# On starry grains

Carl Sagan famously said in his 1980's television series, Cosmos, that there are more stars in the Universe than there are grains of sand on all the beaches on Earth. I was reminded of an article by a friend on this topic while strolling along Woolamai beach on Australia's southern coast. The previous night we had been far enough from Melbourne's midnight glare to see the wonderfully milky strip of light of our own Galaxy which is so prominent in Southern skies. A cold summer night it was and we couldn't stay out for long; but with two friends visiting us from Finland we managed to get night adjusted quickly enough to appreciate the sight. The Milky Way is so bright down under that Northern hemisphere astronomers on their first trip across the equator have even mistaken it for bands of clouds! Back on Woolamai beach, a miniscule fraction of those grains of sand were being whisked into a blizzard by the wind coming in off the Great Southern Ocean and exfoliating my face; small they are but sharp and hard. How small and how many on Earth? Well, if you count a grain of sand as only those which are visible to the naked eye, then small means about a tenth of a millimeter. How many grains of sand on Earth's beaches? That's one of those questions where a first estimate can be made in a few minutes; and give you pause for thought on how to improve it. Take Australia for example: it's a few thousand kilometers across, so its coastline is a few tens of thousands of kilometers long (let's say 30,000 kilometers, or 30,000,000 meters). If it were beach all the way round, the beaches being, say, 50 meters wide and a few meters deep, then we are talking about

30,000,000 x 50 x 5 = 7.5 billion cubic meters of sand.

A grain of sand is about 0.1 mm across, so there are 10,000 per meter or 10,000 x 10,000 x 10,000 (or 1,000 billion) per cubic meter.

So we are talking (oh so very very roughly) about 7500 billion billion grains of sand along our Australian coastline. Call it 10,000 billion billion grains.

So how long are all the coastlines on Earth? Australia is the smallest of seven continents, so they must be at least 10 times longer when taken alltogether. Let's say a 100 times to keep things moving along. If that's right, then we have about

10,000 billion billion x 100 = 1 million billion billion sand grains.

1024 grains of sand. We might be wrong by quite a few orders of magnitude here, but before getting too deeply involved in arguing about that, let's forget the sand and move on to the stars.

How many stars in the Universe? That one is much easier!

We live in a typical galaxy, the Milky Way, and we can count its stars with relative ease --- it's really just a matter of peering into space and counting--- there are about 100 billion. And the Universe is filled with galaxies; recently, the Hubble Space Telescope spent a week staring at a randomly choosen patch of the sky and took a photo of the brightest galaxies along a path between our own Galaxy and the edge of the visible Universe. From that one photo you'd reckon there are at least 100 billion galaxies in the entire Universe. Putting both numbers together, we'd expect there to be

about ten thousand billion billion stars in the Universe

and

about a million billion billion sand grains on the beaches of Earth...

Well! There might be more sand grains on Earth's beaches than stars in the Universe. But certainly, it's too close to call! We cannot possible have estimated either figure so well that we could be sure the stars really are more numerous than the sand grains or vice versa... maybe our coastline estiamte was too small, or the beaches too narrow (or not deep enough). And how many galaxies did we miss with the Hubble images; small faint ones are out there too, there are new ones reported every day...

Google currently lists 2,073,418,204 web pages. Ten years ago there were none. At least we are in no immediate danger of websites outnumbering the stars!

 Chris Flynn The writer is a senior research fellow at Tuorla Observatory in Turku, Finland. He is presently on leave at the Centre for Astrophysics and Supercomputing at the Swinburne University of Technology in Melbourne Australia.

This essay was inspired by a conversation with Glen Mackie, who has presented his view in the article "To see the Universe in a Grain of Taranaki Sand"

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