Fact-checked by Grok 2 weeks ago

Hubble sequence

The Hubble sequence, also known as the Hubble classification or tuning fork diagram, is a morphological scheme for categorizing galaxies based on their visual structure and shape, introduced by astronomer in his 1926 paper "Extra-Galactic Nebulae." This system arranges galaxies along a sequence reflecting increasing complexity from simple, smooth forms to more structured, disk-like appearances, primarily dividing them into ellipticals (E), lenticulars (S0), spirals (S), barred spirals (SB), and irregulars (Irr). Originally derived from observations of about 400 extra-galactic nebulae using photographic plates from the 100-inch Hooker Telescope at , the classification emphasized rotational symmetry and form, with 97% of the sample exhibiting such features. In the sequence, elliptical galaxies (E0 to E7) form the starting branch, appearing as featureless, ellipsoidal systems ranging from nearly spherical (E0) to highly elongated (E7, with axis ratios up to about 3:1), containing older stars and little gas or dust. These transition into lenticular galaxies (S0 or SB0), which have a central bulge and disk but lack spiral arms, serving as an intermediate type between ellipticals and spirals. The main fork then splits into normal spirals (Sa to Sc) and barred spirals (SBa to SBc), where Sa/SBa types feature large central bulges, tightly wound arms, and fewer star-forming regions, while Sc/SBc types have smaller bulges, loosely wound arms, and prominent . Irregular galaxies (Irr), comprising about 3% of the observed sample, do not fit the symmetric sequence and include chaotic forms like the . Although Hubble's system was initially interpreted as an evolutionary progression—with "early-type" galaxies (e.g., Sa) evolving into "late-type" (e.g., Sc)—modern understanding views it as a static morphological tool rather than a timeline, as galaxy evolution involves mergers and environmental factors not captured by shape alone. The classification has been refined over time, notably by Gérard de Vaucouleurs in 1959 with numerical stages (e.g., 1 for Sa, 5 for Sc) and the addition of ring and peculiar features, but the core tuning fork diagram remains foundational for astronomical research and surveys like the Sloan Digital Sky Survey. Galaxies in the sequence typically span 10^7 to 10^12 stars and diameters of 1,000 to 100,000 parsecs, with spirals showing increasing openness, bluer colors, and higher gas content from Sa to Sc.

Historical Development

Edwin Hubble's Original Classification

Edwin Hubble, an American astronomer who joined the staff of the in 1919, developed his galaxy classification system in the 1920s to organize the growing observations of extragalactic "nebulae," building on earlier descriptive schemes by astronomers such as Max Wolf (1908), who used letter designations for amorphous to spiral forms, and James Reynolds (1920), who proposed classes I-V emphasizing spiral variety. Hubble's motivation stemmed from the need for a systematic, physically meaningful sequence to identify patterns in the structural forms of these objects, facilitated by the superior resolution of the observatory's 60-inch and 100-inch reflecting telescopes, which allowed detailed photographic surveys starting around 1926. Hubble first outlined his classification in the 1926 paper "Extra-Galactic Nebulae," published in , where he analyzed approximately 400 objects and introduced four principal types: ellipticals (E), normal spirals (S), barred spirals (SB), and (Irr). For ellipticals, he introduced a numerical subtype from (nearly circular) to E7 (highly flattened), based on the axis ratio derived from photographic appearances. He subdivided the spirals into early (Sa/SBa, with large amorphous nuclei and tightly wound arms), intermediate (Sb/SBb, with brighter nuclei and more resolved arms), and late (Sc/SBc, with small nuclei and loose arms), reflecting a progression in nuclear dominance and arm openness. Irregulars were noted as a small fraction (about 3%) lacking regular structure, exemplified by the . This scheme was fully elaborated in Hubble's 1936 book The Realm of the Nebulae, where he refined the types, including the introduction of galaxies (S0 or SB0) as an intermediate class between ellipticals and spirals, and applied the system to a broader sample that included over 1,000 galaxies by that time. The spiral subtypes (a, b, c) similarly captured variations in arm tightness and resolution, prioritizing gross morphological features over fine details to ensure the sequence's applicability across diverse observations. The classification relied on photographic plates exposed with the Mount Wilson telescopes, which revealed differences in shapes, sizes, central concentrations, and arm resolutions among the galaxies, enabling Hubble to distinguish the types through visual symmetry and structural patterns. A pivotal event underpinning this work was Hubble's 1925 identification of stars in the "nebula," which provided the first reliable distance estimate (about 900,000 light-years), confirming it as an extragalactic system and allowing subsequent classifications to be contextualized within the emerging model of an expanding .

The Tuning Fork Diagram

The tuning fork diagram, introduced by in 1936, visually represents the morphological classification of galaxies in a Y-shaped schematic resembling a . The left arm of the fork begins with elliptical galaxies, denoted as E followed by a numerical index from 0 (nearly spherical) to 7 (highly elongated), progressing through lenticular galaxies (S0) to normal spirals classified as Sa, Sb, and Sc based on increasing openness and tightness of spiral arms. The right arm branches off to represent barred spirals (SBa, SBb, ), distinguished by a central bar structure, while irregular galaxies are positioned separately outside the main fork. This layout illustrates a progression where arm tightness from Sa to Sc corresponds to a decreasing bulge-to-disk ratio, emphasizing visual form over physical properties. Hubble refined this schematic in preparation for a comprehensive atlas, drawing on photographic observations to standardize the sequence. Following Hubble's death in 1953, astronomer completed and published The Hubble Atlas of Galaxies in 1961, incorporating the diagram with enhanced photographic examples and detailed notes on appearances. This posthumous work solidified the diagram as a key tool, building on Hubble's original framework by providing calibrated images that highlighted subtle morphological transitions. The tuning fork diagram became a standardized reference for astronomers, facilitating consistent classification in large-scale surveys such as the Sky Survey, which utilized its scheme to catalog thousands of galaxies based on photographic plates. For instance, the giant Messier 87 is placed as an E0 due to its smooth, nearly round profile, while the (Messier 31) exemplifies an Sb spiral with moderately tight arms and a prominent bulge. Its widespread adoption enabled efficient morphological assessments in early extragalactic studies, influencing galaxy identification across observatories. In the , Sandage introduced revisions to the diagram, incorporating finer structural details such as dust lanes and shells in elliptical to refine subtype distinctions without altering the core sequence. These updates, detailed in subsequent publications, enhanced the diagram's utility for recognizing transitional forms, ensuring its enduring role as a foundational visual aid in .

Morphological Classification

Elliptical Galaxies

Elliptical galaxies are characterized by their smooth, featureless envelopes of , lacking spiral , disks, or significant dust lanes. These galaxies present an ellipsoidal on astronomical images, ranging from nearly spherical to highly elongated forms. In the Hubble sequence, they occupy the initial branch of the tuning fork diagram as the earliest morphological type. The classification of elliptical galaxies spans from , which appear almost perfectly round, to E7, which are markedly flattened, with the subtype determined by the apparent axis ratio b/a of the major and minor axes, where the numerical index approximates $10(1 - b/a). Subtypes to E3 are the most common, comprising the majority of observed ellipticals, while E4 to E7 are rarer and may result from dynamical processes such as mergers that enhance flattening. Physically, elliptical galaxies are dominated by ancient stellar populations, with ages typically exceeding 10 billion years, and contain minimal gas or dust, which suppresses ongoing . Their structure is maintained by random stellar motions rather than organized rotation, leading to high velocity dispersions that can reach hundreds of km/s in the central regions. Representative examples include the giant elliptical M87 in the , a massive system with a prominent , and the cD supergiant NGC 6166, known for its extended envelope surrounding multiple nuclei. Elliptical galaxies constitute approximately 10-20% of the overall in typical environments like loose groups, but they are more prevalent in dense clusters, where they can for a larger to environmental influences favoring their formation and . Observationally, they are as ellipsoidal shapes on photographic plates or digital images, with profiles that closely follow the empirical de Vaucouleurs r^{1/4} law, first formulated in to describe the light distribution in these systems: I(R) = I_e \exp \left\{ -7.67 \left[ \left( \frac{R}{R_e} \right)^{1/4} - 1 \right] \right\}, where I_e is the surface brightness at the effective radius R_e enclosing half the total light.

Lenticular Galaxies

Lenticular galaxies, designated as S0 in the Hubble classification, feature a prominent central bulge embedded in a disk-like structure but lack the spiral arms characteristic of spiral galaxies. This morphology results in a lens-shaped appearance, particularly when viewed edge-on, with the disk appearing smooth and featureless. Some lenticulars display weak bars or faint spiral-like features, leading to subtypes such as S0/a. These galaxies occupy a transitional position between ellipticals and spirals in the Hubble sequence, bridging the morphological divide. Physically, lenticular galaxies contain moderate populations of old stars, with minimal ongoing due to the depletion of gas and dust. Their stellar disks are dominated by low-mass, long-lived stars, and they exhibit low levels of molecular gas, suppressing new star birth. Representative examples include NGC 3115, an edge-on lenticular galaxy showcasing a bright bulge and extended disk about 32 million light-years away, and (also known as M102 or the Spindle Galaxy), which displays a tilted disk with a prominent central ellipsoidal bulge. Classification of galaxies can reveal subtleties such as dust lanes or faint shells, often interpreted as remnants of past mergers that disrupted earlier structures. These features suggest dynamical interactions that may have quenched . Lenticulars constitute approximately 10-15% of galaxies in the nearby and are frequently found in lower-density field environments rather than dense clusters. Lenticular galaxies are distinguished from ellipticals by their thinner disk components and from spirals by their fainter, armless disks. A key metric is their bulge-to-total ratio, typically ranging from 0.5 to 0.8, reflecting a dominance of the bulge over the disk compared to spirals.

Spiral Galaxies

Spiral galaxies are characterized by a prominent central bulge surrounded by a flattened disk containing winding spiral arms, which distinguish them from other morphological types in the Hubble sequence. The spiral arms emerge from the ends of a central bar in some cases or directly from the bulge in others, creating a rotating structure maintained by and density waves. These galaxies exhibit a range of subtypes based on the relative sizes of the bulge and disk, as well as the tightness of the spiral arms: spirals feature tightly wound arms and a large, bright bulge dominating the light profile; Sb spirals have moderately wound arms and a medium-sized bulge; and Sc spirals display loosely wound, patchy arms with a small bulge. Barred spiral galaxies, denoted as SBa, SBb, and , mirror these subtypes but include an elongated central bar that connects to the spiral arms, altering the overall . In SBa types, the bar is prominent alongside a large bulge and tight arms, while types show a weaker bar influence with loose arms and minimal bulge contribution. The winding of spiral arms is quantified by the pitch angle, typically ranging from 5° to 20°, which measures between the arm and the circle centered on the ; tighter arms (smaller pitch angles) are associated with earlier subtypes like and SBa. In the diagram, unbarred spirals occupy the upper prong, while barred variants form the lower prong. Approximately 60% of galaxies in the local universe are spirals (including barred forms), with bars present in about two-thirds of them. Physically, spiral galaxies host active primarily along the arms, where dense gas clouds collapse to form young, massive stars, rendering the arms blue and luminous in optical wavelengths. These disks are gas-rich, containing significant reservoirs of and molecular gas that fuel ongoing star birth, particularly in later subtypes like Sc and SBc. Observationally, the arms reveal prominent dust lanes tracing the spiral structure and numerous H II regions—ionized clouds excited by hot, young stars—visible in emission-line imaging. In barred spirals, the central bar funnels gas inward along its length, driving inflows toward the nucleus and potentially enhancing central or fueling active galactic nuclei. Representative examples include the , classified as SBbc with its moderate bar and loosely wound arms; the (M31), an SAab type with tight arms and a substantial bulge; and M101, an Sc spiral showcasing extended, flocculent arms rich in star-forming regions.

Irregular Galaxies

Irregular galaxies represent a distinct category in the Hubble sequence, characterized by their lack of organized structure and placement outside the primary branches of the diagram. Unlike ellipticals, lenticulars, or spirals, these galaxies exhibit chaotic and asymmetrical morphologies without prominent nuclear bulges, spiral arms, or disk-like features. They are subdivided into two main subtypes: Irr I (or ), which display some underlying structure such as faint bars or patchy concentrations, and Irr II (or amorphous ), which appear highly disrupted with no discernible regularity. Morphologically, irregular galaxies show disordered distributions of stars, gas, and dust, often resolved into individual stars, H II regions, and star clusters only in nearby examples. Irr I galaxies, like the , possess irregular but somewhat coherent shapes with embedded filaments or weak arms, while Irr II examples, such as NGC 4449, present amorphous forms with prominent dust lanes and tidal distortions. These features arise from the absence of symmetric gravitational potentials that define more structured types. While most are dwarf systems, luminous irregulars such as the M82 also exist, exhibiting intense driven by interactions. Physically, irregular galaxies are typically gas-rich, with high neutral hydrogen content supporting intense rates that can reach up to 1.3 M⊙ yr⁻¹ kpc⁻² in compact regions. They often qualify as systems with absolute blue magnitudes typically ranging from -15 to -19, though some luminous irregulars reach brighter magnitudes (e.g., M_B ≈ -21 for M82), dominated by young, massive stars that impart blue colors due to ongoing bursts of activity. Their high gas fractions (up to 5 M⊙ L_B⁻¹) and low metallicities reflect less processed interstellar media compared to spirals. Representative cases include the (Irr I), a satellite of the with active star-forming complexes, and NGC 4449 (Irr II), a starburst irregular exhibiting super star clusters and high gas densities. In the local , irregular galaxies constitute approximately 5-10% of the population, predominantly as dwarf systems or results of interactions; Irr I types are often isolated dwarfs, while Irr II may originate as remnants of mergers or dynamical disruptions. Observationally, irregular galaxies appear blue from their preponderance of young, hot stars, complicating distance estimates via color-magnitude relations, and their fine details—such as resolved stellar populations and H II regions—are typically discernible only in nearby objects like the within 50-60 kpc. Distant irregulars often blend into unresolved, clumpy appearances, requiring or high-resolution imaging to reveal sites obscured by .

Physical Interpretation

Correlations with Galaxy Properties

Elliptical and lenticular (S0) galaxies along the Hubble sequence are characterized by high stellar masses, typically ranging from $10^{11} to $10^{12} M_\odot, and exhibit red colors indicative of predominantly old stellar populations with minimal ongoing star formation. In contrast, spiral galaxies generally possess lower stellar masses, often in the range of $10^{9} to $10^{11} M_\odot, and display blue colors due to active star formation in their disks, with the bulge-to-disk ratio decreasing progressively from early-type spirals (Sa) to late-type spirals (Sc). Specific correlations between Hubble types and physical properties reveal systematic trends in star formation and gas content. Star formation rates decrease along the sequence from late-type spirals (Sc) to ellipticals, with spirals sustaining rates of approximately 1–10 M_\odot yr^{-1} driven by disk-wide activity, while ellipticals exhibit rates below 0.1 M_\odot yr^{-1} due to gas depletion. Similarly, gas content, including atomic and molecular components, is significantly higher in spirals and at 1–10% of total mass, supporting sustained star formation, compared to less than 0.1% in ellipticals and S0 galaxies, where gas is largely exhausted. Environmental density plays a key role in these correlations, with early-type galaxies (E and S0) preferentially inhabiting high-density regions such as galaxy clusters, where interactions may accelerate gas removal and morphological transformation. Late-type spirals () and irregulars, conversely, dominate lower-density field environments, allowing preservation of gas reservoirs and disk structures. Additionally, a luminosity-metallicity relation holds across types, wherein brighter galaxies tend to be more metal-rich, reflecting differences in star formation efficiency and chemical enrichment histories. Large-scale surveys like the (SDSS) have quantified these links through analyses of type-specific distributions, revealing variations in the Schechter function parameters—such as characteristic L_* and faint-end \alpha—that differ by morphological type, with early types showing steeper functions at high .

Implications for Evolution

In his 1936 monograph, proposed that the morphological sequence from elliptical (E) to (S0) to spiral (S) to irregular (Irr) galaxies represented an evolutionary progression driven by the gradual consumption of gas and dust, leading to the development of more complex structures over time. This view posited a linear timeline where early-type galaxies aged into late types as built disks and arms. However, this interpretation has been largely rejected by modern , which views the sequence not as a strict temporal progression but as a reflection of diverse formation histories influenced by , mass, and dynamical processes. Contemporary models interpret the Hubble sequence as capturing distinct pathways in galaxy assembly within the Lambda cold dark matter (ΛCDM) framework, where major mergers of gas-rich progenitors often produce elliptical galaxies by disrupting disks and funneling material into central bulges, while secular processes—such as bar instabilities—drive the evolution of spirals toward earlier types by redistributing and gas inward. There is no universal evolutionary timeline across the sequence; instead, late-type spirals and irregulars can transition to early types through quenching mechanisms that halt , such as feedback or environmental stripping, effectively moving galaxies from the blue cloud to the red sequence. Along the sequence, properties like declining gas content correlate with these transitions, supporting a framework where morphology encodes assembly history rather than age alone. Key evidence for this comes from cosmological hydrodynamical simulations like the , which demonstrate that galaxy morphologies emerge from the interplay between spin, accretion rates, and baryonic physics, with high-spin halos favoring disk-dominated late types and merger-driven loss producing early-type ellipticals. Observations of transforming galaxies in the "green valley"—the color-magnitude region between star-forming blue spirals and quiescent red ellipticals—further illustrate these pathways, revealing morphological changes such as disk fading and bulge growth during phases. In the broader context of hierarchical merging, irregular galaxies serve as gas-rich progenitors that accrete onto halos to form the building blocks of spirals, with subsequent minor mergers preserving disks and major mergers transforming them into ellipticals, thereby populating the sequence from late to early types over cosmic time. Data from the (JWST) in the early 2020s have reinforced this by uncovering massive quiescent galaxies at redshifts z > 4—analogous to early ellipticals—existing just 1-2 billion years after the , indicating rapid and merger-driven evolution far earlier than previously anticipated.

Limitations and Modern Perspectives

Shortcomings of the Classification

One major shortcoming of the Hubble sequence arises from projection effects, where the apparent ellipticity of a depends heavily on its relative to the observer. This leads to potential misclassifications, particularly of disk-dominated systems appearing more elliptical when viewed edge-on, as the classification relies on projected shapes rather than intrinsic three-dimensional structures. For instance, early-type galaxies (S0s) with subtle disks are frequently mistaken for ellipticals in photographic surveys due to these orientation-dependent features. The system also provides incomplete coverage of certain morphological features, failing to systematically account for structures such as rings, polar rings, or tails that are common in interacting or evolving galaxies. Nuclear, inner, and outer rings, which appear in approximately 20% of disk galaxies, are not integrated into the core sequence and often require notations. Similarly, polar rings—orthogonal structures likely formed through accretion or mergers—and extended tails from interactions are recognized but relegated to the irregular category without subtype distinctions, overlooking differences between dwarf irregulars and merger remnants. The Hubble sequence is not inherently evolutionary, lacking any temporal progression among types, yet its diagram has historically been misinterpreted as implying a linear development from ellipticals to spirals. types were instead set by initial formation conditions, such as , with no significant evolution along the sequence occurring post-formation. This non-evolutionary framework introduces biases toward bright, nearby galaxies, as early catalogs suffered from and underrepresented low-surface-brightness systems, skewing the perceived distribution of types. Observational biases further limit the classification's reliability, particularly from pre-Hubble Space Telescope (HST) ground-based surveys, which had insufficient and to detect fine details like weak bars or subtle disk features. Barred spirals, for example, were underrepresented in early counts due to subjective classifications and low signal-to-noise ratios in optical images, with robust detection only becoming feasible through HST's higher- observations up to magnitudes around I_{814W} = 23.2. Quantitatively, the sequence struggles with luminosity evolution and leaves approximately a few percent of galaxies unclassifiable, often peculiar or interacting systems that do not fit neatly into elliptical, lenticular, spiral, or irregular bins. This catch-all approach for irregulars ignores nuanced subtypes, reducing the system's utility for comprehensive morphological analysis.

Extensions and Alternative Systems

One significant extension to the Hubble sequence is the de Vaucouleurs system, introduced in 1959, which expands the original two-dimensional framework into a three-dimensional classification to better accommodate the full range of galaxy structures. This system incorporates additional parameters for bar strength, such as SA for unbarred, SB for strongly barred, and SAB for intermediate bars, while also accounting for inner and outer structures like rings (denoted by 'r') and lenses (denoted by 'l'). A complete notation might read (R)SA(rs)ab, where parentheses indicate optional features, 'R' specifies an outer ring, 'rs' denotes a ring-like structure, and 'ab' represents the inner spiral stage. Alternative systems have addressed specific gaps in the Hubble sequence, particularly for and . The Vorontsov-Velyaminov classification, proposed in 1959, focuses on peculiar and interacting systems, categorizing into types such as amorphous, spiral-like, and fragmented forms to capture merger-induced distortions not emphasized in the Hubble scheme. Multi-wavelength approaches further refine by revealing hidden features obscured in optical light; for instance, observations uncover dust lanes and extended disks, while radio imaging maps neutral hydrogen distributions to identify asymmetric gas structures in spirals and . In the post-2010 era, techniques have enabled automated classifications, with projects like Galaxy Zoo using neural networks trained on labels to reproduce morphological types, achieving accuracies over 90% for distinguishing ellipticals, spirals, and mergers across large surveys. Modern integrations combine morphological with kinematic data to provide deeper insights into dynamics. For elliptical galaxies, a previously proposed fast/slow rotator classifies systems based on the of rotational to velocity dispersion (v/σ), where fast rotators (typically lower-mass, disk-like ellipticals) dominate at luminosities fainter than M_B ≈ -20.5, while slow rotators (more massive, triaxial systems) prevail among brighter examples, refining the early-type sequence. However, a 2025 analysis of 1895 ellipticals from the SDSS-IV MaNGA survey found no evidence for such a bimodality, suggesting properties form a continuum rather than distinct classes. Observations from the (JWST), as of 2025, have illuminated morphological evolution at high redshifts (z > 3), showing that early galaxies exhibit more clumpy, irregular structures that transition to smoother disks by z ≈ 2, challenging the universality of the local Hubble sequence. Advancements in catalogs and quantitative metrics have further enhanced these systems. The 2015 catalog by Buta et al. provides revised de Vaucouleurs classifications for over 2,300 galaxies in the Spitzer Survey of Stellar Structure in Galaxies, incorporating mid-infrared to update stages and features for improved consistency across ~8% of the local volume. Quantitative parameters, such as the applied to near-infrared images, measure light concentration and asymmetry (with values ranging from ~0.4 for smooth ellipticals to ~0.6 for asymmetric irregulars), offering objective proxies for morphological types without relying solely on .

References

  1. [1]
    [PDF] EXTRA-GALACTIC NEBULAE1 By EDWIN HUBBLE ABSTRACT
    EXTRA-GALACTIC NEBULAE1. By EDWIN HUBBLE. ABSTRACT. This contribution gives the results of a statistical investigation of 400 extra-.
  2. [2]
    The Hubble Tuning Fork – Classification of Galaxies - NASA Science
    Oct 6, 1999 · The diagram is roughly divided into two parts: elliptical galaxies (ellipticals) and spiral galaxies (spirals).Missing: authoritative | Show results with:authoritative
  3. [3]
    Galaxy Classification - Las Cumbres Observatory
    Hubble's original classification of galaxy types was published in 1936 in a book called "The Realm of the Nebulae". Since then several people have suggested ...Missing: sequence authoritative sources
  4. [4]
    [PDF] Classification & Stellar Content of Galaxies - A. Sandage
    Soon after publication of Hubble's 1926 paper, Reynolds (1927a, b) criticized the system because of a supposed inadequate number of classification bins.Missing: Edwin | Show results with:Edwin
  5. [5]
    The Realm Of The Nebulae : Hubble,edwin. - Internet Archive
    Jan 24, 2017 · The Realm Of The Nebulae ; Publication date: 1936 ; Topics: Osmania ; Collection: digitallibraryindia; JaiGyan ; Language: English ; Item Size: 599.3 ...
  6. [6]
    The Hubble tuning fork - classification of galaxies
    The diagram is roughly divided into two parts: elliptical galaxies (ellipticals) and spiral galaxies (spirals).Missing: authoritative | Show results with:authoritative
  7. [7]
    Hubble Views the Star that Changed the Universe - NASA Science
    May 23, 2011 · Using all the Cepheids, he obtained a distance of 900,000 light-years. Improved measurements now place Andromeda at 2 million light-years away.
  8. [8]
    Observations of Galaxy Structure and Dynamics - John Kormendy
    Hubble's (1936) original tuning-fork diagram recognized the fundamental distinction between elliptical galaxies, which do not contain disks, and spiral ...<|control11|><|separator|>
  9. [9]
    [PDF] The Hubble Atlas of Galaxies - UT Computer Science
    A system for the classification of galaxies was presented by Hubble to the. I. A. U. in 1925 and was later published in the Astrophysical Journal, 64, 321,.<|control11|><|separator|>
  10. [10]
    The Hubble Atlas of Galaxies - ADS - Astrophysics Data System
    The Hubble Atlas of Galaxies. Sandage, Allan. Abstract. Publication: Washington: Carnegie Institution. Pub Date: 1961 ... GALAXIES;; STARS: CLUSTERS;; STARS ...
  11. [11]
    historical overview - Galaxy Morphology - Ronald J. Buta
    Max Wolf (1863-1932) published a simple system of letters to describe 17 different types of non-galactic nebulae, ranging from amorphous inclined types to ...
  12. [12]
    [PDF] Galaxy morphology and basic properties - Haus der Astronomie
    Diagram after Edwin Hubble, The Realm of the Nebulae, 1935, using images from the Palomar Observatory Sky Survey with individual images credit: Caltech/Palomar.
  13. [13]
    Hubble's Tuning Fork and Galaxy Classification | ASTRO 801
    Edwin Hubble classified galaxies using the types that you see illustrated on the two tuning fork images; the one above and the SINGS image. His main types are: ...Try This With Starry Night! · Spirals · Ellipticals<|control11|><|separator|>
  14. [14]
    Fig. 1.3. The Great Galaxy in Andromeda, a spiral galaxy of type Sb ...
    The sequence is in decreasing order of the importance of the central nucleus or bulge in relation to the surrounding disc. Our galaxy and M31 are of type Sb.Missing: tuning fork diagram
  15. [15]
    Hubble Tuning Fork - SDSS SkyServer DR12
    You can see why this diagram is called the Hubble tuning fork. Hubble believed that galaxies started at the left end of the diagram and evolved to the right. He ...
  16. [16]
    Classification & Stellar Content of Galaxies - A. Sandage
    The 1936 Hubble system is illustrated in the well-known tuning-fork diagram shown in Figure 2, taken from The Realm of the Nebulae. A more complicated diagram ...
  17. [17]
    Episodes in the Development of the Hubble Galaxy Classification
    ... Hubble's famous tuning fork galaxy classification diagram. Over the years he ... 1961, The Hubble Atlas of Galaxies (Washington: Carnegie Institution of ...
  18. [18]
    Galaxy Types - NASA Science
    Oct 22, 2024 · Elliptical galaxies have shapes that range from completely round to oval. They are less common than spiral galaxies.Missing: prevalence distribution Vaucouleurs law
  19. [19]
    https://ned.ipac.caltech.edu/level5/March13/Blanton/Blanton5.html
  20. [20]
    Early-type galaxy speciation: elliptical (E) and ellicular (ES) galaxies ...
    This study delves deeper into the distinction between the ellicular galaxies – characterised by their intermediate-scale discs – and elliptical galaxies.
  21. [21]
    Elliptical Galaxy - ESA/Hubble
    Elliptical galaxies have an even, ellipsoidal shape. They typically contain a much greater proportion of older stars than spiral galaxies do.
  22. [22]
    Observations of Galaxy Structure and Dynamics - John Kormendy
    The best-known example is NGC 6166, which has four nuclei very close to each other in projection deep inside the light distribution (Minkowski 1961). Another ...
  23. [23]
    Galaxies, Properties in Relation to Environment
    In the typical environment (that of loose groups), elliptical and SO galaxies account for only 10-20% of the population. Hubble and Humason, and later (1936) ...
  24. [24]
    [PDF] THE HUBBLE SEQUENCE - EuroVO for education
    M31 is a giant spiral galaxy, similar to the. Milky Way, located at 2.3 million light- years from us. The Magellanic Clouds,. Andromeda, the Milky Way and other ...
  25. [25]
    Classification and Morphology of External Galaxies
    ... weak bar are noted SAB0. The symbol S0, used for SA0 in Hubble's notation, is now used for a lenticular object which cannot be more precisely classified as ...
  26. [26]
    The Hubble Sequence Throughout the Universe's History
    Aug 15, 2013 · This image shows "slices" of the universe at different times throughout its history (present day, and at 4 billion and 11 billion years ago).
  27. [27]
    Galaxy NGC 3115 - NASA
    Jul 27, 2011 · NGC 3115 is located about 32 million light years from Earth and is classified as a so-called lenticular galaxy because it contains a disk and a central bulge ...
  28. [28]
    Full ACS Image of NGC 5866 - NASA Science
    NGC 5866 is an edge-on galaxy that is tilted to our line-of-sight. It is classified as an S0 lenticular, due to its flat stellar disk and large ellipsoidal ...Missing: source | Show results with:source
  29. [29]
    Galaxy Zoo: dust and molecular gas in early-type ... - Oxford Academic
    Two-thirds of these 'dusty ETGs' (D-ETGs) are morphologically disturbed, which suggests a merger origin, making these galaxies ideal test beds for studying the ...
  30. [30]
    Galaxy mergers shed light on galactic evolution model
    Jul 24, 2023 · The dust-rich lenticular galaxies are built from mergers of spiral galaxies. Spiral galaxies can have a small central spheroid plus a disc ...
  31. [31]
    [PDF] Galaxy Morphology1 - Professor Ronald J. Buta
    Sandage (1975) describes the earlier classification systems of Wolf, Reynolds, Lundmark, and Shapley that fell into dis-use more than 50 years ago.
  32. [32]
    Hubble Spies a Spiral Snowflake - NASA Science
    May 13, 2016 · Together with irregular galaxies, spiral galaxies make up approximately 60 percent of the galaxies in the local universe.
  33. [33]
    [1004.0684] On the Fraction of Barred Spiral Galaxies - arXiv
    Apr 5, 2010 · The fraction of barred spiral galaxies is found to be a strong function of stellar mass and star formation history, with a minimum near the ...
  34. [34]
    The role of spiral arms and bars in driving central molecular gas ...
    Observations supporting bar-driven gas inflow have shown that barred galaxies tend to have higher molecular gas concentrations than unbarred galaxies (Sakamoto ...
  35. [35]
    The Pinwheel Galaxy (M101) - The Virtual Astronomy Club
    M101 is an Sc-type galaxy, which means it has a bright central core and well-defined spiral arms emerging from it. As you can see from the image, the galaxy ...
  36. [36]
    Hubble Classification Scheme - an overview | ScienceDirect Topics
    The Hubble sequence is defined as a morphological classification system for galaxies, where irregular galaxies, such as the Magellanic Irregulars, are ...
  37. [37]
    galaxy types: stage, family, and variety
    Elliptical galaxies are smooth, amorphous systems with a continuously declining brightness distribution and no breaks, inflections, zones, or structures.Missing: subtypes prevalence
  38. [38]
    Star Formation Properties of a Large Sample of Irregular Galaxies
    Aug 12, 2004 · The integrated star formation rates of Im galaxies normalized to the physical size of the galaxy span a range of a factor of 10^4 with 10% Im ...
  39. [39]
    Star Formation in Irregular Galaxies
    Because of the high gas content and low abundances of heavy elements, irregulars are thought of as being less evolved than spirals in the sense that less of ...
  40. [40]
    Irregular galaxies
    As many as 25 percent of all galaxies cannot be categorised into either spiral or elliptical and are therefore called irregular galaxies.Missing: prevalence | Show results with:prevalence
  41. [41]
    Structure and Evolution of Irregular Galaxies
    Standard models of stellar populations can yield such blue colors for galaxies of normal cosmological age only with some difficulty which has fostered the idea ...Missing: observational challenges resolved
  42. [42]
    STAR FORMATION IN GALAXIES ALONG THE HUBBLE SEQUENCE
    ABSTRACT. Observations of star formation rates (SFRs) in galaxies provide vital clues to the physical nature of the Hubble sequence and are key probes of ...Missing: Msun/ | Show results with:Msun/
  43. [43]
    [PDF] Lecture 14-15 Spiral Galaxies chapter 5 of S+G - UMD Astronomy
    'Later' types, lower mass, more of baryons in gas, higher specific star formation rates (today):. • Sa -> Sb -> Sc -> Sd in order of decreasing bulge size.
  44. [44]
    The rate and efficiency of high-mass star formation along the Hubble ...
    The rate and efficiency of high-mass star formation along the Hubble sequence Data obtained with IRAS are used to compare and contrast the global star formation ...
  45. [45]
    Physical Parameters Along the Hubble Sequence - ResearchGate
    Aug 10, 2025 · For instance, the morphology of a galaxy has been found to correlate with the gas content (Roberts & Haynes 1994) , star formation rate (Wuyts ...
  46. [46]
    Why does the environmental influence on group and cluster ...
    One well-known example is the morphology–density relation (Dressler 1980) by which early-type galaxies are more common in high-density environments such as the ...
  47. [47]
    The luminosity–metallicity relation in the local Universe from the 2dF ...
    The correlation between galaxy metallicity and luminosity in the local Universe is one of the most significant observational results in galaxy chemical ...
  48. [48]
    Galaxy morphology, luminosity, and environment in the SDSS DR7
    We use the global luminosity density field to define different environments, and analyse the environmental dependence of galaxy morphology and colour. The ...
  49. [49]
    Hubble's Classification of Non-Galactic Nebulae, 1922-1926
    The idea of an evolutionary sequence was obviously on Hubble's mind since he stated that “the observer may well look to Jeans' theory for the thread of ...
  50. [50]
    Secular Evolution of Spiral Galaxies. III. The Hubble Sequence as a ...
    We seek to establish that the Hubble sequence for the classification of galaxies, when viewed in the reverse direction, is essentially a temporal evolution ...Missing: implications original
  51. [51]
    Testing the modern merger hypothesis via the assembly of massive ...
    The modern merger hypothesis offers a method of forming a new elliptical galaxy through merging two equal-mass, gas-rich disc galaxies fuelling a nuclear ...<|separator|>
  52. [52]
    The quenching of galaxies, bulges, and disks since cosmic noon
    We present an analysis of the quenching of star formation in galaxies, bulges, and disks throughout the bulk of cosmic history, from z = 2 − 0.
  53. [53]
    Galaxy morphology & star formation in Illustris Simulation at z=0
    We study how optical galaxy morphology depends on mass and star formation rate (SFR) in the Illustris Simulation.
  54. [54]
    Morphological transformation of galaxies across the green valley
    Jan 12, 2018 · We explore constraints on the joint photometric and morphological evolution of typical low redshift galaxies as they move from the blue cloud through the green ...
  55. [55]
    Evolution of the Hubble Sequence in Hierarchical Models for Galaxy ...
    Feb 17, 1996 · Abstract: We present a model for the broad morphological distinction between the disk and spheroidal components of galaxies.Missing: irregular | Show results with:irregular
  56. [56]
    A massive quiescent galaxy at redshift 4.658 - Nature
    May 22, 2023 · Here we report the spectroscopic confirmation of a massive quiescent galaxy, GS-9209, at redshift, z = 4.658, just 1.25 billion years after the Big Bang.Missing: discoveries elliptical
  57. [57]
    None
    Nothing is retrieved...<|separator|>
  58. [58]
    [PDF] The evolution of barred spiral galaxies in the Hubble Deep Fields ...
    However, an additional 30 per cent of spirals are classed as weakly barred in the RC3, substantially higher than in the UGC or RSA. Quite apart from taxonomical ...
  59. [59]
    De Vaucouleurs' system (Chapter 3) - Galaxy Morphology and ...
    In an attempt to accommodate the entire range of morphological characteristics of galaxies de Vaucouleurs (1959a) introduced a three-dimensional classification ...
  60. [60]
    The Atlas and Catalogue of Interacting Galaxies
    The Atlas and Catalogue of Interacting Galaxies is by B.A. Vorontsov-Velyaminov, published in Moscow in 1959.Missing: classification irregular
  61. [61]
    Quantified H i morphology – I. Multi-wavelength analysis of the ...
    In this paper, we explore six morphological parameters calculated over the extent of the stellar (optical) disc and the extent of the gas disc for a range of ...
  62. [62]
    Galaxy Zoo: reproducing galaxy morphologies via machine learning*
    We present morphological classifications obtained using machine learning for objects in the Sloan Digital Sky Survey DR6 that have been classified by Galaxy Zoo ...
  63. [63]
    Galaxy morphologies at cosmic noon with JWST
    This study establishes a population-wide framework for linking galaxy morphology and dynamics at cosmic noon, providing a key reference for future studies on ...
  64. [64]
    A CLASSICAL MORPHOLOGICAL ANALYSIS OF GALAXIES IN ...
    Apr 24, 2015 · A CLASSICAL MORPHOLOGICAL ANALYSIS OF GALAXIES IN THE SPITZER SURVEY OF STELLAR STRUCTURE IN GALAXIES (S4G). Ronald J. Buta, Kartik Sheth, E.
  65. [65]
    THE GINI COEFFICIENT AS A MORPHOLOGICAL MEASUREMENT ...
    We examine the use of the Gini coefficient as one such measurement. Because it depends on the cumulative distribution of the light of a galaxy, but not the ...