Galaxies are vast, gravitationally bound systems composed of stars, gas, dust, and dark matter. The universe is populated by galaxies that differ fundamentally in shape, size, stellar content, energy output, and environment. Astronomers categorize this diversity to understand the processes that govern their formation and evolution over billions of years. By comparing the traits of different galaxies, scientists gain insights into the physical mechanisms that shape the cosmos.
Classification by Shape and Structure
Galaxies are primarily classified by their shape, a system formalized by Edwin Hubble in his “tuning fork” diagram. This morphological system divides galaxies into three main classes: ellipticals, spirals, and irregulars. A galaxy’s shape reflects its internal dynamics and evolutionary history.
Elliptical galaxies have a smooth, featureless appearance, resembling spheres or elongated shapes. These systems lack a rotating disk and spiral arms, as their stars move on complex, random orbits, resulting in a three-dimensional structure. They are predominantly composed of older, reddish stars and contain very little cold gas or dust. Consequently, star formation has largely ceased in these galaxies.
Spiral galaxies, such as the Milky Way, have a distinctive flat, rotating disk structure with prominent spiral arms. These arms are rich in gas and dust, fueling the continuous creation of new, hot, blue stars. Spiral galaxies are sub-classified based on the size of their central bulge and how tightly their arms are wound. A major subtype is the barred spiral (SB), which features a luminous bar of stars cutting across the central bulge, from which the spiral arms emerge.
Galaxies that do not fit neatly into the elliptical or spiral categories are classified as irregulars. These chaotic systems lack a defined, symmetric structure, often resulting from gravitational disruption or close encounters with other galaxies. Irregular galaxies frequently contain large amounts of gas and dust, exhibiting high rates of star formation. This makes them appear blue due to the presence of many young stars.
Differences in Size, Mass, and Stellar Content
Beyond morphology, galaxies differ significantly in size and stellar populations. Giant galaxies, such as the largest ellipticals, can contain a trillion stars and span hundreds of thousands of light-years across. Conversely, dwarf galaxies are the most numerous type in the universe. They may contain only a few million to a few billion stars and measure just a few thousand light-years in diameter.
The total mass of a galaxy is dominated by dark matter, which astronomers estimate using the galaxy’s rotation rate. Dwarf galaxies often have a higher ratio of dark matter to visible matter compared to larger galaxies. The age and chemical composition of the stars within a galaxy also provide significant differentiation.
Astronomers classify stars into two broad groups based on their “metallicity,” referring to the abundance of elements heavier than hydrogen and helium. Population II stars are the oldest stars, formed when the universe was young and contained very few heavy elements. These metal-poor stars are typically found in the halos of spirals and within elliptical galaxies.
Population I stars are younger, metal-rich stars, like our Sun, that formed from gas clouds enriched by previous supernova explosions. These stars are concentrated in the thin disks and spiral arms of late-type galaxies, where star formation continues. The varying ratio of cold gas and dust determines a galaxy’s ability to form new stars, influencing its current color and future evolution.
Activity Level and Internal Energy
Galaxies are differentiated by their internal activity, defined by energy output not originating from the collective light of stars. Many galaxies are quiescent, meaning they have low rates of star formation and a stable central region. However, a significant fraction are active, displaying high luminosity from their cores.
These active galaxies host an Active Galactic Nucleus (AGN), where a supermassive black hole at the center actively accretes matter. As gas spirals into the black hole, it heats up, releasing large amounts of non-stellar radiation across the electromagnetic spectrum. The resulting light can be so bright that it outshines the entire host galaxy, as seen in examples like quasars.
Starburst galaxies are undergoing a temporary, intense period of star formation at rates hundreds of times higher than a normal galaxy. For example, while the Milky Way forms stars at about three solar masses per year, a starburst galaxy can exceed 100 solar masses per year. This rapid creation of massive, hot stars leads to a high output of ultraviolet light. This light is often absorbed by surrounding dust and re-radiated as intense infrared light, making starburst galaxies some of the most luminous infrared objects in the universe.
The Role of Environment and Interaction
A galaxy’s characteristics are influenced by its environment and the gravitational interactions it experiences. Galaxies are separated into field galaxies, which exist in relative isolation, and cluster galaxies, which reside in massive groups containing thousands of members. The dense environment of galaxy clusters drives significant differences in galactic properties.
Galaxies in clusters are more likely to be ellipticals or lenticulars, while field galaxies are more often spirals or irregulars, a pattern known as the morphology-density relation. The dense intergalactic medium within a cluster can strip gas from a galaxy as it moves through the cluster, a process called ram pressure stripping. This removal of cold gas quickly halts star formation, transforming a star-forming spiral into a gas-poor, quiescent galaxy.
Gravitational forces from nearby galaxies or the cluster as a whole can also cause tidal stripping, pulling stars and dark matter away from a galaxy. Current or recent galactic mergers, which are more common in dense environments, alter a galaxy’s structure. Mergers often destroy spiral disks and create the smooth, featureless appearance of large elliptical galaxies. The history of these gravitational encounters is a major factor in determining a galaxy’s characteristics.