Chapter 15: Galaxies

M87

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 pinwheels 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 ellipticals 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 supermassive black hole. In addition to these unifying characteristics, they also may possess other common structures.

Part of the Taurus Molecular Cloud

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 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 than a lumpy, gravitationally bound blob of star-forming regions, star clusters, and gas that has yet to settle into a spherical or spiral shape.

Irregular galaxy NGC 1427A

• Galactic Bars: literally bars of stars extending 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 apart but 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 poses a central bar.

Prominent dust lanes in the Sombrero Galaxy

The final feature of every galaxy inspected closely so far is a supermassive black hole. All galaxies have gravity potential wells that lead 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.