Re: How many stars are there in the universe?

Hi Tim,

I did a quick estimate on a bit of paper, and I think there are many times more grains of sand on the beaches of the world than there are stars in the universe.

Let's assume that a grain of sand is about 1 micron, or 10^-6m in size. Let's also assume that a typical beach is about 5 km long and 0.5 km wide. Then that corresponds to 5 thousand million grains of sand long by 5 hundred million grains of sand wide. Multiply those together, and we have 2.5 billion billion (or 2.5 times 10¹⁸) grains of sand just on the top layer of sand on the beach. If the sand is, say, 5m (or 5 million grains of sand) deep, then the total number of grains on this one beach alone would be 12.5 million billion billion, or 1.25 times 10²⁵.

The number of galaxies in the observable universe is estimated to be around 100 billion (10¹¹), I think. Each of those contains something like 100 billion stars. Therefore, the total number of stars in the universe is around about 10²², or 10 thousand billion billion.

Comparing these two estimates, the number of stars in the universe is less than one per cent of the number of grains of sand on a single beach. Based on that, I'd say that there are at least many thousand times more grains of sand on the beaches of the world than there are stars in the universe.

Jim O'Donnell, jim@penfold0.demon.co.uk