Updated: Aug 19, 2021
So first of all what is a galaxy? A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. But how do we know this?
Origin of Galaxies
In the early 20th century, there were puzzling objects that filled the sky that came in a variety of shapes. They were called nebulae; nobody really knew what, where and how big they were. Eventually astronomers discovered what they were, and, in a moment, our universe got a lot bigger, a LOT.
In the early 20th century, there were 2 ideas competing, one was that, our milky way was it, the whole universe and everything existed in it. The other was that these spiral nebulae were also galaxies like ours. Harlow Shapley argued that the Milky Way was it while Heber Curtis argued otherwise. Both sides had limited and partial data that turned out false.
The final observation that put an end to this debate was by Edwin Hubble and Milton Humason. They observed a spiral galaxy M31 using the most powerful telescope at the time. They found hundreds of stars in it which changed their brightness in a regular manner, they were called Cepheid variables and they were very important as their luminosity helped us pinpoint their distances. The distance they found to M31 was 900,000 lightyears, which made it pretty clear that they were way outside Milky Way galaxy.
Each galaxy is filled with millions to trillions of stars, gas, and dust. Their range of size can be from around tens of thousands of light years to hundreds of thousands of light years, and that’s pretty big.
Types of galaxies
There are four major types of galaxies, spiral, elliptical, peculiar, and irregular. Spiral galaxies are large rotating discs of stars, gas and dust. We live in one.
Some Elliptical galaxies are nearly spiral where others are elongated. We assume right now that elliptical galaxies are a product of galactic collisions (Entire galaxies colliding!) Galactic collisions are absurd, even if galaxies are colling at hundreds of kilometres per second, they take hundreds of millions of years to complete the event, that’s how big galaxies are. Colliding galaxies form all sorts of bizarre shapes. Although millions or billions of stars are involved in galactic collisions, it’s rare that they actually touch each other and that because compared to the space in the galaxies, stars are very minute. Gas clouds though do collide.
Peculiar galaxies aren’t exactly shapeless, their shape is weird. We think these might be a result of small galaxies colliding and passing through bigger ones. Then we come to truly shapeless galaxies, irregular galaxies. They are pretty small in shape.
Centre of galaxies
Active galaxies throw out immense light, and from this we can conclude that they have immense energy. So, we ask ourselves the question, what could power these super energetic celestial bodies? The answer is something that has a lot of gravity, and what has a lot of gravity? In 1980s scientists assumed that galaxies have super massive black holes in their centre, the Hubble telescope was built to investigate this idea, and the idea was right. Every galaxy has a huge black hole in the centre, even the smaller ones are millions of times of the mass of the sun, and the bigger ones could be even billions! The black holes in the centre feed on the galaxy itself. It grows as the galaxy grows. The objects that are sucked into the black holes create so much friction that they blast out light across the universe.
A blog specially for the almighty black holes coming soon.....
When we look outside a singular galaxy, we can see that galaxies clump together in groups and form clusters. An average cluster can be around tens of millions of light years across and can contain thousands of galaxies. The nearest one to us is the Virgo cluster. Galaxies are bound to their clusters through mutual gravity and rotate in long orbits which can take billions of years to complete. Scientists think that clusters themselves clump together to for superclusters. We are a part of Virgo supercluster which itself is a part of even bigger Laniakea supercluster.
How many galaxies are there?
When we think of these galaxies and cluster and superclusters, a question pops up, how many galaxies are there? In 1990s, scientists pointed the Hubble telescope at the emptiest part of the sky they could, they found out that every point in the data was a whole galaxy! That experiment was repeated in different parts of the sky and the results were same! So, scientists concluded that there are 100,000,000,000 galaxies in just the observable universe. The universe in mind strikingly huge!