More Stars Than Grains of Sand: How Astronomers Arrived at a Number Beyond Imagination

Stand at the edge of the ocean on a long beach and try to imagine counting every grain of sand beneath your feet. Then extend that count to every beach on every continent, every shoreline on the planet, every grain tumbling in the surf or packed into dunes stretching away from the water. By most estimates, that total comes to somewhere around 7.5 quintillion grains — a number written as 7.5 times 10¹⁸. It is a figure so large it defeats ordinary intuition. And yet, astronomers at the European Space Agency and institutions around the world have converged on the conclusion that the observable universe contains roughly 10²⁴ stars — more than a hundred thousand times the total grain count of every beach on Earth combined.

How Astronomers Count What They Cannot Individually See

No one has tallied the stars one by one. What astronomers have done is something far more ingenious: they have combined estimates of the number of galaxies in the observable universe with estimates of the average number of stars each galaxy contains, then multiplied.

The first variable — total galaxies — was long pegged at around 200 billion based on the Hubble Space Telescope's deep field observations. In 2016, a team led by Christopher Conselice at the University of Nottingham revised that figure dramatically upward, suggesting that the observable universe contains at least two trillion galaxies once you account for smaller, dimmer ones that earlier surveys could not detect. The second variable — stars per galaxy — varies enormously depending on a galaxy's size and type. A dwarf galaxy might contain just a few hundred million stars. A large elliptical galaxy like IC 1101, one of the biggest known, may harbor over 100 trillion. The Milky Way, a fairly typical spiral galaxy, holds somewhere between 200 and 400 billion stars.

Multiplying those two estimates together — two trillion galaxies by a conservative average of a few hundred billion stars each — yields a number in the range of 10²⁴. The figure comes with enormous uncertainty. It could be ten times higher or lower. But at virtually any reasonable estimate, it dwarfs the sand count by orders of magnitude.

Why the Observable Universe Is the Relevant Boundary

There is a precise reason astronomers specify the observable universe rather than the universe as a whole. The observable universe is defined by the distance light has been able to travel since the Big Bang approximately 13.8 billion years ago — a sphere roughly 93 billion light-years in diameter when expanded space is taken into account. Beyond that horizon, light from more distant objects has not yet had time to reach us. Those regions almost certainly exist, but we have no observational access to them.

The universe as a whole may be vastly larger than the observable portion — some inflationary cosmological models suggest it is effectively infinite. If that is the case, the total number of stars would be infinite as well, which makes comparison meaningless. The observable universe boundary gives astronomers a meaningful, measurable volume within which to work. Within it, 10²⁴ stars is the best current estimate, and it is the comparison to beach sand that makes the number land with any emotional force at all. Numbers that large require an anchor, and sand grains — something every person has felt slip between their fingers — provide one.

What Those Stars Mean for the Possibility of Other Worlds

The implications of 10²⁴ stars extend well beyond arithmetic. Modern exoplanet surveys, particularly data from NASA's Kepler and TESS missions, suggest that virtually every star hosts at least one planet on average. Some estimates place the average closer to several planets per star. If even a small fraction of those planets fall within the habitable zone of their host star — the range of distances where liquid water could exist on a rocky surface — the potential number of worlds capable of supporting life becomes staggeringly large even if the probability per planet is vanishingly small.

The Drake Equation, formulated by astronomer Frank Drake in 1961, was an early attempt to structure exactly this kind of reasoning. Its variables remain deeply uncertain, and scientists disagree sharply about what values to assign them. But the sheer density of stars and planets the observable universe contains means that even deeply pessimistic assumptions about the emergence of life leave open the possibility of vast numbers of inhabited worlds. A cosmos containing more stars than grains of sand on every beach of a planet that is itself just one pale dot in one unremarkable galaxy is a cosmos that has had every opportunity to generate complexity.

The beach sand comparison was popularized by Carl Sagan, who had an extraordinary gift for translating cosmic scale into human terms. He wanted people to feel the weight of the number, not just read it. Standing on a beach, running your fingers through the sand, understanding that there are more suns burning beyond the sky above you than grains in your hand, in the beach, in every beach in the world — that is one of the few moments where the size of the universe becomes not just a fact but a felt experience.