What is a Galaxy?

by Carson

A galaxy is a collection of stars, gases, and interstellar dust, maintained and held together by gravity. What are the features of galaxies, and how do we classify them? Let’s find out in this article.

The Sizes and Shapes of Galaxies

Describing a galaxy as a collection may underestimate its size. In fact, our Solar System is only a negligible part of our Milky Way Galaxy, which contains billions of stars and their respective planetary systems. However, galaxies can be of different sizes, ranging from containing millions of stars and being thousands of light-years wide, to housing trillions of stars and being hundreds of thousands of light-years in diameter.

However enormous a galaxy is, the universe is much more tremendous. There are at least hundreds of billions of galaxies in the observable universe, and there could be more in other parts of the cosmos, and each of those galaxies is often separated by thousands or millions of light-years. As we don’t know how many distant galaxies we can detect and we don’t know how large the universe is, we don’t have an excellent estimate of the number of galaxies out there.

Moreover, galaxies also take different shapes. While smaller galaxies are irregular (i.e., they do not have a fixed shape), larger ones can be either elliptical or spiral. In a spiral galaxy, a spiral arm is maintained through active star formation, while in elliptical galaxies, no spiral arms are visible, and star formation is often absent. If you would like more information on classifying galaxies based on their appearances, visit this article here about the 3 types of galaxies.

How Do Galaxies Form?

There is a supermassive black hole at the center of almost all sufficiently massive galaxies. The mass of the black hole usually ranges from millions to billions of Solar masses. Although they do not serve the majority of the mass in the galaxy, it helps attract surrounding matter to orbit the black hole and create star-forming regions based on gravity. The combined mass of the black holes and the stars then become so massive that other matter starts to fall in, and this eventually becomes a galaxy containing vast amounts of interstellar dust and stars orbiting around its center of mass.

If every galaxy solely obeyed this process, how can some galaxies be significantly larger than others? Galaxies can collide even though their contents are primarily hollow (star systems are usually light-years away from others). The clumps of matter then interact through gravity, ejecting some stars while usually combining the majority of their contents. When two similarly-sized galaxies collide, or if a larger galaxy absorbs a small one, the resulting galaxy grows in size. Thus, galaxies can emerge extraordinarily massive.

Galaxy Clusters

Planets and stars belong to planetary systems, and those systems belong to galaxies. Do those galaxies belong to another, larger structure? The answer is yes, because galaxies belong to structures known as galaxy clusters, which are collections of galaxies bound by gravity. Galaxy clusters can consist of thousands of galaxies and can be as many as millions of light-years in diameter.


In this article, we’ve explained what a galaxy is, the classifications of it, the formation of galaxies, and the larger structures that galaxies belong to. If we missed any points we should have mentioned, please leave them in the comments below. If you want to learn more about related knowledge, please visit the webpage in the references below.


  1. Adam Mann. (2021, August 25). What is a galaxy? Retrieved September 1, 2022, from https://www.livescience.com/galaxy
  2. (n.d.). What Is a Galaxy? Retrieved September 1, 2022, from https://spaceplace.nasa.gov/galaxy/en/
  3. (n.d.). Galaxy. Retrieved September 1, 2022, from https://astronomy.swin.edu.au/cosmos/g/galaxy
  4. (n.d.). Supermassive Black Hole. Retrieved September 1, 2022, from https://astronomy.swin.edu.au/cosmos/s/supermassive+black+hole
  5. (n.d.). Galaxy Clusters. Retrieved September 1, 2022, from https://www.cfa.harvard.edu/research/topic/galaxy-clusters
  6. (2021, September 23). Clusters of Galaxies. Retrieved September 1, 2022, from https://imagine.gsfc.nasa.gov/science/objects/clusters.html

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