Hubble-de Vaucouleurs Galaxy Morphology

Physical & Observational Principles

In 1926 and 1936, Edwin Hubble organized extragalactic nebulae along a 'tuning-fork' sequence based on B-band photographic structure. On the handle sit smooth, featureless Elliptical galaxies (E0 to E7), where the integer n_E = 10(1 − b/a) denotes projected sky flattening from circular E0 (such as central Virgo giant Messier 87, E0-1 pec) to maximally flattened E7 (b/a = 0.3). At the fork junction sit Lenticular galaxies (S0), which possess both a central spheroidal bulge and a stellar disk but lack active spiral arms. The fork then splits into two parallel prongs of Spiral galaxies—normal unbarred spirals (Sa, Sb, Sc) and Barred spirals (SBa, SBb, SBc)—followed by irregular galaxies (Irr). Moving from 'a' to 'c' along either spiral prong, three concerted changes occur: the central bulge-to-disk luminosity ratio B/T shrinks from ~0.4 in Sa (Sombrero Galaxy M104, SA(s)a) through ~0.2 in Sb (Andromeda M31, SA(s)b, and Bode's Galaxy M81, SA(s)ab) to <0.05 in Sc/Sd (Triangulum Galaxy M33, SA(s)cd); spiral arms open from tightly wound to wide pitch angles; and H II star-forming knots increasingly resolve along the arms.

In 1959, Gérard de Vaucouleurs expanded Hubble's scheme into a continuous numerical Stage T (from T = −5 for E galaxies, T = −2 for S0, T = +1 for Sa, T = +3 for Sb, T = +5 for Sc, T = +7 for Sd, to T = +10 for Magellanic irregulars Im) while adding two orthogonal classifiers: bar family—SA (unbarred), SB (strongly barred), and SAB (intermediate weak bar)—and inner-ring/spiral variety—'(r)' when arms emerge tangentially from an inner ring, '(s)' when two grand-design S-shaped arms spring directly from the nucleus or bar ends (as in M51, SA(s)bc pec), and '(rs)' for transitional forms.

Quantitatively, galaxy photometrists decompose two-dimensional images using José Luis Sérsic's (1963) radial intensity law I(R) ∝ exp[−b_n (R / R_e)^(1/n)], where R_e is the half-light effective radius. Classical elliptical galaxies and early-type spiral bulges follow the de Vaucouleurs R^(1/4) law (Sérsic index n = 4), whereas rotational spiral disks follow a pure exponential profile (n = 1).

Worked Calculation: Classifying Ellipticity and De Vaucouleurs Notation for M87, M104, and M31

1. Messier 87 displays outer isophotes with major axis a = 8.3′ and minor axis b = 6.6′: projected ellipticity is 10 × (1 − 6.6 / 8.3) = 10 × 0.205 ≈ 2, giving Hubble type E0–E1 pec (with 'pec' flagging its relativistic synchrotron jet).
2. The Sombrero Galaxy (M104) is classified SA(s)a (de Vaucouleurs stage T = +1): an unbarred ('SA') spiral with tightly wound s-variety arms, a dominant n ≈ 4 spheroidal bulge, and a prominent edge-on dust lane.
3. The Andromeda Galaxy (M31) is classified SA(s)b (T = +3) with major/minor axes 178′ × 63′; from its disk axial ratio b/a = 63 / 178 = 0.354, its disk inclination to our line of sight is i ≈ arccos(0.354) ≈ 77° (accounting for intrinsic disk thickness q₀ ≈ 0.15).

Common Systematic Pitfalls & Misconceptions

• Interpreting Hubble's 'early-type' (E, S0, Sa) to 'late-type' (Sc, Sd, Irr) sequence as an evolutionary timeline: galaxies do not evolve from ellipticals into spirals; in fact, major mergers of gas-rich spirals build giant ellipticals.
• Assuming an E7 galaxy is intrinsically flatter than every E0 galaxy: E0–E7 measures only 2D sky projection, so an oblate spheroid viewed face-on is classified E0.

Benchmark Celestial Objects & Interactive Calculators

Authoritative Standards & Sources

• Revised New General Catalogue and Index Catalogue (RNGC/IC) (2024/2026 edition): Steinicke, W. 2010, Observing and Cataloguing Nebulae and Star Clusters (Cambridge Univ. Press); Verga, M. OpenNGC (CC-BY-SA 4.0 / data tables) — Wolfgang Steinicke / OpenNGC
• SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg