In this lesson, you will learn about one of the main types of galaxies found in the universe and how to identify elliptical galaxies based on their defining characteristics.
Definition of Elliptical Galaxies
If you've ever looked up at the night sky in a remote area, you may have seen a band of light across the heavens that gives our home galaxy, the Milky Way, its name. The Milky Way Galaxy is a classic example of a spiral galaxy. Many of the most gorgeous astronomical photos of galaxies used in media and popular culture are of spiral galaxies, but the other common type of galaxy, the elliptical galaxy, can tell us so much more about how the universe has changed over time. An elliptical galaxy is a collection of billions of stars that are gravitationally bound; the stars orbit around the center of mass in an egg-shaped central bulge.
Elliptical galaxy IC 2006
Elliptical galaxy IC 2006
Evolution and Characteristics of Elliptical Galaxies
The large galaxies astronomers observe in the local universe were formed by mergers between other, smaller galaxies. When two similar-sized galaxies merge, their stars do not collide with one another; instead, these stars are thrown into new orbits, and often these orbits are randomized and taken out of the simple plane formed by the disk of gas and dust of a spiral galaxy. Indeed, the stars orbit the center of mass of the newly merged galaxy at all angles, forming a bulge without any patterns (for example, no spiral arms).
In these mergers, most of the gas and dust is lost, flung out into the emptiness between galaxies, leaving the elliptical galaxy barren and unable to form any more stars. More massive blue stars have shorter lifetimes than smaller stars like the Sun or red dwarfs. The blue and white stars in an elliptical galaxy have all gone supernova and left behind dim remnants, and only the yellow and red stars shine on. This makes elliptical galaxies look redder in optical wavelengths. Without the bright blue and white stars, elliptical galaxies are dimmer and therefore more difficult to find and photograph in the universe.
Because elliptical galaxies are formed by mergers and have very little new star formation, they are often considered the 'end point' of galactic evolution. By tracking the sizes of elliptical galaxies near and far from our own Milky Way, astronomers can understand the merger process by looking at snapshots of many different mergers (since the process takes hundreds of millions of years) and the rate at which such mergers occur.
Examples
Galaxies often form in clusters, which can contain thousands of galaxies bound together by gravity. Elliptical galaxy M60 and spiral galaxy NGC 4647 are found in the Virgo Galaxy Cluster. In clusters like Virgo, mergers between galaxies are common. In Image 1, the spiral galaxy NGC 4647 shows off its blue arms and dust lanes, while the elliptical galaxy M60 shines brightly as a formless glowing egg. The shine comes from billions of stars indistinguishable at this distance.