Galaxies are vast cosmic systems composed of stars, stellar remnants, interstellar gas, dust, and dark matter, all held together by gravity. They vary widely in form and size, from those with a few thousand stars to gargantuan ones spanning millions of light-years and containing trillions. This diversity naturally leads to questions about which type of galaxy typically achieves the grandest scale.
Understanding Galaxy Measurement
Astronomers employ a combination of techniques to determine a galaxy’s size. One common approach involves measuring its angular size in the sky and then calculating its physical dimensions based on its distance from Earth. Distance measurements rely on various methods, including observed redshift or “standard candles” like Cepheid variable stars or Type Ia supernovae.
The concept of “size” itself can refer to different metrics, such as the total number of stars, the overall mass (including unseen dark matter), or the physical diameter. Galaxies do not possess sharply defined edges, as their stellar populations gradually thin out into the surrounding space. Therefore, measuring their diameter often involves defining a boundary, such as the extent that encloses a significant portion of their total light or mass. For the purpose of identifying the largest galaxies, physical extent and total mass are often considered together.
Main Galaxy Forms
Galaxies are broadly categorized into three primary forms based on their visual appearance: spiral, elliptical, and irregular. Spiral galaxies, like our Milky Way, are characterized by a flattened disk with prominent spiral arms winding outward from a central bulge. These arms are regions of active star formation, rich in young stars, gas, and dust. Spiral galaxies typically range from 3,000 to 300,000 light-years in diameter.
Elliptical galaxies exhibit a smooth, oval, or spherical shape. They generally contain older, redder stars and very little interstellar gas or dust, indicating minimal ongoing star formation. These galaxies vary significantly in size, encompassing both the smallest dwarf galaxies and the largest known galaxies.
Irregular galaxies, as their name suggests, lack a distinct, organized structure. Their chaotic appearances often result from gravitational interactions or collisions with other galaxies, and they contain a mix of star types, gas, and dust. While many irregular galaxies are relatively small, some can be quite massive.
The Largest Galaxy Category
Elliptical galaxies generally hold the distinction of being the largest type of galaxy in the universe. Their immense sizes are a result of their formation and evolutionary processes, primarily through repeated mergers with other galaxies. When galaxies collide, their individual stars rarely hit each other, but the galaxies’ vast gas clouds and dark matter halos can merge, leading to a larger, more diffuse structure.
The stars within elliptical galaxies move on randomly oriented orbits, contributing to their diffuse, extended appearance. Elliptical galaxies typically have used up most of their gas and dust in early star formation or had it stripped away during mergers, resulting in a population dominated by older, lower-mass stars and very little new star birth. This allows their mass to be spread out over vast distances. Supergiant elliptical galaxies can span hundreds of thousands to millions of light-years in diameter and contain up to 10 trillion solar masses, dominating the centers of galaxy clusters.
Notable Giant Galaxies
Among the largest known galaxies, supergiant ellipticals frequently appear. One of the most colossal is IC 1101, a supergiant elliptical located at the heart of the Abell 2029 galaxy cluster. This immense galaxy has an estimated diameter of up to 5.5 million light-years, making it approximately 40 to 55 times larger than our Milky Way, which spans about 100,000 light-years. IC 1101 is believed to contain over 100 trillion stars.
Another prominent example is Messier 87 (M87), a supergiant elliptical galaxy in the nearby Virgo Cluster. While its main visible body has a diameter of about 132,000 light-years, its diffuse outer envelope extends significantly further, reaching up to 490,000 light-years. M87’s total mass, including its dark matter halo, is estimated to be around 2.4 to 2.7 trillion times the mass of the Sun, making it roughly 200 times more massive than the Milky Way. This galaxy has grown by absorbing smaller galaxies over cosmic time.