What features identify a spiral galaxy in telescope images?

Spiral galaxies are identified in telescope images by several distinct morphological features that reflect their structure and composition:

• Overall Symmetry: They exhibit circular symmetry, characterized by a bright, sharp-peaked central nucleus surrounded by a thin, flat outer disk.1
• Spiral Structure: A superimposed spiral pattern is the defining feature. In "normal" spirals, these arms emanate directly from the nucleus, while "barred" spirals feature a bright, linear bar structure straddling the nucleus, with arms unwinding from the bar's ends.1
• Arm Characteristics: The arms often contain interstellar dust, which can appear as thin, dark lanes. The appearance of these arms varies by galaxy type: early types (Sa) typically have smooth, tightly wound arms with small pitch angles, while later types (Sc) feature more open, fragmented, or chaotic arms.2314
• Coloration: There is a notable color contrast; the central areas of a spiral galaxy appear red, similar to elliptical galaxies, while the disk and spiral arms are characteristically blue.1

These features are best observed optically, though interstellar dust can sometimes obscure the structure, making radio observations of neutral hydrogen or molecular clouds necessary for a clearer view of the spiral pattern.53