The Scale of the Universe


We humans live on a planet called “Earth”. This big rock that we call home orbits, along with seven other planets, around a star called the Sun. Including other objects such as comets and asteroids, all together we make up the “Solar System”. 

But is the Sun the only star there is? Actually the Sun is 1 in a 100 billion stars that together make up our Galaxy, the Milky Way. This is a great island of stars held together by gravity and orbiting a common center. Our Solar System is about halfway between the center and the outskirts of our Galaxy.

Guess what? There are an estimated 100 billion more galaxies, each made of 100 billion stars, each having planets around them. As a result of gravity, most galaxies like to stick together, and are found in groups, which are generally called “Galaxy Clusters”. Our Milky Way is part of one which we call “the Local Group”.

At very large scales, galaxy clusters appear to be arranged in giant chains and filaments with huge voids between them. This beautiful structure of our Universe is called the “Large Scale Structure”, in which galaxies are found in ‘superclusters’, basically clusters of galaxy clusters. Our Local Group lives on the outskirts of the “Local Supercluster”, also known as Laniakea.


Great! Now we know where we live. But how far away is everything from us? If I wanted to  go to the Moon, that’s 363,104 kilometers away. Going with a car at 50 km/hrs, it would be a 300 day trip (one way!). The Sun is at 149,597,870 kilometers, so at the same speed it’ll take me 300 years to reach!

Since astronomical distances get difficult to express using Earth’s systems of measurement, astronomers have established units that make more sense for astronomical sizes. The distance between the Sun and the Earth (149,597,870 kilometers!), is 1 astronomical unit. Another very useful unit is the distance the light travels in one year, going at 300’000 km/s. This is called a light-year and it is equivalent to 10 trillion kilometers!


OK, the speed of light is fast, but it’s not infinite. It still takes a chunk of time for light to travel large distances. In fact, it takes about 8 minutes for light from the Sun to reach the Earth. This means that if the Sun were to turn off right now, we would only realize it in 8 minutes. The closest star to Earth is Proxima Centauri, located 4.2 light years away?, which tells you that  the light from that star takes 4,2 years to reach us. When we look at it now, we are actually seeing what that star looked like 4.2 years ago! The center of our galaxy is 26,000 light years away! Right now we are looking at it as it was 26,000 years ago! Let’s go even further! The closest galaxy to our Milky Way, Andromeda, is 2.5 million light years away. So we see it as it looked 2.5 million years ago! Wanna go “wow” again? Andromeda is 100’000 light years in size, so the light we see from the far side of Andromeda comes from 100’000 years before the light we see from the close side. SHBAM!

The farther we look away in distance, the further back we look in time. So every snapshot of a distant galaxy is a picture of both space and time. 


Have you ever tried making a small scaled version of our Solar System in your home? If you decide that the Sun is 1 meter in diameter, basically you will need a house the length of the Golden Gate Bridge to build it to scale. Not only are astronomical distances large, but also that the planets are very small. So there is a lot of empty “space”. 

Let’s change the sense of scale and say that our galaxy fits in an American football field. The solar system is located about at the 20 yard-line and it’s the size of a minuscule spec of dust. And look, there are 100 billion specs of dust as well in that field, and those are all the stars in our galaxy. How far is the nearest football field (or galaxy)? Well on this scale Andromeda is… 2km away! 

Oh no, wait! Didn’t I say 100 billion stars? That sounds like a lot right? But how much is “a lot”? It’s this much: if you start counting stars, from 1 to 100 billion, considering you say 1 number per second, there are 30 million seconds in a year. So you’ll be counting stars for 3000 years non-stop!! 

And yes you may remember I started all of this saying there are 100 billion galaxies in the observable universe, all made of 100 billion stars.. You do the math now.

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