Galaxies are immense cosmic structures, vast collections of stars, gas, dust, and dark matter held together by gravity. These systems vary dramatically in size, containing anywhere from millions to trillions of stars. The universe hosts billions of galaxies, which come in a wide array of shapes, offering clues about their formation and evolution.
Spiral Galaxies
Spiral galaxies feature a distinct central bulge, a flat rotating disk, and prominent arms that curve outward. These arms are often brighter than the surrounding disk because they are active regions of star formation, populated by young, hot, luminous stars. The central bulge, conversely, typically contains older stars.
There are two main types of spiral galaxies: unbarred and barred. In unbarred spirals, the arms emanate directly from the central bulge. In barred spirals, a bar-shaped structure of stars, gas, and dust extends through the galaxy’s center, and the spiral arms begin from the ends of this bar. Our own Milky Way galaxy is a barred spiral, as is the Andromeda galaxy, our closest large galactic neighbor.
Barred spirals are quite common, making up approximately two-thirds of all spiral galaxies observed. Spiral galaxies collectively account for 60% to 77% of the galaxies seen in the universe today.
Elliptical Galaxies
Elliptical galaxies present a smooth, oval-shaped appearance, ranging from a perfect sphere to a highly elongated football. Unlike spiral galaxies, they lack distinct spiral arms, a flat disk, and widespread active star formation. Their composition primarily consists of older, reddish stars, with very little interstellar gas and dust available for new star birth.
These galaxies exhibit a wide range of sizes, from small dwarf ellipticals containing millions of stars to massive supergiants with trillions of stars. Elliptical galaxies are more commonly found in dense galaxy clusters, suggesting that many may have formed from the mergers of other galaxies. The stars within elliptical galaxies tend to orbit the galactic center in more random directions, contributing to their overall smooth, featureless look.
Irregular Galaxies
Irregular galaxies lack a defined, regular structure like organized spirals or smooth ellipticals. They often appear chaotic, without a clear central bulge or discernible spiral arms, and can be relatively small in size. Many irregular galaxies likely acquired their disordered shapes due to gravitational interactions or collisions with other galaxies. Some may also be primordial, representing early galactic forms that have not yet settled into a more structured shape.
Despite their disorganized appearance, irregular galaxies are often sites of vigorous star formation. This activity can be triggered by the compression of gas during galactic interactions, or by other factors like stellar feedback. These galaxies are thought to comprise about a quarter of all known galaxies in the universe.
How Galaxies Are Classified
Astronomers classify galaxies primarily using the Hubble Sequence, often visualized as a “Hubble tuning fork diagram.” This classification scheme organizes galaxies based on their visual appearance. Elliptical galaxies are categorized by their degree of roundness, ranging from E0 for nearly spherical to E7 for highly elongated.
Spiral galaxies are placed on two prongs of the “tuning fork,” distinguished by the presence or absence of a central bar. They are further subclassified (Sa, Sb, Sc for unbarred; SBa, SBb, SBc for barred) based on the size of their central bulge and how tightly wound their spiral arms are. An intermediate class, called lenticular galaxies (S0), features a central bulge and a disk but lacks spiral arms. Irregular galaxies form a separate category for those that do not fit into the other defined shapes. Galaxy shapes are not static; they can evolve significantly over cosmic timescales, with mergers and gravitational interactions often transforming one type of galaxy into another.