Galaxies are immense cosmic structures, each containing billions to trillions of stars, along with vast clouds of gas, dust, and an enveloping halo of dark matter. These colossal systems come in a wide range of shapes and sizes, from small dwarf galaxies to spirals like the Milky Way. The sheer scale of these stellar islands leads to the question of which one holds the record for size. Determining the “biggest” galaxy requires looking into the dense environments where the largest structures in the universe reside.
The Largest Known Galaxy
The reigning champion for physical extent is IC 1101, a supergiant elliptical galaxy located approximately one billion light-years away in the constellation Virgo. It is classified as a cD supergiant elliptical galaxy and is the brightest cluster galaxy (BCG) in the Abell 2029 galaxy cluster. Its measured dimensions are staggering, with its visible stellar component stretching across a diameter that ranges from 4 million to over 6 million light-years. This tremendous size is a product of its location, as central cluster galaxies grow by consuming their neighbors. The galaxy is estimated to contain up to 100 trillion stars, vastly greater than the few hundred billion stars in the Milky Way.
Its morphology is distinctly different from the spiral shape of our galaxy, appearing as a smooth, featureless ellipsoid. Most of the stars within IC 1101 are older, low-to-medium mass, reddish-yellow stars, characteristic of elliptical galaxies that have largely stopped forming new stars. At its core, the galaxy hosts a supermassive black hole, one of the largest ever discovered.
Measuring Galactic Immensity
Defining the size of a galaxy is a complex challenge because these cosmic structures do not have sharp, defined edges like a planet or a star. A galaxy’s visible component, made up of stars and gas, gradually fades into the background of space the farther one looks from its center. Astronomers therefore rely on different metrics, leading to varying size estimates for the same object.
Stellar Extent
One common measurement is the stellar extent, the farthest distance at which the galaxy’s light can be reliably detected above the background glow of the sky. This is often quantified by the half-light radius, which contains half of the galaxy’s total luminosity. For IC 1101, the half-light radius is approximately 2 million light-years, but the full stellar halo extends much farther, leading to cited diameters of up to 6 million light-years.
Dark Matter Halo
Another crucial factor is the dark matter halo, the invisible, massive sphere that provides most of the gravitational pull. The dark matter halo can extend far beyond the visible starlight, and its size is inferred using gravitational effects such as weak lensing or the motions of satellite galaxies. The total gravitational influence, including the dark matter, makes the true physical extent of a galaxy significantly larger than its visible light suggests.
How Giant Ellipticals Form
The enormous scale of galaxies like IC 1101 is a direct result of a prolonged, violent evolutionary process known as “galactic cannibalism” or major mergers. This growth mechanism is particularly common in the dense centers of galaxy clusters, where galaxies are packed closely together. The largest galaxies are thought to have formed through the continuous gravitational absorption of smaller galaxies over billions of years.
When a smaller galaxy ventures too close to the gravitational well of a giant elliptical, the larger structure’s tidal forces begin to stretch and distort the smaller companion. This process tears stars, gas, and dark matter from the smaller galaxy, creating vast, visible streams of debris known as tidal tails. Over time, the core of the smaller galaxy is fully absorbed, adding its mass to the central giant.
The cluster environment provides the ideal conditions for this growth, with high collision and merger rates driving the evolution toward a single, massive central galaxy. As the galaxy consumes its neighbors, it integrates their dark matter halos, strengthening its gravitational dominance. This history of repeated mergers explains why these supergiant galaxies are almost exclusively elliptical, as the violent gravitational interactions destroy the organized, rotating disks characteristic of spiral galaxies.
Comparing Galactic Scales
The scale of the largest known galaxy requires comparison to more familiar structures. Our Milky Way galaxy, a sizable barred spiral, spans a visible diameter of approximately 100,000 light-years. IC 1101 is tens of times wider than our home galaxy, depending on the measurement method used.
If the center of IC 1101 were placed at the location of the Milky Way, its vast stellar halo would extend far past our nearest large neighbor, the Andromeda galaxy. Andromeda is located about 2.5 million light-years away, and IC 1101’s diameter of up to 6 million light-years would completely engulf Andromeda and much of the space between them.
The entire Local Group, which includes the Milky Way, Andromeda, and many smaller systems, spans roughly 9.8 million light-years. The size of IC 1101 is comparable to a significant fraction of the Local Group itself, demonstrating how these supergiant ellipticals dwarf even the largest spiral galaxies in the universe.