Elliptical galaxies are cosmic structures distinguished by their smooth, often featureless appearance and a shape that can range from nearly spherical to highly elongated, resembling an oval or football. These galaxies represent a significant portion of the universe’s galactic population, yet their immense scale and diverse sizes make defining an “average” size a complex astronomical endeavor. Understanding the scale of these distant systems requires specialized methods to measure their vastness.
Characteristics of Elliptical Galaxies
Elliptical galaxies lack the prominent spiral arms seen in galaxies like our Milky Way, presenting a smooth, uniform, three-dimensional ellipsoidal form devoid of distinct features. These galaxies are predominantly made up of older, lower-mass stars, giving them a redder or yellowish hue. They contain very little interstellar gas and dust, which are the raw materials for new star formation. Consequently, ongoing star formation within elliptical galaxies is minimal or virtually absent.
Measuring Cosmic Distances and Sizes
Astronomers employ sophisticated techniques to determine the immense distances to galaxies, which in turn allows them to calculate their physical sizes. One method involves using “standard candles,” which are astronomical objects with a known intrinsic brightness. By comparing this known true brightness to how bright the object appears from Earth, scientists can calculate its distance. A notable type of standard candle is the Type Ia supernova, an exploding star whose consistent peak luminosity makes it a reliable distance indicator for galaxies millions to billions of light-years away.
For even more distant galaxies, astronomers rely on Hubble’s Law, also known as the Hubble-Lemaître Law. This law states that galaxies are moving away from Earth at speeds proportional to their distance, a phenomenon known as redshift. By measuring the extent to which a galaxy’s light has been stretched to redder wavelengths, scientists can determine its recessional velocity and, subsequently, its distance. Galaxy sizes are commonly expressed in light-years, representing the distance light travels in one year (approximately 9.46 trillion kilometers), or in kiloparsecs (kpc), where one kiloparsec equals about 3,260 light-years.
The Range of Elliptical Galaxy Sizes
The concept of an “average” size for elliptical galaxies is complicated by their enormous variability, encompassing the widest range of sizes among all galaxy types. At the smaller end of the spectrum are dwarf elliptical galaxies, which can be as compact as a few thousand light-years across, with some measuring as little as 0.1 kiloparsecs (about 326 light-years) in diameter. These dwarf ellipticals can contain tens of millions of stars and are the most common type of galaxy in the universe.
Conversely, giant elliptical galaxies span hundreds of thousands of light-years in diameter, frequently reaching up to 700,000 light-years across. Some of the most massive, known as supergiant ellipticals or cD galaxies, can extend well over a million light-years. For example, galaxy M87, a giant elliptical in the Virgo Cluster, is approximately 300 kiloparsecs (nearly one million light-years) across. This vast range means that while dwarf ellipticals might be only slightly larger than globular clusters, the largest ellipticals are thousands of times bigger.
How Elliptical Galaxies Grow
Elliptical galaxies grow primarily through mergers with other galaxies. When two or more galaxies collide and combine, especially spiral galaxies, the resulting system often forms a larger elliptical galaxy. This process can trigger a burst of star formation as gas and dust are consumed. However, after this initial burst, the merged galaxy has very little gas remaining to fuel further star birth, explaining the scarcity of young stars in ellipticals.
The gravitational interactions during mergers can dramatically alter the orbits of stars, transforming the organized motion seen in spiral disks into the more random, three-dimensional stellar orbits characteristic of elliptical galaxies. While major mergers between similarly sized galaxies are a significant mechanism, repeated minor mergers, involving one larger galaxy absorbing smaller companions, also contribute to the growth and evolution of elliptical galaxies. This continual accretion of smaller systems allows ellipticals to accumulate mass and expand their stellar populations over cosmic time.
Elliptical Galaxy Sizes in Perspective
To grasp the immense scale of elliptical galaxies, it is helpful to compare them to more familiar galactic structures. Our own Milky Way galaxy, a spiral galaxy, spans approximately 100,000 light-years in diameter. While the Milky Way is considered a relatively large spiral galaxy, many giant elliptical galaxies dwarf it considerably. For instance, a giant elliptical galaxy can be several times, or even up to ten times, the diameter of the Milky Way.
The largest supergiant ellipticals stretch over a million light-years. Even though dwarf ellipticals are the most numerous type of galaxy, making the “average” galaxy much smaller than the Milky Way, the existence of these enormous elliptical systems highlights the extreme range of sizes found in the universe.