If you’ve seen an image of the Milky Way from above or below, you will certainly notice that it has a spiral structure. Not all galaxies are created equal, though, as there are many, known as elliptical galaxies, that are blob-like, while others have irregular shapes. Ours is of a class of galaxies called barred spirals, because it has a rectangular bar in the middle of the galactic disk.
The Milky Way has four main spiral arms: the Norma and Cygnus arm, Sagittarius, Scutum-Crux, and Perseus. The Sun is located in a minor arm, or spur, named the Orion Spur. The galactic disk itself is about 100,000 light years across, and the bar at the center is estimated to be about 27,000 light years long.
Why is the Milky Way a spiral? This is due to its rotation, or rather, the rotation of matter inside the galactic disk around the center. It’s not as if the stars themselves stay in the spiral arms, and rotate around the center of the galaxy, though: if they did this, the arms would wind in tighter and tighter over time (2 billion years or so), since the stars in the center revolve faster than those further out.
The spirals are actually what is called a density wave or standing wave. The best way to describe this is the analogy of a traffic jam: cars travel on a busy road in a city, bunching up in jams over the course of a day at certain sections. But the cars move through the jam eventually, and other cars pile up behind them in the jam. The wave is at a certain location, with bunches of matter piling up there for a while, then moving on to be replaced by other matter. As dust and gas is compressed in the spirals, it is heated up and results in the formation of new stars. This star formation makes the trailing edge of the spiral brighter, and places the density wave “ahead”, where dimmer, redder stars are starting to be compressed.
When you see an image of the Milky Way like the one above, it’s not actually a photo of our galaxy. Since we inhabit the disk and have no way (currently) of going above or below, images of the Milky Way are generated by computers or artists. Astronomers have determined that the Milky Way is a spiral galaxy by mapping the movements of stars and hydrogen clouds in the disk.
The Milky Way is far from being the only spiral galaxy in the Universe. To view images of other spiral galaxies, go to the aptly-named Spiral Galaxies website, or NASA’s Astronomy Picture of the Day Spiral Galaxy Index.
Source: University of Wisconsin-Madison News