A galaxy is a massive, gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and the enigmatic substance known as dark matter. These colossal structures vary dramatically in size, holding anywhere from ten million to one hundred trillion stars, and spanning diameters up to hundreds of thousands of light-years. The universe contains a staggering diversity of these stellar cities. To bring order to this variety, astronomers developed a framework for organizing galaxies based on their observable physical shape.
The Hubble Classification System
The most recognized framework for organizing galaxies was established in the 1920s by astronomer Edwin Hubble. This system categorizes galaxies based purely on their visible morphological appearance, simplifying the complex task of studying their differences. Hubble’s scheme, often visualized as a “tuning fork” diagram, provides a structure for grouping galaxies into distinct classes.
The diagram places the most featureless, smooth galaxies on one end, with two branches extending outward for the more structured, disk-shaped types. While the original intention was to suggest an evolutionary path, this interpretation is now understood to be incorrect; the system organizes galaxies by shape. Despite later refinements, this foundational classification remains the primary tool for astronomers to communicate and study galactic structures.
Elliptical Galaxies and Spiral Galaxies
Elliptical galaxies, designated with the letter ‘E’, possess a smooth, featureless, three-dimensional spheroid shape. They range from nearly spherical (E0) to highly flattened or cigar-shaped (E7). This numerical designation reflects the galaxy’s apparent degree of flatness as observed from Earth.
These galaxies are predominantly composed of older, redder stars. Ellipticals contain very little interstellar gas and dust, the raw materials for new star formation. Consequently, they exhibit almost no active star formation, with their stars moving in randomly oriented orbits rather than an organized rotation. Ellipticals span the greatest range in size, including small dwarf ellipticals and the largest known giant galaxies.
Spiral galaxies, denoted by the letter ‘S’, feature a characteristic flat, rotating disk, a central spherical bulge, and distinct spiral arms. The disk contains significant amounts of gas and dust, which fuel vigorous star formation, resulting in many young, hot, blue stars. The stars in the central bulge, however, tend to be older and yellower.
Spiral galaxies are sub-classified based on the size of their central bulge and the tightness of their spiral arms, ranging from Sa to Sc. Sa galaxies have a large, prominent central bulge and tightly wound, smooth spiral arms. Conversely, Sc galaxies feature a small central bulge and loosely wound, often clumpy spiral arms, containing a higher proportion of gas and dust that supports high rates of star birth. The intermediate Sb type falls between these two extremes.
Barred Spiral Galaxies and Irregular Galaxies
Barred spiral galaxies, classified as ‘SB’, are a common variation of the standard spiral type, making up about two-thirds of all spirals. They share the same disk and spiral arm structure as normal spirals, but are distinguished by a prominent, linear bar-shaped structure extending across the central bulge. The spiral arms emerge from the ends of this central bar, rather than directly from the center.
This stellar bar acts to channel gas inward toward the galaxy’s core, which can trigger intense star formation and fuel the central region. The Milky Way is a barred spiral galaxy, highlighting the prevalence of this structure. Like their unbarred counterparts, they are sub-classified from SBa to SBc, corresponding to the tightness of the arms and the size of the bulge.
Irregular galaxies, or ‘Irr’, do not fit into the organized categories of elliptical or spiral types, lacking a defined, regular structure. They possess neither a distinct central bulge nor a clear spiral arm pattern, often appearing chaotic. Irregulars are rich in gas and dust, which results in active, localized bursts of star formation.
The chaotic shape of these galaxies is often a result of gravitational forces from nearby, larger galaxies or recent galactic mergers. The Large and Small Magellanic Clouds, small companion galaxies to the Milky Way, are well-known examples. Though they represent a smaller percentage of observed galaxies today, irregular galaxies were much more common in the early universe.