How did they come to the conclusion that there are more stars in the universe than grains of sand on Earth?

Question

How did they come to the conclusion that there are more stars in the universe than grains of sand on Earth?

In: 219

Answer 1

Statistics and estimation of quantities. Does depend on what one calls “sand” a bit, because there are a lot of sediment particles smaller than sand (silt and clay), and particles larger than sand (pebbles/cobbles/boulders) that we ignore when talking about sand.

We geologists have studies sands a lot, like everywhere even sandstone rocks, so we have a decent idea of the average number of sand grains that exist per unit volume in sand. You could even calculate it using basic geometry, imagining a sand grain of 1 mm diameter (or whatever size you decide would be good for the estimate) and how many you could fit into a cubic meter.

Then we look at how much sand there actually is on earth. Not sandstone, because that is rock, but just sand as beaches, delta sediments, and so on. This gives us a volume, and we know how many grains exist in any specific volume, so it is a simple arithmetic problem: We have X cubic meters of sand, and Y grains fit on average in X cubic meters, so just multiply. You get a very big number, and it might be off by a couple zeroes, but not by a lot more than that. We won’t get a great number, a “precise” number for the volume, and our number of grains per unit volume might be a bit off, but it will be in the right ballpark.

Then you turn around and do the same thing for stars: come up with an average number of stars per galaxy, and multiply by how many galaxies are out there that we can see or at least assume based on what we can see. This is also a very big number, with maybe some question as to how accurate it is (maybe off by a couple-few zeroes), BUT it is way bigger than the same number we got for sand grains. Not close.

If it were close, within several orders of magnitude (the amount of error in our estimation), then we couldn’t say with much certainty that more stars exist than grains of sand on earth. We would have to conclude that the numbers seem to be about the same.