Elliptical galaxies stand out as one of the three major morphological types, alongside spirals and irregulars. These systems are characterized by their smooth, featureless, and spheroidal shapes. Determining an “average” size for this group is complex because they exhibit the greatest range in scale, spanning from the smallest dwarf galaxies to the largest known supergiant systems.
Defining Galactic Size
Measuring the size of an elliptical galaxy is not as straightforward as measuring a disk-shaped spiral galaxy, which has relatively distinct edges. Astronomers rely on a standardized method known as the effective radius (\(R_e\)), or half-light radius. This measurement defines the radius within which half of the galaxy’s total light is emitted, providing a consistent metric regardless of the galaxy’s brightness profile. The effective radius is typically expressed in units of kiloparsecs (kpc), where one kiloparsec is approximately 3,260 light-years.
The visible light from elliptical galaxies gradually fades away from the center without a sharp boundary. While the effective radius focuses on the luminous, visible component, the overall size of a galaxy is also defined by its much larger, invisible dark matter halo.
The Average Size Spectrum
Elliptical galaxies are broadly categorized into three distinct groups based on their scale. The most numerous, though faintest, are the Dwarf Ellipticals (dEs), which typically have effective radii smaller than 1 kiloparsec. These small systems may contain as few as ten million stars and have masses around \(10^7\) solar masses, often appearing transparent due to low star density.
The “average” or Normal Ellipticals are significantly larger, with effective radii generally ranging from about 5 to 50 kiloparsecs. These galaxies contain stellar masses from \(10^9\) to \(10^{12}\) times that of the sun. A typical bright elliptical might have an effective radius of around 10 to 20 kpc, making their stellar bodies comparable in size to the disks of large spiral galaxies.
At the top of the scale are the rare Giant Ellipticals or cD galaxies, found predominantly at the centers of dense galaxy clusters. These colossal systems can stretch over hundreds of kiloparsecs, with some of the largest, like IC 1101, measuring up to 6 million light-years in diameter. Their masses can exceed \(10^{13}\) solar masses, placing them among the most massive stellar systems in the universe.
Comparison to Other Galaxy Types
Comparing the size of an elliptical galaxy to a spiral galaxy requires distinguishing between their stellar morphology. A typical large spiral galaxy has a flat stellar disk that spans over 30 to 50 kiloparsecs in diameter. In terms of visible stellar diameter, the average elliptical galaxy is often similar to or even smaller than a large spiral.
The three-dimensional structure is fundamentally different. Spiral galaxies are thin, flat disks, while elliptical galaxies are spheroidal, meaning they occupy a much larger volume of space for a similar visible diameter. The largest giant ellipticals far surpass any spiral galaxy in both stellar mass and diameter, extending their visible stellar halos for hundreds of thousands of light-years. The star density in ellipticals is concentrated toward the center, following a smooth profile, whereas spirals have a flatter density profile within their disks.
Factors Determining Scale
The immense variation in the size of elliptical galaxies is primarily a consequence of their evolutionary history, particularly through galactic mergers. The largest giant ellipticals are built up over cosmic time through “dry merging,” where two or more galaxies collide and combine without significant amounts of cold gas. These mergers randomize the stars’ orbits and swell the resulting galaxy, leading to the large, extended stellar halos characteristic of cD galaxies.
The largest ellipticals are almost exclusively located at the gravitational centers of massive galaxy clusters, which are regions of high galaxy density. The high concentration of galaxies in these clusters increases the frequency of mergers, allowing the central galaxy to effectively absorb numerous smaller systems and continually grow in size and mass. The final size of an elliptical galaxy is a record of its long history of gravitational interactions and cannibalism.