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Galaxy: Definition & Types

Instructor: Damien Howard

Damien has a master's degree in physics and has taught physics lab to college students.

Galaxies are collections of stars, dust, and gas bound together by gravity. This lesson will cover the structures and types of galaxies, including spiral, elliptical, and irregular galaxies.

Galaxy Definition

This chart shows the different types of elliptical and spiral galaxies in the Hubble classification system.
Edwin Hubble classification system showing the types of galaxies

On a clear night, in a place with little light pollution, you can look up at the sky and see it dotted with thousands of stars. However, this is only a small number of the stars that surround us. There are literally billions of them. These stars, including our Sun, are all part of one large structure in space called a galaxy.

A galaxy is a large collection of stars, dust, and gas held together by gravity. The smallest galaxies are known as dwarf galaxies and can contain as few as 100 million stars, while on the other end of the scale are massive galaxies with trillions of stars. It is currently believed that there are 100 to 200 billion galaxies in the observable universe.

Supermassive Black Holes

While there can be exceptions for smaller dwarf galaxies, nearly all larger galaxies contain supermassive black holes at their center. A black hole is an object in space with such a high gravitational pull that not even light can escape it. If you were to get close enough to see one, all you would see is the absence of light. It would look like a hole in space. We refer to the ones at the center of galaxies as supermassive because they are a million to a billion times more massive than black holes found elsewhere in the universe.

Types of Galaxies

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.

Messier 51, a.k.a. the Whirlpool Galaxy, was the first galaxy to be designated as a spiral galaxy.
Messier 51 is an unbarred spiral galaxy

Spiral Galaxies

Spiral galaxies have a central bulge of primarily older stars from which spiral arms containing younger stars extend outwards and a nearly imperceptible spherical halo surrounding the entire galaxy.

Spiral galaxies can be further broken down into Sa, Sb, and Sc type spiral galaxies. The Sa type galaxies have the most tightly wound arms around their center and tend to have the largest bulges. The opposite is true of Sc spiral galaxies, which have the most loosely wound arms and smallest bulges. Type Sb, as you might guess, falls in between the two.

Spiral Galaxy Subtypes

NGC 1300 is a barred spiral galaxy with a diameter of 1,100 light-years, making it roughly the same size as our Milky Way.
NGC 1300 is a barred spiral galaxy

There are a couple of major subtypes of spiral galaxies worth going over. First, there is the barred spiral galaxy. In these galaxies, the spiral arms connect to a bright bar of stars that spans through the middle of its central bulge. They have the same three types as the spiral galaxy, but with a B thrown in for their bar: SBa, SBb, and SBc.

Our galaxy, the Milky Way, happens to be a barred spiral galaxy, with our Earth located in one of its arms. Our Milky Way is an average-sized barred spiral galaxy with a diameter of approximately 100,000 light-years and contains somewhere between 100 and 400 billion stars.

NGC 5010 is a lenticular galaxy located in the Virgo constellation at a distance of approximately 140 million light-years from Earth.
NGC 5010 is a lenticular galaxy

The second major subtype of spiral galaxies are lenticular galaxies. These are designated S0 or SB0, and their shape falls somewhere in between those of spiral and elliptical galaxies. They exhibit the bulge of a spiral galaxy and have a disk extending from it but don't have any spiral arms.

Elliptical Galaxies

Elliptical galaxies do not have the finely defined features of a spiral galaxy but instead appear as very smooth ellipsoids. Like spiral galaxies, they have their own types ranging from E0 to E7 based on how spherical they appear. Elliptical galaxies designated E0 are nearly spheres, where those designated as E7 are flat and elongated. It is believed that elliptical galaxies sometimes originate from the merging of two or more galaxies. These galaxies are primarily composed of older stars and can range in size from very small up to very large galaxies containing a trillion stars.

You can see that this elliptical galaxy is a smooth ellipsoid shape with no disk or spiral arms to speak of.
Elliptical galaxies are one of three types of galaxies

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