The Whirpool is a classic example of a spiral galaxy, but not all galaxies have such a uniformly distinct shape. Image Credits: NASA, ESA, S. Beckwith (STScI) and the Hubble Heritage Team (STScI/AURA)
Galaxies are among the largest and most beautiful objects in the night sky.
These huge swirling masses are where almost every star in the Universe is born, lives and dies.
But it has only been in the past century that astronomers have understood what galaxies are, and come to realise their importance.
Before then, they were thought to be spiral-shaped nebulae on the outskirts of our own Galaxy, the Milky Way.
To many astronomers they were a nuisance, prone to getting in the way of observations and easy to confuse with comets.
But over the years, interest in these objects grew and so did the controversy about their size.
Were they small but nearby, or colossally huge and far away?
On 26 April 1920 two astronomers, Harlow Shapley and Heber Curtis, argued on this topic at what would come to be known as the Great Debate.
Shapley maintained that the Universe consisted of only one galaxy, our own Milky Way, while Curtis said that we lived in just one of many.
In the years that followed, Edwin Hubble went on to measure the distance to the Andromeda Galaxy to be 900,000 lightyears (though this number has since been revised up to 2.5 million lightyears).
This placed it far outside the limits of Shapley’s galaxy.
This meant that galaxies were huge.
In our own Galaxy, there are at least a hundred billion other stars, ranging from tiny red dwarfs to blue supergiants.
They are gravitationally bound together, along with vast clouds of gas and dust, all swirling around a supermassive black hole.
There are three main categories, or ‘morphologies’, of galaxy: spiral, elliptical and irregular.
While the human eye is very good at telling the difference between these types, it is difficult to program computers to recognise them, making large-scale galaxy categorisation a long and laborious process.
This has led to citizen science projects such as Galaxy Zoo, in which thousands of volunteers separate out the different galaxy types.
This is important because a galaxy’s type tells us where it is in its life cycle.
Though there is much contention about how the first galaxies formed, the current leading theory is that in the early Universe, a few hundred thousand years after the Big Bang, dark matter began to clump together.
Hydrogen and helium gas fell towards the huge masses, forming clouds that eventually collapsed to form stars.
It’s thought that these first galaxies were spirals.
As the original clouds of gas were spinning, these galaxies continued to spin as they formed stars, eventually resulting in the structure we see today, majestic arms sweeping from the centre.
Many galaxies have a central bar of stars that these arms stretch out from.
These arms contain huge amounts of gas and dust, the raw products required to create stars, and so they are highly active regions with new stars constantly being formed.
These galaxies are not alone in the Universe and so they frequently bump into each other.
Most often there is a notable size difference, and so the small galaxy is simply subsumed into the much larger one.
Galaxies that have undergone such a process begin to form dense bulges of older stars at their centres.
However, if the two galaxies are around the same size, the process is more dramatic.
The merger can take a billion years and pull the galaxies into some strange shapes.
These become irregular galaxies.
While not all irregulars are undergoing a full merger, they have all interacted with another galaxy and been pulled oddly by gravity.
For those that do merge together completely, the end result is an elliptical galaxy.
This is essentially a giant bulge, with stars moving around randomly within it.
There is little star formation inside, meaning that only older red stars remain, sometimes leading these galaxies to be called ‘red and dead’.
There are also lenticular galaxies, that bridge the gap between spiral and elliptical.
The theory is that they were once spirals, whose arms have faded away.
Dr Elizabeth Pearson is BBC Sky at Night Magazine’s news editor.