Galaxies are vast collections of stars, gas, dust, and dark matter, held together by gravity. These immense cosmic structures vary significantly in size, from dwarf galaxies containing millions of stars to giants with trillions. Astronomers categorize galaxies into distinct types to better understand their formation and evolution. This classification system helps to organize the diverse appearances observed across the universe.
Elliptical Galaxies
Elliptical galaxies are smooth, featureless, oval-to-round shapes. They range from nearly spherical to highly elongated. These galaxies contain older, redder stars, indicating their star formation occurred long ago.
With very little interstellar gas and dust, new star formation is minimal or absent. The stars within them orbit the galactic center in various random directions, contributing to their largely unstructured appearance. They vary greatly in size, from dwarf ellipticals smaller than a typical globular cluster, to giants spanning over a million light-years and containing trillions of stars. They are found in the central regions of galaxy clusters.
Spiral Galaxies
Spiral galaxies are characterized by their distinctive spiral arms that extend from a central bulge. These arms are regions rich in gas and dust, making them active sites where new stars are continuously forming. The newly formed stars in these arms are younger and bluer, contributing to the bright, luminous appearance of the spiral patterns.
The central bulge is a dense concentration of older stars. Spiral galaxies are further divided into two main subtypes: normal spirals and barred spirals. In normal spirals, the arms emerge directly from the central bulge, while barred spirals feature a prominent bar-shaped structure of stars across their center, with the arms beginning at the ends of this bar. Our own Milky Way galaxy is classified as a barred spiral galaxy.
Irregular Galaxies
Irregular galaxies lack a distinct shape, appearing chaotic. Unlike the smooth ellipses or structured spirals, they do not fit neatly into the typical galaxy classifications. Their unusual appearances result from gravitational interactions or collisions with other galaxies, which distort their original forms.
These galaxies contain abundant gas and dust. This rich supply fuels new star formation throughout their irregular structures. Examples include the Large and Small Magellanic Clouds, smaller galaxies orbiting our Milky Way.