Hubble’s Galaxy Classification: Spiral, Elliptical, Irregular
Linas JuozėnasShare
Knowledge Ark · Universe · Chapter 03 / Article 02
Hubble’s galaxy classification
Spirals, ellipticals, lenticulars, and irregulars: learning to read the shapes of galaxies—and the clues their appearances can leave out.
What do you notice first in a galaxy?
Perhaps two winding arms. Perhaps a bright, rounded center. Perhaps scattered blue knots that refuse to form a neat pattern. These first impressions are the beginning of galaxy morphology—the study of visible form.
The previous article explored the larger gravitational setting. Here, the focus moves to the luminous galaxy: the arrangement of stars, gas, and dust that we can image.
Hubble’s classification gives us a shared language for those appearances. Its real value comes from using that language alongside measurements of motion, stellar populations, and the supply of gas.
How do you read Hubble’s tuning fork?
Edwin Hubble published his influential photographic classification in 1926 and presented the familiar tuning-fork arrangement in The Realm of the Nebulae in 1936. Later astronomers refined it, including the description of lenticular galaxies and more detailed ways to record bars, rings, and spiral structure.[1], [3]
Read the left-hand sequence as ellipticals, the junction as lenticulars, and the two right-hand branches as unbarred and barred spirals. The arrangement makes similarities and differences easier to compare.
“Early” and “late” do not tell you a galaxy’s age.
These terms describe positions in the classification. Hubble explicitly separated the descriptive scheme from theories of evolution. An elliptical is not a young spiral waiting to unfold, and a galaxy does not have to travel along the fork.[2]
Elliptical or lenticular: is there a disk?
Elliptical galaxies
An elliptical has a smooth, rounded or elongated light distribution, without obvious spiral arms. Its label runs from E0, nearly round in projection, to E7, strongly elongated.
Many are dominated by older stellar populations and have relatively little ongoing star formation. They range widely in mass and structure; a giant elliptical is only one member of the family.[13]
Stellar motions can include substantial organized rotation. A smooth image alone cannot tell us how much rotation or disk-like structure is present.[4]
Lenticular galaxies
A lenticular galaxy has a disk, usually with a central bulge, but little prominent spiral structure. Its name comes from its lens-like appearance. Lenticulars can also contain bars or rings.
Many have relatively low star formation, which can make them resemble ellipticals in color. Looking for the disk is therefore more useful than judging by redness alone.
Some may be faded or gas-depleted spirals; others show evidence of mergers or different assembly histories. Observations support multiple routes to an S0 appearance.[6]
Smooth does not mean empty of gas. Molecular-gas surveys detect cold gas in a subset of elliptical and lenticular galaxies. Some also contain dust, small regions of new star formation, or extended hot gas. Broad tendencies should not be turned into definitions with no exceptions.[5]
What do the numbers E0 to E7 measure?
n ≈ 10 × (1 − b/a)
If b/a = 0.7, the approximate label is E3. The number describes projected shape, not size or age. An elongated three-dimensional galaxy viewed from a suitable direction can look round, so E0 does not establish that the galaxy is spherical.[1]
What distinguishes one spiral from another?
A spiral galaxy has a stellar disk with recognizable winding arms. Many also have a central bulge. Bright young stars and star-forming gas can make the arms conspicuous, although the disk contains a substantial older stellar population as well.
Along the familiar Sa–Sb–Sc sequence, three visual features tend to change together. They do not always agree perfectly, so classification involves judgment.[1], [18]
| Type | Central bulge | Spiral arms |
|---|---|---|
| Sa | Often relatively prominent | Typically tightly wound and comparatively smooth |
| Sb | Often intermediate in prominence | Moderately open, with more visible structure |
| Sc | Often less prominent relative to the disk | Typically more open and patchy with bright star-forming regions |
What does the B mean?
In SBa, SBb, or SBc, the B identifies a visible stellar bar crossing the central region. A bar is an elongated pattern of stellar orbits, distinct from the surrounding bulge.
Bars can redistribute angular momentum and help gas move inward, sometimes supporting central star formation. A visible bar does not by itself establish that the central black hole is actively accreting.[8]
Do all arms work the same way?
No. Some spirals have two clear, sweeping arms; others have many patchy segments. These appearances are often called grand-design and flocculent structure.
Arms can be associated with density waves, evolving patterns within a disk, bars, or the pull of a neighboring galaxy. Their presence does not require one universal mechanism.[7]
Why do some catalogs use SA, SAB, or extra letters?
Later extensions add more detail. SA usually denotes an unbarred system, SAB an intermediate or weakly barred one, and SB a clearly barred one. Other symbols can record rings and transitional features. These are refinements to the basic vocabulary rather than separate kinds of matter or fixed evolutionary stages.[3]
What makes a galaxy irregular?
An irregular galaxy lacks the regular appearance needed to fit comfortably into the main elliptical, lenticular, or spiral categories. It may show scattered star-forming regions, an off-center concentration of light, or an asymmetric outline.
That appearance does not mean its stars and gas have no organization. Radio measurements reveal ordered gas rotation in many dwarf galaxies whose visible light looks uneven.[9]
Interactions can distort a galaxy, while patchy star formation and stellar feedback can also shape its appearance. Many small irregulars form stars at a modest pace. A starburst is a particularly intense episode, not a requirement for belonging to the irregular family.[10]
“Irregular,” “interacting,” and “starbursting” therefore answer different questions: what the galaxy looks like, whether it is being disturbed by another system, and how intensely it is making stars.
Does shape tell us how many stars a galaxy is making?
Galaxy shape and star formation are related statistically. Nearby spiral disks account for much of the ongoing star formation, while many ellipticals and lenticulars are relatively quiet. Within each family, however, activity varies.[11]
Star formation rate, or SFR, describes how much gas becomes new stars per unit time, commonly in solar masses per year. Specific star formation rate, or sSFR, divides that rate by the galaxy’s existing stellar mass. It lets us compare recent activity in galaxies of very different sizes.
| Family | A common tendency | What the label does not guarantee |
|---|---|---|
| Elliptical | Older stellar populations; little recent star formation | Complete absence of cold gas or young stars |
| Lenticular | A disk with relatively low star-forming activity | One specific formation history |
| Spiral | Star formation within a gas-bearing disk | A high rate in every spiral, or the same rate across its disk |
| Irregular | Unevenly distributed star-forming regions | A starburst or a recent major merger |
Later spiral types often have higher star formation relative to stellar mass. That does not require them to make more stars in absolute terms than every larger, earlier-type galaxy.[11]
A small example: absolute versus relative activity
Consider two illustrative galaxies. Galaxy A contains 100 billion solar masses in stars and forms one solar mass of new stars per year. Galaxy B contains one billion solar masses in stars and forms 0.1 solar masses per year.
A makes more new stellar mass each year. B has a specific star formation rate ten times higher: its current activity is greater relative to the stellar mass already present. These numbers are an example, not observations of named galaxies.
Color is another clue with limits. Young, massive stars can make a galaxy look blue. Older populations often look redder, but dust can also redden an actively star-forming galaxy. Astronomers use spectra and several wavelength bands to separate those possibilities.[13]
How can a galaxy change its classification?
A galaxy’s structure can change as material is added, removed, or rearranged. Those changes need not happen at the same time as a rise or fall in star formation.
Rearrange and add material
Interactions can produce tails, disturb stellar orbits, and drive gas flows. Major mergers can build spheroidal systems, but the outcome depends on gas supply and encounter conditions. Disks can sometimes survive or form again.[12]
Change over many orbits
Bars and other disk patterns redistribute angular momentum and move material. Over time, they can build rings or central stellar concentrations without a major collision.[8]
There is consequently no single compulsory route from spiral to lenticular to elliptical—or in the opposite direction. Morphology records an outcome at the time of observation, while stellar ages, motions, and gas properties help reconstruct how it arose.
The next article on collisions and mergers examines encounters more closely. A later article compares secular and merger-driven evolution across a galaxy’s history.
What can the image hide from us?
A photograph is a particular view through particular filters, with finite depth and resolution. Before treating an apparent difference as a physical one, astronomers ask how those choices affect what can be seen.
-
01 · Viewing angle
A face-on disk exposes its arms; an edge-on view can hide them. A round-looking galaxy may be intrinsically elongated or contain a disk seen from above.
-
02 · Wavelength and dust
Ultraviolet light highlights short-lived young stars. Longer wavelengths can reveal a smoother underlying population, while dust obscures different regions by different amounts.
-
03 · Resolution
If a galaxy spans only a few resolution elements, its bar, arms, or separate star-forming regions may blend into one smooth-looking source.
-
04 · Image depth
Faint disks and tidal features can vanish in a shallow exposure. Deeper observations can change a classification by revealing structures that were there all along.
Studies therefore compare galaxies at similar effective resolution and emitted wavelengths, and test which features their images would recover. Deeper Galaxy Zoo DECaLS imaging, for example, exposed weak bars, arms, and tidal structures that were less apparent in earlier images.[14], [17]
A clumpy image is not a complete history.
At high redshift, light emitted in the visible part of the spectrum reaches us in the infrared. Webb observations have revealed disk-like structures in early galaxies that broaden the picture obtained from earlier observations.
These are classifications of the light distribution. Establishing how much a distant galaxy is supported by ordered rotation requires measurements of its motions. An uneven appearance alone cannot tell us that a galaxy recently merged or lacks an underlying disk.[15]
How do astronomers go beyond visual labels?
The tuning fork remains useful because it is quick to understand. Modern studies add measurements that can be applied consistently across large samples.
Sérsic index
This parameter describes the shape of a galaxy’s surface-brightness profile. It helps characterize structure, but no single value uniquely proves that a galaxy is a disk or an elliptical.[14]
Gini coefficient
Gini measures how unevenly the galaxy’s light is distributed among image pixels. A high value means a relatively small set of pixels carries a large share of the light.[16]
M20
M20 measures how the brightest 20% of the light is distributed relative to the galaxy’s center. It can help identify structures with several bright concentrations or off-center light.[16]
Asymmetry and clumpiness
These measurements describe uneven structure and small bright patches. They can flag useful candidates for closer study, although several physical processes can produce similar signals.[14]
Human classifications and machine learning can complement these measurements. Galaxy Zoo DECaLS combined volunteer judgments with trained models to describe large numbers of galaxies, while retaining information about uncertain classifications.[17]
The aim is to connect a visible pattern with a testable physical explanation. That becomes more reliable when images, spectra, stellar motions, and environmental information point toward a consistent story.
Start with the shape. Keep asking what lies behind it.
A disk, a bar, a smooth glow, or a scattered collection of bright regions gives us something concrete to describe. Hubble’s classification makes those descriptions comparable across galaxies.
Its greatest value comes from the questions it opens: where is the gas, how are the stars moving, what happened before, and what might change next?
Sources and further reading
Historical texts, observational studies, and research reviews. The diagrams are conceptual guides to morphology; their colors and relative sizes are illustrative.
- Hubble (1926) — Extra-galactic NebulaeThe original photographic classification, its notation, and Hubble’s explanation of its descriptive purpose.
- Baldry (2008) — What Hubble really meant by late and early typeHistorical clarification of the labels and the misconception that they prescribe galaxy evolution.
- Buta — Galaxy Morphology: historical overviewThe 1926 scheme, 1936 tuning fork, lenticular galaxies, and later refinements.
- Cappellari et al. (2011) — The ATLAS3D project VII: A new look at the morphology of nearby galaxiesWhy similar-looking early-type galaxies can have different stellar motions and disk structures.
- Young et al. (2011) — The ATLAS3D project IV: The molecular gas content of early-type galaxiesCold molecular gas in a subset of elliptical and lenticular galaxies.
- Deeley et al. (2020) — The SAMI Galaxy Survey: A Range in S0 Properties Indicating Multiple Formation PathwaysEvidence that lenticular galaxies develop through more than one route.
- Dobbs & Baba (2014) — Dawes Review 4: Spiral Structures in Disc GalaxiesSpiral patterns, their possible origins, and their relationship to gas and star formation.
- Kormendy & Kennicutt (2004) — Secular Evolution and the Formation of Pseudobulges in Disk GalaxiesBars, angular momentum transport, and gradual changes within galactic disks.
- Oh et al. (2015) — High-resolution mass models of dwarf galaxies from LITTLE THINGSOrdered gas rotation and dynamical measurements in dwarf galaxies.
- Lee et al. (2009) — Dwarf Galaxy Starburst Statistics in the Local VolumeWhy a strong starburst is a special episode rather than the normal state of every dwarf galaxy.
- González Delgado et al. (2016) — Star formation along the Hubble sequenceHow star formation and growth relative to stellar mass vary across nearby galaxy types.
- Hopkins et al. (2009) — How Do Disks Survive Mergers?The roles of gas fraction and encounter conditions in the survival or rebuilding of disks.
- Blanton & Moustakas (2009) — Physical properties and environments of nearby galaxiesGalaxy populations, colors, gas, star formation, and environmental trends.
- Conselice (2014) — The Evolution of Galaxy Structure over Cosmic TimeVisual and quantitative morphology, observational limits, and galaxy evolution.
- Ferreira et al. (2023) — The JWST Hubble Sequence: The Rest-Frame Optical Evolution of Galaxy Structure at 1.5 < z < 8Early Webb results on disk-like, spheroidal, and peculiar structures at high redshift.
- Lotz, Primack & Madau (2004) — A New Non-Parametric Approach to Galaxy Morphological ClassificationThe Gini coefficient, M20, and the information contained in the distribution of galaxy light.
- Walmsley et al. (2022) — Galaxy Zoo DECaLS: Detailed Visual Morphology Measurements from Volunteers and Deep Learning for 314,000 GalaxiesCombining human classifications and machine learning while revealing features in deeper images.
- NASA / ESA — The Hubble tuning fork: classification of galaxiesAn accessible visual introduction to the main branches of the classification.
All articles in this chapter
- Dark Matter Halos: Galactic Foundations
- Hubble’s Galaxy Classification: Spiral, Elliptical, Irregular — you are here
- Collisions and Mergers: Drivers of Galactic Growth
- Galaxy Clusters and Superclusters
- Spiral Arms and Barred Galaxies
- Elliptical Galaxies: Formation and Features
- Irregular Galaxies: Chaos and Starbursts
- Evolutionary Paths: Secular vs. Merger-Driven
- Active Galactic Nuclei and Quasars
- Galactic Futures: Milkomeda and Beyond