Galaxy: Definition & Explanation

Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

Galaxies are collections of stars, dust and gas rotating around super massive black holes. This lesson will cover the structures an types of galaxies. Afterwards there is a quiz to test your understanding.


It is believed that there are 200 million or more galaxies in the known universe. A galaxy can range in the number of stars as well as its size.

Small galaxies, such as dwarf galaxies can contain as little as tens of millions of stars, while on the other end of the scale, there are massive galaxies with hundreds of trillions of stars. The diameter of an average galaxy can reach 326,000 light years, meaning that it would take light 326,000 years to cross the average galaxy.

In 1929, Edwin Hubble began to organize and classify the different types of galaxies that were being observed by astronomers. The result was the Hubble Classification System, which categorizes galaxies into three main categories; spiral, elliptical, and irregular.

Sombrero galaxy
sombrero galaxy

Types of Galaxies

Spiral Galaxies

Spiral galaxies are broken down into two main types, normal and barred. Our galaxy, The Milky Way is thought to be a barred spiral galaxy with a black hole at its center. Spiral galaxies make comprise up to half of all the galaxies in the known universe.

At the center of spiral galaxies are black holes. Surrounding the center is a bulge of older stars, with dim stars creating a halo effect around the bulge. while in the spiral arms, younger stars reside.

NGC1300 is a barred spiral galaxy

Elliptical Galaxies

Elliptical galaxies range in shape between being spherical to flat and elongated. Primarily composed of older stars, elliptical galaxies are usually smaller in number, but can reach one trillion stars in number.

Elliptical galaxies make up only 10% - 15% of all galaxies in the universe. Although their formation is not fully understood, it is believed that elliptical galaxies are sometimes formed from the merging of two or more galaxies.

Elliptical Galaxy
elliptical galaxy

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