Galaxies are vast collections of stars, gas, dust, and an invisible substance called dark matter, all held together by gravity. These cosmic structures vary greatly in size, from a few thousand stars to trillions. Billions of such galaxies exist in the universe.
Spiral Galaxies
Spiral galaxies are characterized by a flattened, rotating disk, a central bulge of stars, and prominent spiral arms that curve outwards from the center. This disk component contains significant amounts of gas and dust, which are the raw materials for new star formation. Younger, hotter, and brighter stars are found within these spiral arms, giving them a bluish appearance. The central bulge of a spiral galaxy consists of older, redder stars. Surrounding the main disk is a fainter, roughly spherical halo containing old stars and globular clusters. Our home galaxy, the Milky Way, is classified as a barred spiral galaxy, a subtype where a bar-shaped structure of stars extends from the central bulge before the arms begin. The Andromeda Galaxy, our nearest large galactic neighbor, is another example of a spiral galaxy.
Elliptical Galaxies
Elliptical galaxies exhibit smooth, oval shapes, ranging from nearly spherical to highly elongated. Unlike spiral galaxies, they lack distinct features such as spiral arms or a flattened disk. These galaxies are predominantly composed of older, low-mass stars, which contribute to their reddish color. Elliptical galaxies contain very little interstellar gas and dust, which explains their minimal star formation activity. Their stars orbit the galactic core in random directions, contributing to their smooth, featureless appearance. These galaxies show a wide range in size, from dwarf ellipticals with tens of millions of stars to supergiant ellipticals containing over a hundred trillion stars.
Irregular Galaxies
Irregular galaxies do not possess a defined, symmetrical shape like spirals or ellipticals. They lack a central bulge or clear spiral arm structure. These galaxies are rich in interstellar gas and dust, which fuels active star formation across their extent. The irregular shapes of these galaxies can result from various processes, including gravitational interactions or collisions with other galaxies. Some irregular galaxies may have once been spiral or elliptical galaxies that were deformed by these powerful gravitational forces. Notable examples include the Large and Small Magellanic Clouds, which are satellite galaxies orbiting our Milky Way.
Classifying Galaxies
Astronomers categorize galaxies primarily based on their visual appearance, a system first proposed by Edwin Hubble in 1926. This classification scheme, often conceptualized as the Hubble sequence or “tuning fork” diagram, divides galaxies into the broad categories of ellipticals, spirals, and irregulars. The system assigns specific designations based on characteristics like ellipticity for elliptical galaxies or the tightness of spiral arms for spiral galaxies. This classification helps scientists organize galaxies observed across the universe. By categorizing galaxies based on their shapes and structural properties, astronomers gain insights into their formation processes and how they evolve over cosmic timescales. The visual morphology provides clues about a galaxy’s history and its current state of star formation.