Hubble tuning fork
Definition: The Hubble tuning fork is a purely morphological classification that nonetheless correlates strongly with real physical differences in gas content, stellar population age, and star-formation activity across galaxy types.
Why it matters here: The Hubble tuning fork is a purely morphological classification that nonetheless correlates strongly with real physical differences in gas content, stellar population age, and star-formation activity across galaxy types.
Relationship: Classify a galaxy image using the Hubble tuning fork (elliptical, spiral, barred spiral, irregular).
Boundary or common confusion: Interpreting the tuning-fork diagram as a literal evolutionary sequence, with ellipticals evolving into spirals (Hubble’s original, incorrect, "early type"/"late type" terminology, still used today purely as a label). Modern evidence instead suggests many ellipticals form from mergers of spirals, the reverse of Hubble’s original intuition.
elliptical galaxy
Definition: Classify a galaxy image using the Hubble tuning fork (elliptical, spiral, barred spiral, irregular).
Why it matters here: The Hubble tuning fork is a purely morphological classification that nonetheless correlates strongly with real physical differences in gas content, stellar population age, and star-formation activity across galaxy types.
Relationship: Edwin Hubble’s 1926 galaxy classification scheme, though originally conceived (incorrectly) as an evolutionary sequence from ellipticals to spirals, remains in near-universal use today as a purely morphological classification, precisely because morphology correlates strongly with physically meaningful properties: gas content, ongoing star formation, and stellar population age.
Boundary or common confusion: Interpreting the tuning-fork diagram as a literal evolutionary sequence, with ellipticals evolving into spirals (Hubble’s original, incorrect, "early type"/"late type" terminology, still used today purely as a label). Modern evidence instead suggests many ellipticals form from mergers of spirals, the reverse of Hubble’s original intuition.
spiral galaxy
Definition: M31 (Andromeda Galaxy), type SA(s)b, and M87, type E0/cD, look completely different: one is a flattened disk with prominent spiral arms and active star formation, the other a smooth, featureless ball of old, red stars with no ongoing star formation.
Why it matters here: The Hubble tuning fork is a purely morphological classification that nonetheless correlates strongly with real physical differences in gas content, stellar population age, and star-formation activity across galaxy types.
Relationship: M101 (Pinwheel Galaxy) (SAB(rs)cd): a nearly face-on spiral, 6.4 Mpc away, rich in cold gas and dust with prominent blue, young star-forming regions along its spiral arms.
Boundary or common confusion: Interpreting the tuning-fork diagram as a literal evolutionary sequence, with ellipticals evolving into spirals (Hubble’s original, incorrect, "early type"/"late type" terminology, still used today purely as a label). Modern evidence instead suggests many ellipticals form from mergers of spirals, the reverse of Hubble’s original intuition.
barred spiral galaxy
Definition: Classify a galaxy image using the Hubble tuning fork (elliptical, spiral, barred spiral, irregular).
Why it matters here: The Hubble tuning fork is a purely morphological classification that nonetheless correlates strongly with real physical differences in gas content, stellar population age, and star-formation activity across galaxy types.
Relationship: Spiral galaxies (Sa-Sc, or barred SBa-SBc) are flattened, rotating disks with a central bulge, spiral arms traced by young blue stars and star-forming gas, and a bulge-to-disk ratio that decreases from Sa to Sc.
Boundary or common confusion: Interpreting the tuning-fork diagram as a literal evolutionary sequence, with ellipticals evolving into spirals (Hubble’s original, incorrect, "early type"/"late type" terminology, still used today purely as a label). Modern evidence instead suggests many ellipticals form from mergers of spirals, the reverse of Hubble’s original intuition.
irregular galaxy
Definition: M82 (Cigar Galaxy) (Irregular (starburst)): 3.5 Mpc away, Gravitationally disturbed by M81; intense starburst with galactic superwind
Why it matters here: The Hubble tuning fork is a purely morphological classification that nonetheless correlates strongly with real physical differences in gas content, stellar population age, and star-formation activity across galaxy types.
Relationship: Classify a galaxy image using the Hubble tuning fork (elliptical, spiral, barred spiral, irregular).
Boundary or common confusion: Interpreting the tuning-fork diagram as a literal evolutionary sequence, with ellipticals evolving into spirals (Hubble’s original, incorrect, "early type"/"late type" terminology, still used today purely as a label). Modern evidence instead suggests many ellipticals form from mergers of spirals, the reverse of Hubble’s original intuition.
bulge-to-disk ratio
Definition: Spiral galaxies (Sa-Sc, or barred SBa-SBc) are flattened, rotating disks with a central bulge, spiral arms traced by young blue stars and star-forming gas, and a bulge-to-disk ratio that decreases from Sa to Sc.
Why it matters here: The Hubble tuning fork is a purely morphological classification that nonetheless correlates strongly with real physical differences in gas content, stellar population age, and star-formation activity across galaxy types.
Relationship: Nearly face-on grand-design spiral, low bulge-to-disk ratio
Boundary or common confusion: Interpreting the tuning-fork diagram as a literal evolutionary sequence, with ellipticals evolving into spirals (Hubble’s original, incorrect, "early type"/"late type" terminology, still used today purely as a label). Modern evidence instead suggests many ellipticals form from mergers of spirals, the reverse of Hubble’s original intuition.