One of astronomy’s most captivating thoughts, famously shared by Carl Sagan, claims there are more stars in the universe than grains of sand on all of Earth’s beaches. This mind-boggling comparison begs the question: is it actually true? Let’s delve into the science and math behind this cosmic conundrum.
Calculating the Stars in the Observable Universe
Estimating the number of stars requires a multi-step approach. First, consider our Milky Way galaxy, home to hundreds of billions of stars. While obscured by dust and gas, a conservative estimate places this number around 200 billion.
Next, we multiply this by the estimated number of galaxies in the observable universe. Astronomers in 2016 estimated this to be roughly two trillion galaxies. However, this estimate includes galaxies with a stellar mass more than a million times that of our sun, significantly smaller than the Milky Way.
This discrepancy in galactic size is addressed by considering that smaller, less massive galaxies are far more numerous. Furthermore, most stars are red dwarfs, much smaller than our sun. Accounting for these factors, a reasonable average is about 10 million stars per galaxy.
Therefore, the estimated total number of stars is 10 million stars/galaxy * 2 trillion galaxies = 20 quintillion stars (2 x 1019). That’s an astronomical number, but how does it compare to grains of sand?
Estimating Grains of Sand on Earth’s Beaches
To calculate the number of sand grains, we need to determine the volume of sand on beaches and the number of grains per volume.
A cubic meter of sand, assuming an average grain size of 1 millimeter, contains approximately one billion (109) grains.
Estimating the total volume of sand is more complex. Assuming an average beach width of 50 meters, a depth of 10 meters, and a total global sandy shoreline length of 750,000 kilometers (based on studies estimating 30% of non-icy shorelines are sandy), the total sand volume is roughly 375 billion cubic meters.
Multiplying this volume by the number of grains per cubic meter yields an estimated 375 quintillion (4 x 1020) grains of sand.
The Verdict: Sand Wins
Surprisingly, the initial estimate suggests there are roughly 20 times more grains of sand than stars in the observable universe. This challenges the famous aphorism.
However, these are rough estimates. Variations in grain size could significantly impact the sand count. Smaller grains would drastically increase the total. Conversely, while unlikely, a much higher average star count per galaxy could shift the balance.
Considering the vastness of deserts and the ocean floor, which contain far more sand than beaches, it’s safe to say sand likely outnumbers stars by a considerable margin.
While this exercise highlights the limitations of estimating such immense quantities, it demonstrates the power of using scientific reasoning and mathematical approximations to explore seemingly impossible questions. The universe, while filled with an incredible number of stars, may be dwarfed by the sheer abundance of sand grains on our own planet.
