Galaxies are massive, gravitationally bound systems categorized into three main morphological classes: spiral, irregular, and elliptical. While spiral galaxies, like our own Milky Way, are known for their distinct arms and flattened disks, elliptical galaxies present a much smoother, featureless appearance. These stellar systems are defined by their shape and composition, and they represent a significant portion of the universe’s galactic population. This article focuses on defining the physical characteristics, internal dynamics, formation history, and typical cosmic locations of elliptical galaxies.
Defining the Elliptical Shape
Elliptical galaxies are characterized by their smooth, three-dimensional, ellipsoidal shape, which gives them a soft, amorphous appearance. Unlike spiral galaxies, they completely lack spiral arms, a central disk, or any significant structure. Their light profile is uniform, fading gradually outward from a bright center.
The classification of these galaxies uses the system developed by Edwin Hubble, which assigns them a numerical designation from E0 to E7. The number indicates the apparent degree of elongation or flattening when viewed from Earth. An E0 galaxy appears nearly spherical, while an E7 galaxy is highly elongated and lens-shaped.
This classification is based on the galaxy’s two-dimensional appearance on the sky, not its true three-dimensional shape. The shape of an elliptical galaxy is not maintained by the orderly rotation found in spirals. Instead, the overall form is determined by the random, non-ordered motion of its constituent stars.
Internal Composition and Dynamics
The stellar population within an elliptical galaxy is dominated by old, red stars, often referred to as Population II stars. This composition gives the galaxy an overall reddish-yellow color, contrasting with the bluer hue of star-forming spiral galaxies. These systems contain very little cool gas and dust, the raw materials necessary for the birth of new stars.
Because of the near-total lack of star-forming gas, elliptical galaxies exhibit extremely low rates of stellar birth, which leads to their description as “quenched” or “red and dead.” The motion of stars within the galaxy is highly randomized, moving in complex, three-dimensional orbits that are not confined to a single plane. This random movement, known as velocity dispersion, is the primary source of internal support against gravitational collapse.
The overall flattening of an elliptical galaxy is caused by the difference in the random velocities of stars along different axes. Nearly all massive elliptical galaxies harbor a supermassive black hole at their core, whose mass is closely correlated with the overall mass of the host galaxy.
How Ellipticals Form and Grow
The most widely accepted model for the formation of large elliptical galaxies involves major mergers. These are the gravitational collision and coalescence of two or more smaller galaxies, often spirals. When two spiral galaxies of comparable size collide, the delicate, ordered orbital paths of their stars are violently disrupted. This chaotic interaction randomizes the stellar orbits, erasing the original disk structure and creating the smooth, three-dimensional shape characteristic of an elliptical galaxy.
The immense gravitational shockwaves triggered by the merger compress the remaining gas and dust, leading to a rapid, short-lived burst of star formation. This starburst quickly uses up or expels the remaining gas, effectively shutting down future star formation in the newly formed remnant. The result is a galaxy dominated by old, red stars.
Once formed, massive elliptical galaxies can continue to grow through a process sometimes called “galactic cannibalism.” This involves the repeated absorption of smaller satellite galaxies that venture too close to the gravitational well of the larger elliptical. This continuous accretion of stellar material contributes significantly to the immense size of the most massive ellipticals.
Where Ellipticals Reside in the Cosmos
Elliptical galaxies show a strong preference for high-density environments in the cosmos. They are overwhelmingly found in the centers of massive structures like galaxy clusters and compact groups of galaxies. This distribution contrasts sharply with spiral galaxies, which are more common in the lower-density “field” environments outside of major clusters.
The largest known galaxies in the universe, the giant elliptical galaxies—sometimes classified as cD galaxies—are typically found sitting at the gravitational center of these massive galaxy clusters. Their central location allows them to easily accrete material and consume smaller galaxies, contributing to their enormous size. The high density of matter in these regions provides the frequent gravitational interactions and mergers required to form and evolve these featureless stellar systems.