What is a Galaxy? How Does Galaxy Formation Work?

Last Updated May 27, 2023 by . First Published in 2010.

Astronomical Objects Series
  1. Constellation Guide and Associated Mythology
  2. What are Asteroids, Meteors and Comets?
  3. Binary Stars and Double Stars
  4. What are Variable Stars and How to Observe Them
  5. Supernova and Supernovae
  6. What Nebula and Nebulae, What are the Types of Nebula?
  7. What Is a Black Hole? Black Holes Explained - From Birth to Death
  8. Quasars (QUAsi-Stellar Radio Source)
  9. Pulsars - Natures Lighthouses Key to Astronomy
  10. What is a Neutron Star? What is Inside a Neutron Star?
  11. Gamma Ray Bursts
  12. Kuiper Belt
  13. What is an Exoplanet? How Can We Detect Exoplanets?
  14. What is a Galaxy? How Does Galaxy Formation Work?
  15. The Messier Catalogue of Objects To Observe
  16. The Caldwell Catalogue
  17. 25 Stunning Sights Every Astronomer Should See
What is a Galaxy? How Does Galaxy Formation Work?

A look at what galaxies are, the types of galaxies that we can see, the "tuning fork" and galaxy formation - where do galaxies come from?

Billions of galaxies populate our universe, and every one is an enormous group of stars that exist in combination in space. We are living in one of these galaxies known as the Milky Way, named after the path of milky light that its stars make across the Earth's sky.

At the start of the twentieth century, we believed that the Universe consisted of just the Milky Way galaxy. It wasn't until 1924 that Edwin Hubble discovered that some of the nebulae previously observed were distant galaxies. Since then, we have discovered hundreds of thousands of galaxies. We estimate there are between 100-125 billion galaxies in the visible Universe. A single telescope image can come with dozens of them and it's simple to assume that the universe is brimming over with them, In fact, however, the galaxies are separated by nonvenomous expanses of empty space. Multiply the diameter of a galaxy by about 20 and that is the distance between it and another one.

What is a Galaxy?

By any standard, galaxies are massive. Even dwarf galaxies are measured in thousands of light years (1 light year = 9.46 million, million kilometres). By comparison, the Milky Way is about 100,000 light-years across, and the biggest galaxies are over a million light-years across. The smallest galaxies contain a few million stars, whereas the Milky Way contains 500 billion and the biggest contains a million stars. each star follows its path around the galaxy's centre. The sun goes around the centre of our galaxy roughly once in 250 million years; that's one galactic year.

M51 Whirlpool Galaxy
M51 Whirlpool Galaxy 

The galaxies are held together by gravity and also contain gas, dust and large amounts of invisible "dark matter". Recent research suggests that at the very centre of many galaxies, there exists a black hole.

What is the Milky Way?

Stars surround Earth whichever way we look out. These stars all belong to our galaxy, the Milky Way Galaxy. The Milky Way is a spiral galaxy. The spiral arms have an abundance of bright young stars and gas clouds and are the major location for star formation. The entire Milky Way galaxy is situated within a spherical halo consisting of globular clusters which contain some of the oldest stars in the galaxy.

Artists concept of the Milky Way galaxy
Artists concept of the Milky Way galaxy 

The Milky Way Galaxy is an enormous selection of stars, gas, and dust, which in combination make up a disc-shaped body that measures about 100,000 light-years across and about 4,000 light-years thick.

We know there are hundreds of billions of stars in the Galaxy however, we don't know precisely how many. The number of stars has been estimated using a function for the Galaxy's mass. This is regarded as about 1,000 billion solar masses. This figure not only covers the visible stars, gas, and dust but also additional material that hasn't been discovered yet. This "dark matter" is thought to be up to 90% of the Milky Way's overall mass, and we don't know what it is made from, we only know it is there.

The overall mass and our knowledge of the part of the Galaxy we are living in are then used to estimate the number of stars in the Milky Way galaxy. Most astronomers agree between 200


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