Persistence of Vision Wand.")
In this presentation we will ponder the impermanent, illusory nature of the Milky Way's spiral arms, looking at the evidence that the Milky Way has spiral arms in the first place, then discussing theories that explain how and why the spiral arms exist. (Evidence for the known shape, size, and mass of the Milky Way were discussed in a previous presentation.)
Also note that besides the main spiral arms, the Milky Way has fragmented, discontinuous spurs, which we will also investigate.
First, mapping the spiral arm structure of the Milky Way.
Recall that the interstellar medium greatly obscures our view of the overall Milky Way. However, by plotting the positions for very bright objects in our vicinity such as massive stars, or very large luminous objects such as emission nebulae, there is an indication of "lanes" with relatively empty space between them. This small portion of the entire Milky Way is as much as we can map using visible light, so establishing its spiral arm structure using this evidence is by no means a slam-dunk.
However longer wavelengths than visible light--such as infrared, and radio waves--are not blocked by the gas and dust clouds of the interstellar medium, such that an overall map of nearly the entire Milky Way can be made, and the spiral arm structure becomes apparent. Now let's shift the question from does the Milky Way have spiral arms, to why does the Milky Way have spiral arms, as not all thin disk galaxies have spiral arms as beautiful as ours.
Second, the how and why of the Milky Way's spiral arms.
So the spiral arms of the Milky Way are a similar persistence of vision illusion--like the lights on PimpstarTM rims, short-lived massive stars are born and die at certain intervals as they orbit around.
Our Milky Way notably has a mixture of dozens of different population stars sprinkled within, presumably from dozens of dwarf galaxies that have been pulled in and swallowed to trigger density waves. Don't worry about those poor dwarf galaxies--such is the price for beauty, and our Milky Way, with its plentiful supply of nearby dwarf galaxies is still going to remain beautiful and spiral-armed for some time. (Video link: "The Sagittarius Impact as an Architect of Spirality and Outer Rings in the Milky Way: Movie #2.")
The smaller, fragmented spurs of the Milky Way have a simpler explanation that was discussed in a previous presentation. When a supergiant explodes as a type II supernova, its shockwave will propagate through space, and will compress nearby gas and dust clouds to trigger star formation. This new generation of stars will eventually live and die, and these massive stars will then explode as type II supernovae, triggering the next generation of stars, etc. Like a wildfire burning through a forest, setting new trees ablaze, it may not necessarily have a continuous unbroken line, but will more or less continue in the same direction in search of new fuel.
To recap, density waves (which must be maintained by swallowing nearby dwarf galaxies) trigger the coordinated birth of stars of the spiral arms, and in other ares of the Milky Way subsequent generations of massive stars exploding in type II supernovae set off the self-sustaining star formation of the spurs. Keep in mind that both these features--spiral arms and spurs--are impermanent and illusory in nature.