One of the basic working rules in empirical science is the Donald Duck Factor, that when something novel looks, waddles and sounds like a duck, then in all probability it might also be a duck, or at least of the same species. And when a similar structure or shape is found in two disparate phenomena, there is a good chance that both might be formed by a similar process. Such might be in the case of tropical cyclones and spiral galaxies.
It is cyclone season in the northern hemisphere of the equatorial regions and yet again humanity has discovered how puny it is in the face of a tropical cyclone when Cyclone Nargis devastated parts of Burma recently. However tropical cyclones, or hurricanes, are not well understood weather phenomena and the formation of tropical cyclones is the topic of extensive ongoing research.
A spiral galaxy is a one belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the Nebulae” and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk of stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters.
The origin of the spiral arms in galaxies is not as easy to identify as one may first think. After all, whatever it is that creates the spiral arms must be able to account for a wide variety of spiral structures. Some galaxies have a well-organised spiral structure (grand design spirals), while others are patchy (flocculent spirals). The arms may be wound tightly around the galaxy or may be more open. Some spiral arms originate at the end of bars, others directly from the galactic bulge.