Last week, I described my struggles to see the spiral structure of the Whirlpool Galaxy.
The name is apt. The spinning collection of perhaps 100 billion stars is seen “face-on,” i.e., from the top. Thus, it looks very much like a child’s pinwheel as the stars slowly rotate. You’ve seen that spiral structure before — in cake batter as you stir it or in your morning coffee as you pour in in the cream.
Go ahead. Try it. Give the black coffee a good stir until it forms a sort of vortex. Then slowly pour in the cream. Instant spiral galaxy.
The universe is made up of trillions of “island universes,” as galaxies are sometimes called. Our Milky Way is but one of them.
Spiral galaxies like the Milky Way and the Whirlpool make up the lion’s share, over 70 percent, of galaxies. It seems likely that galaxies have some basic quality or qualities that make them want to be spirals.
Why are most galaxies shaped like pinwheels and some others shaped like the otherwise unstructured eggs we call elliptical galaxies? How can a spinning mass of stars maintain its spiral structure over millions — nay, billions — of our pathetically short earthly years?
In fact, astronomers aren’t exactly sure what causes some galaxies to form into spirals. However, as with many of the processes of the universe, gravity has a lot to do with it.
Galaxies and the stars in them form out of giant clouds of mostly hydrogen gas. The newly formed stars and the gas that has not formed into stars collapse toward the center because of their mutual gravitational force. As the collapse occurs, many galaxies will flatten out and begin to rotate.
The stars at the center congregate into a bulge with a flattened disk rotating it. Clearly, the stars out at the edge of the disk have a lot farther to go than the stars near the bulge. As we move away from the bulge, the stars have an increasing distance to go and thus lag behind an increasing amount.
The same principle applies to your morning cup of coffee. The cream at the edge of the cup has a lot more distance to go than the cream at the center, so you get the spiral shape.
The principle is called differential rotation, and it causes the galaxy to form into a spiral.
Now let’s go back to your cup of coffee. Quite soon, the spiralized cream merges into the coffee and the spiral disappears.
Similarly, a galaxy ought to lose its spiral structure relatively quickly. As the galaxy rotates, the arms should get tighter and tighter around the bulge. After a few galactic rotations, the spiral arms ought to tighten up and disappear, to “wind up” into an amorphous disk and lose their pinwheel structure.
But they don’t. The spiral structure seems to last for a very long time. Thus, differential rotation explains the spiral structure, but it doesn’t explain why it last so long.
Gravity seems to have one more trick up its sleeve. It manifests itself in waves. The process resembles a rock thrown into a quiet pond.
Go out and try it on some calm spring day. You’ll see the ripples emanate from the point of impact. The waves are more energetic but have a shorter wavelength when they are close to the point of impact. As the ripples move outward, they lose energy and spread out into longer waves. Note that the water isn’t moving outward much. You are seeing the water undulate with the force of the impact.
And so it is with galaxies. Gravity bends space into so-called “density waves,” which act like ripples in the fabric of space. The stars of a galaxy get caught up in those waves.
I was reminded of density waves as my wife and I drove into Chicago and ran into traffic jams that my daughter says never go away. She dubbed them “permajams,” and they help to explain why galaxies keep their spiral structure.
We’d be humming along until we got to an area of dense traffic where we slowed down. Once we broke free of the denser area, we’d hum along for a while until we hit the next area of traffic density.
Since the gravitational density waves emanate out from the centers of galaxies like ripples on a pond, one might thus expect that the stars should be arranged in rings.
However, remember that the galaxy is rotating.
As the density waves spread out into the galaxy, they get caught up in the rotation of the galaxy. That rotation bends the density waves into spirals. The stars then get caught up in the spiralized density waves like the cars in a Chicago permajam.
In short, the structure of a spiral galaxy is the physical manifestation of rotating ripples in the fabric of space.
Of course, the process is far more complicated than the simple (grin) model we have been discussing. The first stars in a galaxy formed out of the initial clumping together of clouds of dust and gas. As a galaxy evolves, it creates new stars out of the leftover clouds of gas. Those stars exert gravitational force on each other, which helps to maintain the power of the density waves.
Similarly, as new cars enter the highway, the traffic jams get denser, and the cars move even more slowly through the areas of density. In the case of a galaxy, the spiral structure is maintained by the formation of new stars.
Once all the available gas clouds are used up and all the possible stars are formed, we might expect that eventually the spiral structure would wind itself up and disappear.
Apparently, it does. A small percentage of galaxies have the required disk shape, but they have lost much of their spiral structure. Significantly, such lens-shaped “lenticular” galaxies are made up almost completely of very old stars. Apparently, lenticulars have lost their ability to form new stars, which helped to maintain the power of their density waves. No new cars are entering the highway, and thus the traffic jams have broken up.
But there may be even more to the process than density waves and star formation.
Spiral galaxies often have smaller “satellite” galaxies hovering around them. For example, our own galaxy has two notable fellow travelers, the Large and Small Magellanic Clouds.
As it turns out, M51 has at least one satellite. A medium-sized telescope reveals a smaller, fainter smudge near the Whirlpool. It’s close enough that long-exposure images show the Whirlpool sucking the stars out of the smaller satellite along one of the Whirlpool’s spiral arms.
The outward pull of the satellite galaxies may therefore be contributing to a galaxy’s spiral staying power.
Of course, spiral galaxies can lose their structure in other ways. The universe abounds with egg-shaped elliptical galaxies, some of which contain trillions of stars and therefore dwarf a normal spiral galaxy many times over.
Two significant facts emerge about such large ellipticals. They are composed of mostly older stars. Also, they are usually not rotating.
Some astronomers speculate that ellipticals form when two or more spirals merge together. That process probably happened quite a lot when the universe was younger and smaller, and the galaxies were closer together.
The merger of two spirals wipes out the spiral structure of the two galaxies. The lack of rotation and the relative lack of new star formation make the re-emergence of a spiral structure unlikely.
If that process seems meaningless and theoretical to you, please take note. Our own spiral Milky Way galaxy and the nearby Andromeda galaxy, also a spiral, are careening toward each other at 68 miles per second. In four billion years or so, our beautiful pinwheel will be part of a larger but far less beautiful cosmic egg.