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-===== Galaxy Classification =====
-• The "Island Universe" Hypothesis: Historically, the nature of "elliptical nebulae" was a subject of intense debate. Immanuel Kant suggested these were distant systems similar to the Milky Way, which he termed "island universes".
-• The Shapley-Curtis Debate (1920): This famous scientific disagreement centered on whether these nebulae resided within our Galaxy (Shapley's view) or were extragalactic (Curtis's view). The debate was largely resolved when Edwin Hubble identified Cepheid variables in M31, proving it was far outside the Milky Way.
-• The Hubble Sequence: Hubble categorized galaxies into three primary morphological groups: Ellipticals (E), Spirals (S and SB), and Irregulars (Irr).
-• The Tuning-Fork Diagram: This visual representation shows a sequence from spherical ellipticals (E0) to flattened ones (E7), then split into two parallel tracks of normal spirals and barred spirals.
-• Terminology: Galaxies on the left of the diagram (ellipticals) are called "early types," while those on the right (spirals/irregulars) are "late types," though this is not an evolutionary sequence.