The universe: Distances and proportions

Ever wondered how far away all the stars that you see at night are? Hundreds of kilometres away? Thousands? Millions? The closest thing to us in space is the moon.

It's 384,000 kilometres from earth. Is that a long distance or a short one? Compared to anything on earth, it's pretty far. Here are earth and the moon to scale. In the space between them, the earth could fit 30 times.

Or you could line up all the other planets of the solar system and still have 8,000 kilometres to spare. The sun is even further away - almost 400 times the distance to the moon. If you could fly there in a jumbo jet, it would take 19 years. It's 150 million kilometres away. The sun itself is pretty big.

Its diameter is 109 times that of earth. But that's nothing compared to the biggest stars we know of. Canis Majoris, for example, has a diameter 1,400 times greater than that of the sun, or 2 billion kilometres. Kilometres are not very good units to measure distances anymore. Instead, we use units based on the speed of light. ​ ​Light travels at almost 3,000 billion meters per second.

A beam of light from the sun reaches earth in just over eight minutes. The distance from earth to the sun is eight light minutes. Compare that to the distance to the moon. It's just 1.3 light seconds away from earth. See the pattern?

A light hour is the distance light travels in one hour. A light year is then the distance light travels in a year. That's 9.46 trillion kilometres, or 9.46 x 10 to the power of 15 meters, using scientific notation and the SI unit. Okay, let's put Mr. Light Year to use.

Our solar system is eight light hours in diameter, so it takes four hours for a ray of sunshine to reach the outermost planet, Neptune. And our solar system is only a small part of the galaxy called the Milky Way. The Milky Way, with its spiraling arms, is home to a couple of hundred billion stars. Its diameter - 100,000 light years. The Milky Way is so big that even that giant star, Canis Majoris, is an insignificant little white dot.

But it doesn't end there. Galaxies stick together in groups, and groups of galaxies appear in larger groups called clusters. ​ ​Our galactic group is part of the Virgo Supercluster which has a diameter of around 110 million light years. And the Virgo Supercluster has neighbors too. Go out even further and there are millions of superclusters in the universe. So how big is the universe?

We just don't know. It could be infinite. But we don't know because we can't see all of it. To see something, light has to travel from the object we're looking at to our eyes. If we look at a star that is one light-year away, we don't see that star as it is right now, but as it was a year ago.

And since the universe is only 13.8 billion years old, we cannot see further than 13.8 billion light years. The part of the universe we can see - the observable universe - is a bubble around us with a diameter of about 92 billion light years. Any light shining in our direction from outside of that bubble hasn't reached us yet, even if it started at the dawn of time - 13.8 billion years ago. So when you look at the stars at night, you're actually seeing them as they looked thousands of years ago. You're looking into the past.