Damien has a master's degree in physics and has taught physics lab to college students.
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.
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
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.
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 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.
The third major type of galaxies are irregular galaxies. Irregular galaxies are simply defined as those galaxies that do not have a well-defined structure to them. We believe that these galaxies are often formed when spiral or elliptical galaxies distort by colliding with another galaxy or passing too close to one so that gravitational pull alters their shape.
There are two main categories of irregular galaxies, Irr I and Irr II. Irr I irregular galaxies have at least a little bit of structure to them but not enough to be classified as another type of galaxy, and Irr II irregular galaxies display no structure whatsoever. There is also a third subcategory called dIrr, which simply stands for dwarf irregular galaxies.
A galaxy is a large collection of stars, dust, and gas held all together by gravity. There are 100 to 200 billion galaxies in the observable universe, and they can range in size from 100 million stars to up to as large as trillions of stars. At the center of almost all large galaxies lies a supermassive black hole, which is an object in space with so much mass even light can't escape its gravitational pull.
In 1929, Edwin Hubble organized galaxies into three main categories based on their appearance.
- Spiral galaxies have a central bulge with spiral arms extending off of them and a near translucent spherical halo surrounding them.
- Elliptical galaxies have a smooth ellipsoid shape that can range from spherical to flat and elongated.
- Irregular galaxies are galaxies without enough structure to be defined as either spiral or elliptical.
Spiral galaxies also have two major subtypes.
- Barred spiral galaxies are a subtype of spiral galaxies that have bright bars of stars stretching across the center of their bulge to which the spiral arms attach.
- Lenticular galaxies are another subtype of spiral galaxies that have a central bulge with a flat disk extending from it, instead of spiral arms.
After completing this lesson on galaxies, you should be ready to:
- Define galaxy
- Identify and explain the different types of galaxies
- Briefly describe the Milky Way galaxy
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