06 Oct2015

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**To read the original article in ABC Science Click here**

Astronomers actually set out to answer this question about a decade ago. It’s a tricky problem to solve, but it’s slightly easier if you throw in a couple of qualifiers — that we’re talking about stars in the observable universe; and grains of sand on the entire planet, not just the beaches.

The scientists started by measuring the luminosity density of a section of the universe — this is a measurement of how much light is in that space.

They then used this measurement to estimate the number of stars required to create that amount of light. This was quite a mathematical challenge!

“You have to assume that you can have one type of star represent all types of stars,” says astronomer Simon Driver, Professor at the International Centre for Radio Astronomy Research in Western Australia and one of the scientists who worked on the question.

“Then let’s assume, on average, this is a typical mass star that gives out the typical amount of light, so if I know that a portion of the universe is generating this amount of light, I can now say how many stars that would equate to.”

Now equipped with an estimate of the number of stars within a section of the universe, the next challenge was to work out the size of the universe.

Given we know that the universe is 13.8 billion years old, we can assume that we exist in a sphere 13.8 billion light years in volume. But there’s a catch: the universe is potentially infinite in size.

“We know that it has a finite age — we know it started 13.8 billion years ago — but spatially, in terms of its extent, it could be infinite,” Driver says. However we also know that because of its age, we exist in a bubble within that infiniteness, and that bubble is called the ‘observable’ universe.

After all these calculations and caveats, Driver and colleagues came up with a figure of seven followed by 22 zeros, or 70 thousand million, million, million stars in the observable universe.

**That’s 70,000,000,000,000,000,000,000 stars!**

“That was almost harder to work out than the number of stars,” Driver says.

Luckily, someone suggested starting with the Sahara Desert, which is home to around half of all the grains of sand on Earth.

“That made it easier; I then just had to work out how many grains of sand are in the Sahara, and I didn’t have to worry about every beach on the planet,” Driver recalls.

He found the total size of the Sahara, the average depth of sand across the Sahara and from that was able to calculate the approximate number of grains of sand in the Sahara.

“Once I got all that I could put all those numbers together and got a number that was remarkably close to the figure for the number of stars, but just a little bit less,” he says. “It was about a factor of 10 smaller, but there’s easily a factor of 10 error in both of those estimates so it could just as easily be the other way around.”

That’s 70,000,000,000,000,000,000,000 stars versus 7,000,000,000,000,000,000,000 grains of sand!

So even though it’s an impossible question to answer definitively, it seems that the mind-bending possibility of so many stars existing in the universe is actually true.

*Professor Simon Driver spoke with Bianca Nogrady*