Spiral Galaxies: The Starry Snowflakes of the Cosmos

spiral galaxy
Two spiral galaxies as seen by Hubble Space Telescope; one is seen edge-on and is called NGC 4302; the other is seen nearly face-on and is called NGC 4298. These show us what our own Milky Way would look like from outside the galaxy. NASA/STScI

In the realm of the galaxies, the most photogenic types are the spiral galaxies. Like snowflakes, no two are exactly alike. They generally have graceful-looking arms that stretch out from their cores, embedded with wispy clouds of gas and dust. Our own Milky Way is a spiral galaxy with a "bar" of stars, gas and dust stretching across the middle. Spirals make up about 60 percent of the known galaxies, particularly in our "local" universe.

They exist as parts of clusters of galaxies, although very few are found in the cores of clusters.

The Structure of the Spiral

The lovely arms of spiral galaxies are not solid, but rather are made up of stars and clouds of gas and dust. The formation of new stars takes place in the spiral arms, embedded in starbirth nurseries. But, how do the spiral arms themselves form? Although astronomers know a lot about galaxies, the origin and evolution of the spiral arms is still tough to understand. Spiral galaxies are flat — what astronomers call "disk" galaxies. The material in the disk rotates around the core, but at different speeds, depending on where it lies. Material closer to the center rotates faster than stars and gas and dust in the outer regions. Disturbances in the disk ultimately forms spiral structures which are maintained by gravitational forces which result in the arms really being density waves of material.

Think of them like ripples in a pond moving out, but in spiral form. The ripples carry along the material: stars, gas, and dust. The arms are thick with material while the space between the arms has less material.

So, what causes the density waves? That's still a puzzler. It's possible that interaction with the central bar could send material outward to form a wave of material that ultimately becomes a spiral arm.

Or, a companion galaxy might exert enough influence to send material out into a wave that becomes a spiral arm. However, they form, the spiral patterns of the density waves actually remove gravitational energy from a galaxy.

The arms of a spiral appear to lead back to the core of the galaxy. Some cores are solid, bright, and tightly confined. Others, like the Milky Way's core, appear to be more of a long bar stretching across the middle. The bar is thought to be a way to transport energy and material out from the central region. In most galaxies, there's also a central supermassive black hole (or two), which exerts a strong gravitational effect on the innermost regions.

A spiral doesn't just have arms, it also has a core, and a sphere of stars orbiting the core. As with most other galaxies, a spiral also has a shell of the mysterious dark matter surrounding it, which affects the rotation rates of the stars and the arms.

Observing Spirals

There are countless spirals throughout the universe and they began forming not long after the Big Bang. The oldest is about 11 billion years old (the MIlky Way is about 10 billion years old), and they can be observed in many orientations. A galaxy that is "face on" makes it easy to spot the spiral structure.

Some are seen "edge on", and tracing their spiral arms is more difficult. Generally, astronomers look for evidence of starbirth regions, which give off characteristic glows in both infrared and ultraviolet light. Some spirals have very tightly wound arms while others are more loosely wrapped. The degree of winding and the number of arms give clues to the galaxy's activity and evolution. Astronomers commonly assign letters to a galaxy type, such as Sa for spiral galaxy with tightly wound arms, Sb for medium-wound, or Sc for loosely wound arms. A barred spiral would be labeled SBa, SBb, or SBc, to indicate that it has a bar and how tightly wound its arms appear to be. Galaxy-watching is a favorite activity among both amateur and professional astronomers. Good backyard-type telescopes can reveal galaxies in the nearby universe, and of course, such giants as Hubble Space Telescope can find all kinds of galaxies, including spirals, in the VERY distant cosmos.

Merging Spirals

The future of a spiral galaxy is almost always the same: it will likely merge with a nearby galaxy to form an elliptical galaxy. That makes spirals a sort of "intermediate" form. Galaxies have been colliding and merging since the first ones formed shortly after the Big Bang. Astronomers speak of a sort of "hierarchical model" where small wisps of protogalaxies get together to form larger ones, with the spiral shape being one result. They can see smaller dwarf spheroidal galaxies merging with the Milky Way, for example, and those stars are simply swept into the stream of stars that make up the Milky Way.

Ultimately, however, our galaxy will collide with the Andromeda Galaxy, a nearby large spiral. They will end up as an elliptical galaxy, but not before a lot of starbirth activity takes place in the wake of countless shock waves. The arms will ultimately disappear after millions of years of star formation as a result of the collision. The black holes in both galaxies could merge, as well, after a lengthy orbital dance. In most cases, the spirals disappear into the collision, and the resulting elliptical then begins its own aging process over billions and billions of years.