Are there more stars in the universe than grains of sand on Earth?

From ABC Science Online, 19 August 2015:

It may hurt your brain to think about it, but it seems that the answer is likely to be yes, or at least the numbers are roughly in the same ballpark.

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. Read more.

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