When my son, Niklas, was about 10 years old, he asked me one of these little questions:

“Dad, how big is heaven?”

Wow, that’s a difficult one, I said. I have no idea. Only God knows, I guess. However, at least I can tell you how big the universe is.

“OK, how big is the universe then?”

Very big! Let’s start in our neighbourhood. The moon is actually very close – all the stars are much further way. The distance between our home and the moon is about 390.000 km or 240.000 miles.

Just imagine, you could take a car to the moon – it would take you 135 days! However, then you would not be allowed to make any stops whatsoever. With a rocket, you would be much faster, of course. It took the Apollo astronauts only three days to get to the moon half a century ago.

“Ooh, and the sun? Can I go there as well”, Niklas asked.

It is pretty hot there, I said, and it is 150 million kilometers or 90 Million miles away. With your imaginary space car you would have to drive 140 years non-stop.

“I don’t want to do this!”

Very smart, I replied. Even light takes 8 minutes to get from the sun to us – that is as fast as anything can ever go, but the other stars are still much further away.

“How far?”

Look, I said, every star is a sun like our own sun. Some of these suns can actually be pretty bright – even ten thousand times brighter than our sun. The only reason why they seem to be so faint in the night sky is because they are really far away. The closest star to us – we call it Proxima Centauri – is 40 million million kilometers away. That is 25 trillion miles!

It would take you 75.000 years to get there, even with the fastest rocket we have today!

“Puh, that is far!”, Niklas found.

Indeed, this is why astronomers do not measure distances in kilometers or miles any more, but in light years! If you would switch on a big flash light at Proxima Centauri, then it would still take more than four years before the light would arrive at earth. Hence, we say that Proxima Centauri is four light years away.

Niklas didn’t know that a star can be so far away and wanted to know more about the other stars.

I explained to him, that with the naked eye you could see about 3000 stars, but that our Milky Way would probably contain much more than hundred thousand million stars. On average they would be as far away as 30.000 lightyears and more.

Niklas was impressed: “Is the universe really that big?”

No, bigger! There are many more Milky Ways out there in the universe with that many or even more stars. One of them is very close: the Andromeda galaxy. That Milky Way is 2.5 million light years away. One of my students measured that with radio telescopes.

Andromeda_Galaxy_with_h-alpha

However, most other galaxies are actually billions of light years away.  We suspect that more stars exist than there are grains of sand at the beach. There is no rocket you could think of, that could take us to all these galaxies – ever. You would have to travel longer than the age of the universe.

“That is really big!”, he marveled, so his last question was: “Dad, why did God make the universe to terribly big?”

I have no clue, I told him; maybe, God wants to simply tell us that He, who created everything, is bigger and more amazing than even this unbelievably big and fascinating universe.

Heino Falcke, children’s sermon in Groesbeek, NL, February 2010

See also: “Six thousand versus 14 billion: How large and how old is the universe?”

Dutch version of this text: “Papa – hoe groot is de hemel?”

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