De Vaucouleurs system
 | This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations. (August 2026) |
The de Vaucouleurs system, also known as the revised Hubble system or the de Vaucouleurs–Sandage system, is a comprehensive galaxy morphological classification scheme developed by French astronomer Gérard de Vaucouleurs in 1959. It extends Edwin Hubble's original tuning-fork classification into a three-dimensional parameter space that describes the structural complexity of galaxies.
Historical development
[edit]Origins (1950s)
[edit]Gérard de Vaucouleurs (1918–1995) began developing his classification system while working at Mount Stromlo Observatory in Australia (1951–1957), continuing at Lowell Observatory (1957–1958) and Harvard College Observatory (1958–1960). His seminal 1959 paper "Classification and Morphology of External Galaxies," published in Handbuch der Physik, introduced a fundamental revision to Hubble's 1926 scheme.
De Vaucouleurs recognized that Hubble's two-dimensional "tuning fork" diagram could not adequately represent the full diversity of galaxy morphologies. In particular, Hubble's system did not explicitly account for:
- Barred structures in lenticular galaxies
- Ring structures in both barred and unbarred galaxies
- Transition types between ellipticals and spirals
- Late-type spirals beyond Hubble's Sc category
- The three-dimensional nature of morphological variation
Reference catalogues (1960s–1990s)
[edit]In 1960, de Vaucouleurs joined the University of Texas at Austin as one of the first five faculty members in the newly formed astronomy department, where he spent the remainder of his career. Working with his wife and collaborator Antoinette de Vaucouleurs (1921–1987), he applied his classification system to thousands of galaxies observed at McDonald Observatory.
The Reference Catalogue of Bright Galaxies (RC1) was published in 1964, classifying 2,599 galaxies. This was followed by the Second Reference Catalogue (RC2) in 1976 and the Third Reference Catalogue (RC3) in 1991, each expanding the sample and refining the classification criteria. These catalogues became essential tools for extragalactic astronomers worldwide.
The modern atlas (2007)
[edit]In 2007, astronomers Ronald J. Buta, Harold G. Corwin, and Stephen C. Odewahn published The de Vaucouleurs Atlas of Galaxies through Cambridge University Press. This comprehensive work illustrated 400 nearby galaxies using modern CCD imagery, updating de Vaucouleurs's original photographic plates for the digital era. The atlas demonstrated that the de Vaucouleurs system remained highly applicable to modern electronic imaging.
The three-dimensional classification volume
[edit]
The fundamental innovation of the de Vaucouleurs system is its representation of galaxy morphology as a **three-dimensional classification volume**, in contrast to Hubble's two-dimensional tuning fork.
| Axis | Parameter | Symbol | Physical Meaning |
|---|---|---|---|
| X (horizontal) | Stage | T | Position along spiral sequence |
| Y (vertical) | Family | A, AB, B | Bar strength |
| Z (depth) | Variety | r, rs, s | Inner ring structure |
This volume allows for systematic placement of galaxies based on their structural features, with the understanding that these parameters are not entirely independent—correlations exist between stage, barredness, and ring structure.
Stage parameter (T)
[edit]The stage parameter provides a numerical index along the morphological sequence, enabling quantitative analysis and computer database applications.
Stage index values
[edit]| T | Type | Description | Typical Colors | Gas Content |
|---|---|---|---|---|
| −5 to −3 | E | Ellipticals | Red (B−V ~ 0.9) | <1% mass |
| −2 | S0⁻ | Early lenticulars | Red-white | <5% mass |
| −1 | S0 | Lenticulars | White | ~5% mass |
| 0 | S0/a | S0–spiral transition | White-blue | ~10% mass |
| 1 | Sa | Early spirals | Blue-white | ~15% mass |
| 2 | Sab | Sa–Sb intermediate | Blue | ~20% mass |
| 3 | Sb | Moderate spirals | Blue | ~25% mass |
| 4 | Sbc | Sb–Sc intermediate | Blue | ~30% mass |
| 5 | Sc | Late spirals | Blue (B−V ~ 0.5) | ~35% mass |
| 6 | Scd | Very late Sc | Very blue | ~40% mass |
| 7 | Sd | Extremely late | Very blue | ~45% mass |
| 8 | Sdm | Magellanic spirals | Blue | ~50% mass |
| 9 | Sm | Magellanic irregulars | Blue | >50% mass |
| 10 | Im | Irregulars | Very blue | >50% mass |
Physical correlations
[edit]The T index correlates strongly with physical galaxy properties:
- Color index: Earlier types (T < 2) have redder B−V colors (~0.9 for ellipticals); later types (T > 5) are bluer (~0.5 for Sc galaxies)
- Gas content: Neutral hydrogen (HI) mass fraction increases from <1% in ellipticals to >50% in irregulars
- Star formation: Specific star formation rate correlates positively with T; late types show vigorous star formation
- Mass concentration: Bulge-to-total luminosity ratio decreases from ~0.9 in ellipticals to ~0.1 in late spirals
- Metallicity: Mean stellar metallicity decreases along the sequence from [Fe/H] ~ +0.2 in bulges to [Fe/H] ~ −0.7 in irregulars
Family parameter (Barredness)
[edit]The family parameter describes the presence and strength of a central bar structure, which plays a crucial role in galaxy dynamics by funneling gas toward the center and driving spiral structure.
Family classifications
[edit]| Symbol | Name | Bar Strength | Arm Origin | Fraction | Example |
|---|---|---|---|---|---|
| SA | Nonbarred | None | Nucleus | ~30% | Messier 81 |
| SAB | Weakly barred | Intermediate | Bar ends/nucleus | ~40% | Messier 83 |
| SB | Strongly barred | Prominent | Bar ends | ~30% | NGC 1300 |
Approximately 60–70% of spiral galaxies show some bar structure, making the family parameter essential for complete classification.
Visual comparison
[edit]
SA: Messier 81 — unbarred spiral with arms from nucleus
SAB: Messier 83 — weakly barred "Southern Pinwheel"
SB: NGC 1300 — prototypical strongly barred spiral
Lenticular galaxy families
[edit]For lenticular galaxies (S0), the family notation indicates:
- SA0: Unbarred lenticular — smooth disk without bar
- SB0: Barred lenticular — shows bar in disk
- S0: Indeterminate bar (usually edge-on orientation)
Variety parameter (ring structure)
[edit]The variety parameter characterizes the presence of inner ring structures, which form at resonance locations in the galactic disk.
Variety classifications
[edit]| Symbol | Name | Structure | Arm Attachment | Physical Origin |
|---|---|---|---|---|
| (r) | Ringed | Complete inner ring | Arms begin tangent to ring | Resonance at inner Lindblad resonance |
| (rs) | Pseudoring | Partial/broken ring | Arms begin near ring feature | Transient or forming resonance |
| (s) | Spiral | No ring | Arms begin at center or bar ends | No resonance feature |
Ring structures are most prominent at intermediate stages (S0/a to Sb) and become less distinct at both early (E, S0) and late (Sc, Irr) stages. The presence of rings correlates with resonant orbits in the galactic disk and provides constraints on galaxy mass distributions.
Extended notation
[edit]Outer rings (R)
[edit]Outer ring structures, which form at the outer Lindblad resonance, are denoted by prepending to the classification:
- R: Definite outer ring (e.g., RSB(r)bc)
- (R): Outer pseudoring or outer lens structure (e.g., (R)SA(rs)ab)
Approximately 10–15% of spiral galaxies show prominent outer rings.
Arm classes
[edit]De Vaucouleurs originally distinguished arm morphology:
- m: Massive arms — broad, diffuse, relatively low surface brightness contrast (e.g., Messier 33)
- f: Filamentary arms — thin, knotty, high contrast (e.g., Messier 101)
These append the stage letter: Sbm, Scf. While less commonly used today, this distinction relates to spiral arm generation mechanisms.
Lenses
[edit]Lens-like structures (regions with shallow brightness gradients) are denoted:
- (l): Inner lens (e.g., S0(l))
- (L): Outer lens
Notable galaxy examples
[edit]| Galaxy | Classification | T | Distance (Mly) | Notes |
|---|---|---|---|---|
| Messier 87 | E+0-1 | −4 | 54 | Giant elliptical; Virgo Cluster cD galaxy |
| NGC 4346 | SB0 | −1 | 50 | Barred lenticular in Canes Venatici |
| Messier 81 | SA(s)ab | 2 | 12 | Grand-design unbarred spiral; Bode's Galaxy |
| Milky Way | SAB(rs)bc | 4 | 0 | Weak bar; pseudoring; intermediate stage |
| Andromeda Galaxy | SA(s)b | 3 | 2.5 | Unbarred; nearest major spiral |
| Messier 83 | SAB(s)c | 5 | 15 | Weakly barred; "Southern Pinwheel" |
| NGC 1300 | SB(s)bc | 4 | 61 | Prototypical barred spiral |
| NGC 1512 | (R)SB(r)ab | 2 | 38 | Double-ringed barred galaxy |
| Large Magellanic Cloud | SB(s)m | 9 | 0.16 | Magellanic type; satellite of Milky Way |
| Small Magellanic Cloud | SB(s)m pec | 9 | 0.20 | Peculiar Magellanic irregular |
Comparison with other classification systems
[edit]| Feature | Hubble (1926) | de Vaucouleurs (1959) | Yerkes (MK) (1960s) | DSS (1990s) |
|---|---|---|---|---|
| Dimensions | 2D tuning fork | 3D volume | 2D + luminosity | Automated |
| Stages | E, S0, Sa–Sc, Irr | Extended T (−5 to +10) | Modified stages | Photometric |
| Barredness | SB only | SA, SAB, SB | Similar | Bar/No bar |
| Rings | Implicit | Explicit (r, rs, s) | Implicit | Not included |
| Numerical index | No | Yes (T) | Yes | Yes |
| Luminosity class | No | No | Yes (I–V) | No |
| Physical basis | Morphological | Morphological | Physical | Photometric |
The de Vaucouleurs system provides the most comprehensive purely morphological classification, while the Yerkes system adds physical parameters through luminosity classes.
Scientific applications
[edit]Galaxy evolution studies
[edit]The de Vaucouleurs system provides a framework for studying galaxy evolution through morphological transformation:
- Ram-pressure stripping: Late-type spirals (Sc–Sd) lose gas in dense clusters and transform to S0 galaxies
- Morphology–density relation: Early types (E, S0) dominate cluster cores; late types (Sc–Irr) populate the field
- Merger-induced changes: Disturbed morphologies in interacting systems
- Bar formation and dissolution: Bars evolve over cosmic time, affecting family classification
The T parameter serves as a proxy for evolutionary state, with galaxies generally evolving from late to early types through environmental and internal processes.
Modern surveys
[edit]The de Vaucouleurs system remains standard in large-scale galaxy surveys:
- Sloan Digital Sky Survey (SDSS): Morphological classifications for >10⁶ galaxies
- Galaxy Zoo: Citizen science project using de Vaucouleurs types; >10⁸ classifications by volunteers
- 2MASS (Two Micron All Sky Survey): Near-infrared morphologies compared to optical de Vaucouleurs types
- Spitzer S4G (Survey of Stellar Structure in Galaxies): Mid-infrared morphologies at 3.6 and 4.5 μm
Machine learning applications
[edit]Modern machine learning algorithms trained on Galaxy Zoo data can automatically assign de Vaucouleurs T types with >90% accuracy for bright galaxies. These automated classifications enable morphological studies of millions of galaxies in upcoming surveys like LSST and Euclid.
Limitations and criticisms
[edit]Despite its widespread use, the de Vaucouleurs system has recognized limitations:
- Subjectivity: Visual classification depends on observer experience and training; different classifiers may assign different types to the same galaxy
- Wavelength dependence: Morphology varies with observation wavelength; optical (B-band) classifications differ from infrared (K-band)
- High-redshift effects: Cosmological evolution and surface brightness dimming complicate classification of distant galaxies (z > 0.5)
- Peculiar galaxies: Interacting and merging systems often resist standard classification
- Environmental bias: Field galaxies are easier to classify than crowded cluster galaxies
- Surface brightness limit: Low surface brightness features (outer rings, faint bars) may be missed
Alternative and complementary systems include:
- Yerkes (MK) system: Adds luminosity classes
- van den Bergh system: Distinguishes luminosity classes for spiral galaxies
- Galaxy Zoo decision tree: Simplified classification for citizen science
- CAS parameters: Quantitative structural parameters
Legacy
[edit]Gérard de Vaucouleurs's classification system revolutionized extragalactic astronomy by providing a comprehensive framework that captures the complexity of galaxy morphology. The system remains the dominant classification scheme in professional astronomy more than six decades after its introduction, a testament to its utility and flexibility. The 2007 atlas ensures that de Vaucouleurs's work continues to inform 21st-century galaxy studies.
See also
[edit]References
[edit]- Buta, R., Corwin, H., & Odewahn, S. (2007). The de Vaucouleurs Atlas of Galaxies. Cambridge University Press. ISBN 978-0-521-82048-6
- de Vaucouleurs, G. (1959). "Classification and Morphology of External Galaxies." Handbuch der Physik, 53, 275–310
- de Vaucouleurs, G., & de Vaucouleurs, A. (1964). Reference Catalogue of Bright Galaxies. University of Texas Press
- de Vaucouleurs, G., de Vaucouleurs, A., & Corwin, H. G. (1976). Second Reference Catalogue of Bright Galaxies. University of Texas Press
- de Vaucouleurs, G., et al. (1991). Third Reference Catalogue of Bright Galaxies. Springer-Verlag
- Sandage, A., & Tammann, G. A. (1981). A Revised Shapley-Ames Catalog of Bright Galaxies. Carnegie Institution
- Buta, R. (2013). "Galaxy Morphology." In Planets, Stars and Stellar Systems, Volume 6, Springer
External links
[edit]- Galaxy Morphology and Classification — R. Buta, NASA/IPAC
- The de Vaucouleurs Atlas — Cambridge University Press
- Galaxy Zoo — Citizen science classification project
- SIMBAD — Astronomical database with morphological types