Galaxies are vast systems of stars, gas, dust, and dark matter, held together by gravity. They can contain hundreds of billions of stars, and the universe is home to an estimated 200 billion to 2 trillion galaxies. Typical galaxies span tens to hundreds of thousands of light-years across. Their diversity in size and composition leads to a wide range of appearances.
Understanding Galaxy Classification
To organize the vast array of galaxies, astronomers rely on classification systems based on their visual appearance. The most widely recognized system is the Hubble Sequence, often called the “Hubble tuning-fork diagram,” devised by Edwin Hubble in 1926. This scheme groups galaxies primarily by their morphology.
The Hubble Sequence categorizes regular galaxies into three main classes: ellipticals, lenticulars, and spirals. A fourth class includes galaxies with an irregular appearance. While the diagram arranges galaxies in a sequence, it does not imply an evolutionary path where one type necessarily transforms into another over time. Instead, it serves as a descriptive tool to characterize the different forms galaxies take.
Spiral Galaxies
Spiral galaxies are characterized by a flattened, rotating disk from which prominent spiral arms emerge. These arms typically wind outwards from a dense central region known as a galactic bulge. Surrounding this disk and bulge is a fainter, more spherical halo, containing older stars and star clusters.
The spiral arms are regions rich in gas and dust, providing material for active star formation, which gives them a bluer appearance due to young, hot stars. In contrast, the central bulge tends to harbor older, redder stars. Spiral galaxies are further divided into two main subtypes: normal spirals (S) and barred spirals (SB). Barred spirals feature a straight, bar-shaped structure of stars extending from the central bulge, with the spiral arms beginning at the ends of this bar. Our home galaxy, the Milky Way, is a barred spiral galaxy, as is the Andromeda Galaxy.
Elliptical Galaxies
Elliptical galaxies present a smooth, featureless appearance, ranging in shape from nearly spherical to highly elongated. Unlike spiral galaxies, they lack a distinct disk or spiral arms and typically contain very little interstellar gas and dust. This scarcity of star-forming material means elliptical galaxies have minimal ongoing star formation.
Their stellar populations are predominantly composed of older, redder stars, which orbit the galactic center in more random directions compared to the organized rotation seen in spirals. Elliptical galaxies exhibit a wide range in size, from small dwarf ellipticals containing millions of stars to massive giant ellipticals found at the centers of galaxy clusters, which can contain trillions of stars. They are believed to be the result of collisions and mergers between other galaxies, such as spirals.
Irregular Galaxies
Irregular galaxies do not fit neatly into regular classifications due to their chaotic shapes. They often lack a clear central bulge or discernible spiral arm structure, appearing as amorphous blobs of stars, gas, and dust. Their distorted forms are frequently a result of gravitational interactions or collisions with other galaxies, which can disrupt their original structures.
These galaxies are rich in gas and dust, supporting vigorous star formation, which gives them a bluer color compared to the arms and disks of spiral galaxies. Irregular galaxies are generally smaller than most spirals and ellipticals. Prominent examples include the Large and Small Magellanic Clouds, satellite galaxies of our Milky Way.
Other Galaxy Types
Beyond the three primary classifications, other galaxy types exist that represent transitional forms or unique phenomena. Lenticular galaxies (S0) are considered an intermediate type between ellipticals and spirals. They possess a central bulge and a flattened disk, similar to spirals, but lack distinct spiral arms and contain very little gas or dust, resembling ellipticals in their older stars and minimal star formation.
Peculiar galaxies have unusual features, shapes, or compositions that deviate from typical classifications. These often result from gravitational interactions, mergers, or an active galactic nucleus. Their unique morphologies can include distorted arms, tidal tails, or ring structures. Dwarf galaxies are small, faint galaxies, containing only a few billion stars, significantly fewer than their larger counterparts. They are the most abundant type of galaxy in the universe and can be found in various forms, including dwarf ellipticals, dwarf irregulars, and even rare dwarf spirals, often orbiting larger galaxies.