Galaxies are immense collections of stars, gas, dust, and dark matter, held together by gravity. These cosmic structures vary significantly in size, ranging from dwarf galaxies with a few million stars to giant galaxies boasting trillions. Astronomers classify these vast stellar systems primarily based on their visual appearance.
Spiral Galaxies
Spiral galaxies have flattened, rotating disks and spiral arms extending from a central bulge. These arms are regions of active star formation, appearing bright blue due to hot, young stars, gas, and dust. The central bulge is a dense concentration of older, redder stars, often spherical or football-shaped. A larger, invisible halo of dark matter envelops the entire structure.
Many spiral galaxies, including our Milky Way, feature a bar-shaped structure of stars across their central bulge. These are known as barred spiral galaxies, where the spiral arms emerge from the ends of the bar. Normal spiral galaxies, in contrast, have arms that originate directly from the central bulge without a bar. The Andromeda Galaxy (Messier 31) is an example of a normal spiral galaxy, while the Milky Way is a barred spiral.
Elliptical Galaxies
Elliptical galaxies have a smooth, featureless appearance, lacking spiral arms. Their shapes range from nearly spherical to highly elongated, like an American football. These galaxies vary greatly in size, from dwarf ellipticals with millions of stars to giant ellipticals containing trillions.
Their stellar populations are predominantly old, with little gas and dust for new star formation. This gives them a reddish or yellowish hue, due to cooler, older stars. Due to low star-forming activity, elliptical galaxies are often considered “red and dead” compared to active star-forming regions in spiral galaxies. Messier 87 (M87), a supergiant elliptical galaxy, exemplifies this type.
Irregular Galaxies
Irregular galaxies do not fit into the classifications of spirals or ellipticals, lacking a defined shape. These galaxies have a chaotic appearance, rich in gas and dust, fueling active star formation. This stellar birth can make them appear bright in certain regions.
Their irregular forms can arise from various dynamic processes. Some irregular galaxies were once spirals or ellipticals that have been distorted by powerful gravitational interactions, such as close encounters or mergers with other galaxies. These tidal forces can rip apart a galaxy’s organized structure, leading to its irregular shape. Other irregular galaxies are smaller, truly chaotic systems that have never formed a coherent structure. The Large Magellanic Cloud and Small Magellanic Cloud, satellite galaxies of the Milky Way, are prominent examples of irregular galaxies, showcasing their often dynamic and star-forming nature.