, and the occasional addition of a black hole from a neighboring in a merger. Even though the black holes are ubiquitous they spend most of their time quiescent and starved of fuel, only bursting into activity every ten million years or so. The mass of the black hole is proportional to the mass of the old stars in the galaxy. These central black holes are a small percentage of the galaxy's overall mass.