There are trillions, if not more, galaxies in the Universe. However, they can be mostly classified into just three groups. Find out what these categories consist of and how we can classify a galaxy.
The Types of Galaxies
There are three types of galaxies: Elliptical galaxies, spiral galaxies, and irregular galaxies. Let’s explain what their features are for us to classify them.
Firstly, some galaxies look either round or elliptic, and they are called elliptical galaxies. There is little to no ongoing star formation in elliptical galaxies, so they generally host older stars. The largest galaxies are elliptical galaxies, which are denoted by an “E” in the classification. For example, E0 galaxies are almost completely round, while E7 galaxies are very elongated.
Examples of elliptical galaxies include IC 1101 and Messier 87.
Spiral galaxies are galaxies where materials and stars are mainly concentrated in spiral arms orbiting the galactic center. They maintain active star formation. Moreover, spiral galaxies can be barred, meaning that there is also a bar-shaped dense region of materials in the center of the galaxy. The classifications of these galaxies start with the letter S, while SB is for barred spiral galaxies.
These galaxies can be further split into three groups: “Sa”, “Sb”, and “Sc”. While galaxy types with a prefix of “Sa” has a large central bulge, spiral galaxies with a prefix of “Sc” has a small central bulge, and “Sb” represents a medium-sized bulge. Keep in mind that “SB” and “Sb” are not the same, and these subcategories for barred spiral galaxies are represented by “SBa”, “SBb”, and “SBc”.
Examples include the Milky Way Galaxy and the Pinwheel Galaxy.
Irregular galaxies are galaxies that are neither elliptical or have spiral arms. They are just irregularly-shaped clumps of stars and interstellar dust that orbits a galactic center. Irregular galaxies are the smallest among the three classes. Examples include the small and large Magellanic clouds.
Why Do Spiral Galaxies Have Spiral Arms?
The most interesting type of galaxies among the three is spiral galaxies. This is because they can sustain spiral arms even though stars orbit the galactic center at different speeds. One theory to explain this phenomenon is the density wave theory, which requires active star formation in the galaxy.
If the density of the regions is high, more massive stars can be formed. These stars compress the materials in the spiral arms, facilitating the formation of more massive stars. This process continues as the stars form and die. Therefore, we see spiral arms as regions of bright stars and a sign of ongoing star formation.
In this article, we’ve mentioned the three main types of galaxies, the features of these categories, and an intriguing property of spiral galaxies. If we’ve missed any crucial features, feel free to leave them in the comments below.
References and Credits
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- Nola Taylor Redd. (2018, November 19). What Is a Spiral Galaxy? Retrieved October 23, 2021, from https://www.space.com/22382-spiral-galaxy.html
- Jean Tate. (2009, September 15). Barred Spiral Galaxy. Retrieved October 23, 2021, from https://www.universetoday.com/40300/barred-spiral-galaxy/
- (n.d.). Spiral Galaxies. Retrieved October 23, 2021, from http://astronomy.nmsu.edu/geas/lectures/lecture29/slide03.html
- (1999, October 21). What process creates and maintains the beautiful spiral arms around spiral galaxies? I’ve been told that density waves are responsible–so where do the density waves come from? Retrieved October 23, 2021, from https://www.scientificamerican.com/article/what-process-creates-and/
- (n.d.). Density Wave Model. Retrieved October 23, 2021, from https://astronomy.swin.edu.au/cosmos/D/density+wave+model
- Deborah Byrd. (2011, January 4). How do spiral galaxies keep their shape? Retrieved October 23, 2021, from https://earthsky.org/space/how-do-spiral-galaxies-keep-their-shape/
- (2012, May 15). The Galaxy Next Door. Retrieved October 23, 2021, from https://www.nasa.gov/mission_pages/galex/pia15416.html