The fact that the Milky Way has a spiral shape tells us straight away that our Galaxy is situated in the suburbs. As an analogy, some people choose to live in the suburbs given the (typically) larger accommodations per unit cost and the relatively easy access to resources.
Similarly, galaxies situated in groups (the ‘suburbs’) have relatively light interactions with neighboring galaxies as well as reasonable access to “galaxy food” (hydrogen) infalling from their surroundings. By contract, the galaxies packed into tight spaces (clusters) must cope with some rather aggressive interactions which usually involve two galaxies tearing material off of each other by a process called “ram pressure stripping.” This has the effect of wearing away that beautiful spiral pattern in Galaxy images.
At worst, the galaxy interactions lead to mergers, in which one galaxy joins with another one. In this case, the spiral pattern disappears utterly. We cannot leap out of the Milky Way and look down onto our own galaxy, and never will do. Yet we still manage to know what the Milky Way looks like thanks to the information given to us by the humble hydrogen atom.
Hydrogen is the most abundant element in the Milky Way. As long as the hydrogen is cool, it emits a very long wavelength color of light that penetrates the entire galaxy. These multitudes of tiny beacons are easily detected by our radio telescopes. By recording the positions of all of these beacons, we are able to construct maps which show us how many arms there are, and what are their shapes.
But even in the suburbs we are not safe from damaging galaxy interactions. In about one billion years, the nearest large spiral galaxy to us, The Andromeda Galaxy, will collide with the Milky Way. The spiral arms will go away, possibly forever. Our days of sporting that elegant spiral shape are numbered, and fortunately for humans, this colossal merger will not affect our way of life in the slightest.
