A galaxy is a massive system of stars, stellar remnants, interstellar gas, dust, and dark matter that is all held together by gravity. These cosmic islands vary widely in size, from dwarfs containing a few million stars to supergiants with trillions. Astronomers classify these structures primarily by their visual appearance, or morphology, a system often referred to as the Hubble Sequence. This classification, developed by Edwin Hubble in the 1920s, organizes galaxies into distinct types based on their shape and structural features.
Elliptical Galaxies
Elliptical galaxies are characterized by their smooth, featureless, three-dimensional shape. They are classified with the letter “E” followed by a number from 0 to 7. E0 represents an almost circular appearance, and E7 indicates a highly elongated, football-like shape. This classification number reflects the galaxy’s observed flattening, not necessarily its true physical shape in space.
The stellar population within elliptical galaxies is predominantly composed of older, redder stars, and they possess very little interstellar gas or dust. This lack of star-forming material means that new star birth is minimal. Elliptical galaxies are frequently found in dense galaxy clusters. Many are believed to form from the collision and merger of two or more spiral galaxies. During a galactic merger, the turbulent gravitational forces disrupt the ordered structure, causing the stars to settle into more random, ellipsoidal orbits.
Spiral Galaxies
Spiral galaxies are defined by a distinct, organized structure that includes a flat, rotating disk, a dense central bulge, and prominent spiral arms that wind outwards. The central bulge is a concentration of older stars at the galaxy’s core, while the disk contains younger stars, gas, and dust. These galaxies are sub-classified using the letters “Sa,” “Sb,” and “Sc.” This system distinguishes them based on how tightly wound their spiral arms are and the size of their central bulge.
Type Sa galaxies have tightly wound arms and a large, bright central bulge. Sc galaxies feature much looser, more open arms and a smaller bulge. The spiral arms themselves are regions where interstellar gas and dust are compressed, triggering active star formation. This ongoing star birth populates the arms with many young, hot, blue stars, which is why these regions appear brighter than the surrounding galactic disk.
Barred Spiral Galaxies
Barred spiral galaxies share the same fundamental components as regular spiral galaxies, including a flat disk, a central bulge, and spiral arms. They are distinguished by a straight, bar-shaped structure composed of stars that cuts across the center of the galaxy. The Milky Way galaxy is classified as one of these barred spirals, a type that accounts for approximately two-thirds of all observed spiral galaxies.
This stellar bar actively influences the galaxy’s internal dynamics by transporting material. The bar structure creates density waves that funnel gas and star-forming material from the disk inward toward the central region. This process can enhance star formation near the galaxy’s core and may also feed the supermassive black hole thought to reside at the very center.
Irregular Galaxies
Irregular galaxies are those that lack any distinct, symmetrical, or organized structure. They often appear chaotic, with no clear central bulge or defined spiral arm pattern. Astronomers believe that the odd shapes of these galaxies frequently result from gravitational interactions or collisions with nearby, larger galaxies.
Despite their disorderly appearance, irregular galaxies often contain significant amounts of gas and dust, and they exhibit vigorous star formation activity. The Small and Large Magellanic Clouds, which are satellite galaxies orbiting the Milky Way, are well-known examples. Many of these systems may have once been spiral or elliptical galaxies that were tidally distorted by the gravity of a more massive neighbor, a process that stripped away their original ordered structure.