arms are subdivided into two categories: massive and filamentary. In the first instance, the spiral arms are wide, diffuse, and do not contrast significantly with the space between them. In contrast, in the second instance, the spiral arms are narrow and clearly defined.

Shape and pitch angle

The shape of the arm is usually parameterised by the pitch angle . The pitch angle is the angle between the tangent to spiral arm at a given point and the perpendicular to the radius drawn to that point. In the majority of spiral galaxies,

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