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Elliptical Galaxy Facts & Definition

Elliptical Galaxy NGC 1316
This image from NASA's Galaxy Evolution Explorer shows the galaxy NGC 1316, located about 62 million light-years away in the constellation Fornax. The elliptical-shaped galaxy may be in the late stages of merging with a smaller companion galaxy. The Galaxy Evolution Explorer data was taken in December 2003.
Credit: NASA/JPL-Caltech/CTIO

Elliptical galaxies are the most abundant type of galaxies found in the universe. However, because of their age and dim qualities, they are frequently outshone by younger, brighter collection of stars.

Elliptical galaxies lack the swirling arms of their more well-known siblings, spiral galaxies. Instead, they bear the rounded shape of an ellipse, a stretched-out circle. Some stellar collections are more stretched than others.

Classification and characteristics

In 1926, Edwin Hubble devised a system to classify galaxies. Known as the Hubble sequence, or the "Hubble tuning-fork", it organizes galaxies based on their shape. Under this organization, elliptical galaxies are classified by how stretched they are. Galaxies classified as E0 appear to be almost perfect circles (remember, a circle is an ellipse!), while those listed as E7 seem much longer than they are wide. It is worth noting, however, that a galaxy's appearance is related to how it lies on the sky when viewed from Earth. A galaxy having the  E7 shape but seen head on would appear as an E0, for instance, because observers would not see its stretched shape, which lies "behind" it.

Elliptical galaxies have a broader range in size than other types of galaxies. The smallest are dwarf elliptical galaxies, which can be less than 10 percent the size of the Milky Way and contain only ten million times the mass of the sun. But ellipticals can also stretch to more than a million light-years across, and contain more than ten trillion stars. M87, identified as one of the largest galaxies in the universe, is classified as an elliptical galaxy — E0, to be exact. Scientists recently announced that the largest galaxy, IC 1101, was 50 times the size of the Milky Way and 2000 times as massive.

Astronomers have identified more spiral galaxies than ellipticals, but that is simply because they are easier to spot. While spiral galaxies are bright, elliptical galaxies are dim. Spiral galaxies are hotbeds of star formation, but elliptical galaxies are deader. They contain less gas and dust, which means fewer new (and brighter) stars are born. The existing stars inside an elliptical galaxy tend to be older, giving off more red light than younger stars.

If fewer elliptical galaxies have been spotted, why do astronomers think they dominate? Because when specific regions of the sky are studied in depth, more elliptical galaxies appear. Astronomers think such counts are consistent throughout the universe.

Giant Elliptical Galaxy Centaurus A
The giant elliptical galaxy Centaurus A shows a split personality because it hides a gaseous spiral at its core. When Centaurus A collided with a spiral galaxy 300 million years ago, it slurped up the spiral's gases, which formed a new spiral inside the larger galaxy. Image released Oct. 22, 2012.
Credit: ESO

History and formation

Because elliptical galaxies contain older stars and less gas, scientists think that they are nearing the end of the evolution line for galaxies. The universe is a violent place, and collisions between galaxies are frequent — indeed, the Milky Way is due to crash into the Andromeda Galaxy in a few billion years. When two spirals collide, they lose their familiar shape, morphing into the less-structured elliptical galaxies.

A supermassive black hole is thought to lie at the center of these ancient galaxies. These gluttonous giants consume gas and dust, and may play a role in the slower growth of elliptical galaxies.

Born from collision, elliptical galaxies are more commonly found around clusters and groups of galaxies. They are less frequently spotted in the early universe, which supports the idea that they evolved from the collisions that came later in the life of a galaxy.

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