Galaxies

A galaxy is a gravitationally bound system of stars, interstellar gas, cosmic dust, stellar remnants, and dark matter, ranging from ultra-faint dwarf systems of a few hundred thousand stars to giant cD cluster galaxies containing several trillion solar masses of stellar matter. The observable universe contains hundreds of billions of individual galaxies distributed across an interconnected cosmic web of sheets, nodes, and filaments. Astronomers classify these systems by their optical morphology, a scheme that began with Edwin Hubble's work in the 1920s and 1930s and was later expanded into a three-dimensional classification volume by Gérard de Vaucouleurs. This hub explains the physical basis of galaxy classification and links every object in the CRV Science Deep Sky Galaxy Guide.

🔭 Galaxy Guide: Browse all 30 objects in the CRV Science Deep Sky Galaxy Guide, filter them by morphological class, and test your classification skills in the Deep Sky Classification game.

Until the early 1920s, astronomers debated whether the spiral "nebulae" cataloged by Charles Messier and William Herschel were gas clouds inside the Milky Way or separate "island universes" at extragalactic distances. During the 1920 Great Debate at the Smithsonian Museum of Natural History, Harlow Shapley argued that spiral nebulae belonged to the Milky Way, whereas Heber Curtis argued they were independent galaxies. In 1923 and 1924, Edwin Hubble used the 100-inch Hooker telescope at Mount Wilson Observatory to resolve individual Cepheid variable stars in the Andromeda Nebula (M31) and Triangulum Nebula (M33). By applying Henrietta Swan Leavitt's period-luminosity relation to measure their intrinsic luminosities, Hubble demonstrated that Andromeda lay hundreds of thousands of parsecs outside the Milky Way, establishing extragalactic astronomy.

Galaxies serve as the visible tracers of cosmic structure. On scales of megaparsecs, gravity assembles galaxies into bound groups of a few dozen members, rich clusters containing thousands of galaxies embedded in X-ray-emitting intracluster plasma, and superclusters spanning hundreds of millions of light-years. The Milky Way is a barred spiral (type SAB(rs)bc) roughly 30 kiloparsecs (100,000 light-years) in diameter with a total virial mass near 1.2 × 10¹² solar masses (M_☉), making it one of the two dominant galaxies in the Local Group alongside Andromeda. Most objects in the Galaxy Guide reside at distances between 0.05 and 150 megaparsecs, meaning their photons traveled across intergalactic space for millions of years before reaching Earth.

Hubble arranged galaxies along a "tuning fork" sequence published in The Realm of the Nebulae (1936), which Gérard de Vaucouleurs expanded in 1959 to incorporate ring structures, weak bars, and late-type Magellanic irregulars. The composite notation encodes structural components into a concise morphological symbol:

• E0 to E7: Elliptical galaxies, numbered by their apparent sky-projected ellipticity n = 10 × (1 − b/a), where a and b are the semi-major and semi-minor isophotal axes. An E0 galaxy appears circular (b/a = 1.0), whereas an E7 system reaches the maximum stable flattening (b/a = 0.3).
• S0 (Lenticular): Transitional disk galaxies possessing a central spheroidal bulge and an extended stellar disk, but lacking spiral density waves or appreciable cold molecular gas.
• SA, SAB, SB: Spiral families separated by bar strength: unbarred spirals (SA), intermediate or weakly barred spirals (SAB), and strongly barred spirals (SB).
• (r), (rs), and (s): Inner variety codes indicating whether spiral arms emerge tangentially from an inner resonance ring (r), directly from the nucleus or bar ends in an S-shaped sweep (s), or show an intermediate broken ring (rs).
• a, b, c, d, m: Stage letters along the spiral sequence tracking three correlated parameters: bulge-to-disk luminosity ratio (B/T), spiral arm pitch angle, and the resolution of H II star-forming knots. Stage a spirals have prominent bulges and tightly wound arms (pitch angles ~10°), whereas stage c and d spirals have small bulges and open arms (pitch angles 20°–35°).
• pec: Peculiar systems exhibiting tidal tails, bridges, warps, or polar rings caused by gravitational interactions or recent mergers.
• Irr I and Irr II (Im): Irregular galaxies lacking rotational symmetry; Magellanic irregulars (Sm and Im) retain faint bars and rudimentary single-arm spirals.

Morphology reflects dynamical and star-formation history. Spiral disks are rotationally supported systems (ratio of ordered rotational velocity to velocity dispersion v/σ > 1) enriched in neutral atomic hydrogen (H I) and cold molecular clouds (H₂), sustaining ongoing star formation that colors their disks blue. Elliptical galaxies are pressure-supported triaxial spheroids (v/σ < 1) sustained by anisotropic stellar velocity dispersions; depleted of cold gas, their spectra are dominated by long-lived red giant and K/M main-sequence stars. In dense galaxy clusters, ram-pressure stripping and gravitational harassment convert gas-rich spirals into gas-poor lenticulars and ellipticals, producing Alan Dressler's morphology-density relation.

Unbarred Spiral Galaxies (SA)

In unbarred spiral galaxies (Hubble-de Vaucouleurs family SA), logarithmic spiral arms wind directly outward from a central axisymmetric bulge without an intervening linear stellar bar. Quasi-stationary density waves propagate through the disk, compressing interstellar gas along the inner edges of the arms to ignite OB stellar associations and pink H II emission nebulae. This class includes a classic two-armed grand-design spiral interacting with a dwarf companion, the Whirlpool Galaxy (M51a), an asymmetric giant disk spanning 170,000 light-years, the Pinwheel Galaxy (M101), a nearly edge-on early-type spiral encircled by a prominent dust lane and a massive central bulge, the Sombrero Galaxy (M104), a symmetric face-on grand-design spiral mapped across mid-infrared and optical wavelengths by Webb and Hubble, the Phantom Galaxy (M74), and a multi-armed flocculent spiral whose sheared star-forming segments lack a single global density wave, the Sunflower Galaxy (M63).

Barred Spiral Galaxies (SB)

Approximately two-thirds of local spiral galaxies in near-infrared surveys contain a central stellar bar (SB and SAB families), formed when self-gravitating disk instabilities lock stellar orbits into elongated x₁ resonant trajectories. The rotating non-axisymmetric gravitational potential of the bar exerts gravitational torques on interstellar gas, stripping angular momentum and funneling molecular clouds inward along dark dust lanes toward circumnuclear starburst rings and central supermassive black holes. Representative examples in the guide include the archetypal grand-design barred system NGC 1300, the face-on multi-armed Southern Pinwheel Galaxy (M83), the Great Barred Spiral NGC 1365 hosting a variable Seyfert active nucleus in the Fornax Cluster, the nearby intermediate dust-rich starburst Sculptor Galaxy (NGC 253), and the theta-shaped barred spiral Messier 109 in Ursa Major.

Elliptical and Lenticular Galaxies (E / S0)

Elliptical (E) and lenticular (S0) galaxies—collectively termed early-type galaxies—are dominated by old, metal-rich stellar populations whose surface brightness profiles follow Sérsic laws with high concentration indices (n ≈ 2–8). Built through hierarchical major and minor galaxy mergers that scramble coherent disk rotation into three-dimensional random orbital velocity dispersions, these systems contain minimal cold star-forming gas but frequently retain massive halos of X-ray-emitting plasma at temperatures of 10⁷ K. The guide covers the Virgo Cluster cD giant Messier 87 (whose 6.5 × 10⁹ M_☉ black hole was imaged by the Event Horizon Telescope), the edge-on lenticular Spindle Galaxy (NGC 3115), the post-merger radio-loud lenticular Fornax A (NGC 1316), the nearest active radio galaxy Centaurus A (NGC 5128) bisected by a warped dust lane, and the giant elliptical Messier 60 paired with spiral neighbor NGC 4647.

Ring and Irregular Galaxies

Ring and irregular systems depart from the standard Hubble spiral-elliptical dichotomy through either resonant orbital dynamics, head-on bullseye collisions, or low-mass gravitational potentials where turbulent supernova feedback prevents thin-disk settling. Collisional ring galaxies form when a compact intruder plunges perpendicularly through the center of a gas-rich disk, launching an outward-propagating radial density wave that triggers a circular ring of blue supergiants. The guide highlights the detached polar-like ring system Hoag's Object, the collisional bullseye systems Cartwheel Galaxy and Lindsay-Shapley Ring (AM 0644-741), the nearby M81-group irregular starburst Cigar Galaxy (M82) driving a biconical superwind of ionized hydrogen, and the Milky Way's prototype Magellanic satellite, the Large Magellanic Cloud (LMC).

Interacting and Merging Galaxies

In hierarchical ΛCDM cosmology, galaxies grow through gravitational encounters and mergers with neighboring halos. When two disk galaxies pass within a few tens of kiloparsecs at relative velocities comparable to their internal circular velocities (200–400 km/s), mutual tidal forces strip outer stars and neutral hydrogen into extended bridges and curving tidal tails while driving strong bar instabilities that funnel inner gas into nuclear starbursts. This class includes the colliding Antennae Galaxies (NGC 4038/4039) producing thousands of young super-star clusters, the long-tailed Mice Galaxies (NGC 4676) captured shortly after first pericenter passage, the rose-shaped unequal-mass pair Arp 273 (UGC 1810/1813), the compact galaxy group Stephan's Quintet featuring a 1,000 km/s intergalactic shock front, and the grazing ocular spiral pair NGC 2207 and IC 2163.

Nebulae and Supernova Remnants

Although historical catalogs grouped all fuzzy telescopic patches together as "nebulae," true nebulae and supernova remnants are interstellar clouds of ionized, neutral, or shocked gas located inside our own Milky Way galaxy, typically hundreds to thousands of light-years away rather than millions. They are included in the Galaxy Guide because they represent the other major class of deep-sky observational targets and illustrate the stellar life cycle that powers galactic evolution. The guide contrasts core-collapse and thermonuclear supernova remnants—the pulsar-wind Crab Nebula (M1) and oxygen-rich shell Cassiopeia A—against asymptotic-giant-branch planetary nebulae Shed by dying low-mass stars, the Ring Nebula (M57) and multipolar Cat's Eye Nebula (NGC 6543), alongside the nearest high-mass star-forming H II region, the Orion Nebula (M42).

Galaxies evolve continuously across cosmic time through secular internal torques, environmental stripping, and major mergers. Numerical N-body and hydrodynamic simulations demonstrate that major mergers between gas-rich spirals—such as the ongoing collision in the Antennae and the future merger between the Milky Way and Andromeda in ~4.5 billion years—destroy thin stellar disks through violent relaxation, funnel cold gas into central ultra-luminous infrared starbursts, and leave behind a relaxed elliptical remnant.

At the dynamical centers of nearly all bulge-dominated galaxies sit supermassive black holes with masses ranging from 10⁶ to 10¹⁰ M_☉, scaling tightly with the host bulge velocity dispersion (the M_BH–σ relation). When cold gas or hot halo plasma accretes onto these black holes, the resulting Active Galactic Nucleus (AGN) radiates across the electromagnetic spectrum and can launch relativistic magnetohydrodynamic jets spanning hundreds of kiloparsecs, as observed in Messier 87 and Centaurus A. Combined with supernovae and stellar winds driven by solar radiation and OB stars, AGN feedback heats and expels circumgalactic gas, quenching star formation and regulating the stellar mass growth of galaxies as studied across extragalactic astrophysics and planetary science.

Why is Hubble's tuning fork diagram not an evolutionary sequence?

When Edwin Hubble introduced the tuning fork diagram, he borrowed the terms "early-type" (for ellipticals and tightly wound Sa spirals) and "late-type" (for loosely wound Sc spirals and irregulars) from earlier stellar evolution models, strictly as labels for morphological complexity from left to right. Modern observations and cosmological simulations show that galaxies evolve in the opposite direction: gas-rich disk and irregular galaxies form early in the universe, whereas massive gas-poor elliptical galaxies assemble later through repeated gravitational mergers of spiral progenitors and active galactic nucleus quenching.

How do spiral galaxies maintain their arms without winding up tightly?

Because galactic disks rotate differentially—with inner stars completing an orbit in far less time than outer stars—material arms composed of a fixed group of stars would wind up into a tight, indistinguishable spiral within a few hundred million years (the "winding dilemma"). As formulated by C. C. Lin and Frank Shu in 1964, grand-design spiral arms are quasi-stationary density waves: compression zones that rotate at a constant pattern speed Ω_p. Stars and interstellar gas clouds overtake the wave, compress to trigger short-lived blue star formation within the arm, and exit on the downstream side.

What is the difference between a galaxy and a planetary or emission nebula?

A galaxy is an independent, gravitationally bound island system containing millions to trillions of stars, dark matter, and interstellar gas located hundreds of thousands to billions of light-years beyond the Milky Way. Conversely, planetary nebulae (such as the Ring Nebula), emission nebulae (such as the Orion Nebula), and supernova remnants (such as the Crab Nebula) are localized clouds of gas and dust measuring 1 to 100 light-years across that reside inside the Milky Way, illuminated or shocked by one or several stars.

1. Hubble, E. P. (1926). "Extragalactic nebulae." The Astrophysical Journal, 64, 321–369. doi:10.1086/143018
2. de Vaucouleurs, G. (1959). "Classification and Morphology of External Galaxies." Handbuch der Physik, 53, 275–310. doi:10.1007/978-3-642-45932-0_7
3. Dressler, A. (1980). "Galaxy morphology in rich clusters: implications for the formation and evolution of galaxies." The Astrophysical Journal, 236, 351–365. doi:10.1086/157753
4. Kormendy, J., & Ho, L. C. (2013). "Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies." Annual Review of Astronomy and Astrophysics, 51(1), 511–653. doi:10.1146/annurev-astro-082708-101811