The universe contains billions of galaxies, ranging from small dwarf galaxies to colossal systems stretching across millions of light-years. Determining the “biggest” galaxy requires understanding how astronomers measure these immense, ill-defined objects. The vastness of space continually challenges our perception of scale, driving the search for the most extreme structures created by the cosmos.
How Astronomers Define Size
Determining the largest galaxy is complicated because “size” in astronomy can refer to several different metrics. The most intuitive measure is the physical diameter or stellar extent, which is the distance across the visible body of the galaxy. Since galaxies lack sharp edges, astronomers often define this size using an isophotal diameter, which measures the extent down to a specific, faint level of brightness.
Another definition of size focuses on mass, which includes the vast, invisible halo of dark matter surrounding every galaxy. While the Milky Way contains about 100 billion visible stars, its total mass is substantially greater due to dark matter. For many astronomers, a galaxy’s true size is best represented by its total gravitational influence.
A third, specialized measure considers the immense radio lobe extent produced by active galaxies with powerful central black holes. These galaxies blast jets of plasma millions of light-years into space, forming giant lobes visible only to radio telescopes. For instance, the galaxy Alcyoneus has radio structures spanning over 16 million light-years, making it the largest known single-galaxy structure in the universe.
Identifying the Largest Known Galaxy
When defined by the extent of its stellar light, the largest known system is the supergiant elliptical galaxy IC 1101. This colossal structure resides at the center of the Abell 2029 galaxy cluster, about one billion light-years from Earth. IC 1101 is classified as a cD galaxy (central dominant galaxy), known for its vast, diffuse envelope of stars.
Estimates of its diameter vary, but the galaxy’s visible stellar halo stretches between 4 and 6 million light-years across. For perspective, the Milky Way is only about 100,000 light-years in diameter, meaning IC 1101 is dozens of times wider. It is estimated to contain over 100 trillion stars, dwarfing the Milky Way’s population.
IC 1101 achieved this immense size through galactic cannibalism, where a galaxy at the center of a dense cluster absorbs smaller galaxies. Over billions of years, numerous mergers built this elliptical behemoth, leaving it with a yellowish-red hue indicative of an older stellar population. This accumulation of matter is typical for the most massive galaxies found in the densest regions of the universe.
Calculating Cosmic Distances
Determining the physical size of IC 1101 requires astronomers to first accurately measure its distance from Earth using the cosmic distance ladder. The initial rung of this ladder uses standard candles, astronomical objects with a known intrinsic brightness. For relatively nearby galaxies, astronomers use Cepheid variable stars, whose pulsation period is directly related to their absolute luminosity.
For more distant objects like IC 1101, Type Ia supernovae serve as the preferred standard candle. These stellar explosions reach a consistent peak luminosity, allowing scientists to calculate distance by comparing this known absolute brightness to their observed apparent brightness. The difference reveals the distance, based on the inverse square law of light.
For the most remote galaxies, the primary method relies on measuring redshift and applying Hubble’s Law. Redshift is the stretching of light waves toward the red end of the spectrum as an object moves away, a consequence of the universe’s expansion. The greater the redshift, the faster the galaxy is moving away, and according to Hubble’s Law, the greater its distance.
Once the distance is accurately established, astronomers use simple trigonometry, relating the galaxy’s observed angular size in the sky to its calculated distance, to determine its physical diameter in light-years. This combination of standard candles to calibrate the distance-redshift relationship allows for the precise measurement of the most distant and largest galaxies.