Published online by Cambridge University Press: 05 September 2017
For many astrophotographers, the spiral galaxy M 31 was their first extragalactic subject – the first image remains in memory. This is a good point of introduction to the astrophysics of these spiral systems.
THE CLASSIFICATION OF SPIRAL GALAXIES
Spiral galaxies with their spiral arms are prototypical galaxies. These impressive forms allow even those with little knowledge of astronomy to instantly identify spirals as “galaxies” in their images. Spiral arms have thus played a deciding role in the various classification schemes for this type of galaxy. Note that all classification schemes are based on the view onto the main plane of the galaxy. In other words, the “face-on” view is what matters for the classification. Galaxies seen from an oblique inclination are rotated in order to simulate a face-on view. The classification of edge-on galaxies is difficult.
An edge-on spiral galaxy allows a view into its galactic plane with large amounts of gas and dust. However, this makes it more difficult to say anything about the spiral arms. This dilemma also applies to the classification of our Milky Way, since Earth is located within the main plane which thus doesn't afford a view onto this plane. Further conclusions require complex measurements in wavelengths which are able to penetrate the thick clouds of gas and dust. Today, the consensus among astronomers is an SB classification with four spiral arms. However, the Milky Way did not always have this four-arm structure; it is suspected that tidal interactions induced disturbances in the plane of the disc which led to the splitting of two dominant spiral arms.
The Hubble classification, which was mentioned in the Introduction, has proved useful over many years and distinguishes spiral types based on the compactness of the pattern of the arms. The nomenclature uses, in addition to the letter S, the suffixes a, b, and c, for example describing as Sa a galaxy the arms of which appear very tight and as a rule can be followed over one or two full revolutions. Sb denotes types the arms of which are not as tightly wound and more coarsely structured compared to Sa.