37.2 Trillion: Galaxies or Human Cells?


How many galaxies are there in the observable universe? How many cells are there in the human body?

We know both of these numbers must be really big. But which is bigger?

No one has counted all the cells and all the galaxies one by one, so any number will be an estimate. But estimates are not just guesses.

Scientists actually have counted the number of cells in some multicellular organisms. A tiny transparent worm called Caenorhabditis elegans, a common laboratory animal, was the first multicellular organism to have its complete genome sequenced.

Researchers have learned a lot about this worm — enough for several Nobel Prizes — and they know that there are exactly 1,031 cells in the adult male and 959 in the adult hermaphrodite (there is no female C. elegans).

But counting cells in humans is more difficult.

In a paper published in 2013, Eva Bianconi of the University of Bologna in Italy and her colleagues outlined a method for estimating the number of cells in a “standard human being,” which they defined as a 30-year-old weighing 154 pounds, standing 5 feet 7 inches tall, and possessing a body surface area of 20 square feet.

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Human cells vary in size, and they are at different densities in different places. So the scientists had to account for the varying volumes and surface areas of body parts — skin, blood, internal organs and everything else — and how tightly packed the cells were in each.

Previous estimates had put the number of cells anywhere from 1.0 x 1012 to 1.0 x 1020 — a large range. This newest estimate, probably the best we have, falls closer to the low end: Dr. Bianconi and her colleagues concluded that there were 3.72 x 1013 cells in each of us. That is, 37.2 trillion.

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The method for measuring the number of galaxies in the universe is, by comparison, pretty straightforward, but it is not easy. Astronomers pointed the Hubble telescope at a portion of the sky of known size, counted the number of galaxies they could see, and then multiplied to estimate the number of galaxies in the observable universe.

There are, of course, complications. Galaxies merge over time; the universe expands; the distance we can see with improved technologies increases. And of course, we are talking only about the observable universe — the part we can actually see.

But the best estimate now is that there are between 100 billion and 200 billion galaxies in the universe.

So, more galaxies or more cells? This is not a close call. Even using the highest estimate for galaxies (200 billion) and the lowest estimate for human cells (1 trillion), there are at least 800 billion more cells in your body than there are galaxies in the known universe.



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