Spiral galaxies are among the most beautiful and plentiful galaxy types in the cosmos. When artists draw galaxies, spirals are what they first visualize. This is likely due to the fact that the Milky Way is a spiral; as is the neighboring Andromeda Galaxy. Their shapes are the result of long galactic evolution activities that astronomers are still working to understand.
Characteristics of Spiral Galaxies
Spiral galaxies are characterized by their sweeping arms which stretch out from the central region in a spiral pattern. They are subdivided into classes based on how tightly the arms are wound, with the tightest classified as Sa and those with the most loosely wound arms as Sd.
Some spiral galaxies have a "bar" passing through the center of which the spiral arms extend. These are classified as barred spirals and follow the same sub-classification model as "normal" spiral galaxies, except with the designators SBa - SBd. Our own Milky Way is a barred spiral, with a thick "ridge" of stars and gas and dust passing through the central core.
Some galaxies are classified as S0. These are galaxies for which it is impossible to tell if a "bar" is present.
Many spiral galaxies have what is known as a galactic bulge. This is a spheroid packed with lots of stars and contains within it a supermassive black hole that binds together the rest of the galaxy.
From the side, spirals look like flat disks with central spheroids. We see many stars and clouds of gas and dust. However, they also contain something else: massive haloes of dark matter. This mysterious "stuff" is invisible to any experiment that has sought to directly observe it. Dark matter plays a role in galaxies, which is also still being determined.
The spiral arms of these galaxies are filled with lots of hot, young blue stars and even more gas and dust (by mass). In fact, our Sun is sort of an oddity considering the type of company it keeps in this region.
Within the central bulge of spiral galaxies with looser spiral arms (Sc and Sd) the population of stars is very similar to that in the spiral arms, young hot blue stars, but in much greater density.
In contracts spiral galaxies with tighter arms (Sa and Sb) tend to have mostly old, cool, red stars that contain very little metal.
And while the vast majority of the stars in these galaxies are found either within the plane of the spiral arms or the bulge, there exists a halo around the galaxy. While this region is dominated by dark matter, there are also very old stars, usually with very low metallicity, that orbit through the plane of the galaxy in highly elliptical orbits.
The formation of spiral arm features in galaxies is mostly due to the gravitational effect of material in the galaxy as waves pass through. This posits that pools of greater mass density slow down and form "arms" as the galaxy rotates. As gas and dust pass through those arms it gets compressed to form new stars and the arms expand in mass density further, enhancing the effect. More recent models have attempted to incorporate dark matter, and other properties of these galaxies, into a more complex theory of formation.
Supermassive Black Holes
Another defining characteristic of spiral galaxies is the presence of supermassive black holes at their cores. It is not known if all spiral galaxies contain one of these behemoths, but there is a mountain of indirect evidence that virtually all such galaxies will contain them within the bulge.
It was actually of spiral galaxies that first suggested the possibility of dark matter. Galactic rotation is determined by the gravitational interactions of the masses present within the galaxy. But computer simulations of spiral galaxies showed that the rotation velocities differed from those observed.
Either our understanding of general relativity was flawed, or another source of mass was present. Since relativity theory has been tested and verified on virtually all scales there has so far been resistance to challenging it.
Instead, scientists have postulated that an as-yet-unseen particle exists that does not interact with the electromagnetic force - and most likely not the strong force, and perhaps not even the weak force (though some models do include that property) - but it does interact gravitationally.
It is thought that spiral galaxies maintain a dark matter halo; a spherical volume of dark matter that permeates the entire region in and around the galaxy.
Dark matter has yet to be detected directly, but there is some indirect observational evidence for its existence. Over the next couple of decades, new experiments should be able to shed light on this mystery.
Edited and updated by Carolyn Collins Petersen.