Is there more galaxies than grains of sand?

Are there really more stars in our universe than there are grains of sand on all the beaches on Earth?

It may hurt your brain to think about it, but it seems that the answer is likely to be yes, or at least the numbers are roughly in the same ballpark.

Astronomers actually set out to answer this question about a decade ago. It's a tricky problem to solve, but it's slightly easier if you throw in a couple of qualifiers — that we're talking about stars in the observable universe; and grains of sand on the entire planet, not just the beaches.

The scientists started by measuring the luminosity density of a section of the universe — this is a measurement of how much light is in that space.

They then used this measurement to estimate the number of stars required to create that amount of light. This was quite a mathematical challenge!

"You have to assume that you can have one type of star represent all types of stars," says astronomer Simon Driver, Professor at the International Centre for Radio Astronomy Research in Western Australia and one of the scientists who worked on the question.

"Then let's assume, on average, this is a typical mass star that gives out the typical amount of light, so if I know that a portion of the universe is generating this amount of light, I can now say how many stars that would equate to."

Now equipped with an estimate of the number of stars within a section of the universe, the next challenge was to work out the size of the universe.

Given we know that the universe is 13.8 billion years old, we can assume that we exist in a sphere 13.8 billion light years in volume. But there's a catch: the universe is potentially infinite in size.

"We know that it has a finite age — we know it started 13.8 billion years ago — but spatially, in terms of its extent, it could be infinite," Driver says. However we also know that because of its age, we exist in a bubble within that infiniteness, and that bubble is called the 'observable' universe.

After all these calculations and caveats, Driver and colleagues came up with a figure of seven followed by 22 zeros, or 70 thousand million, million, million stars in the observable universe.

That's 70,000,000,000,000,000,000,000 stars!

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So what about grains of sand on Earth?

"That was almost harder to work out than the number of stars," Driver says.

Luckily, someone suggested starting with the Sahara Desert, which is home to around half of all the grains of sand on Earth.

"That made it easier; I then just had to work out how many grains of sand are in the Sahara, and I didn't have to worry about every beach on the planet," Driver recalls.

He found the total size of the Sahara, the average depth of sand across the Sahara and from that was able to calculate the approximate number of grains of sand in the Sahara.

"Once I got all that I could put all those numbers together and got a number that was remarkably close to the figure for the number of stars, but just a little bit less," he says. "It was about a factor of 10 smaller, but there's easily a factor of 10 error in both of those estimates so it could just as easily be the other way around."

That's 70,000,000,000,000,000,000,000 stars versus 7,000,000,000,000,000,000,000 grains of sand!

So even though it's an impossible question to answer definitively, it seems that the mind-bending possibility of so many stars existing in the universe is actually true.

Professor Simon Driver spoke with Bianca Nogrady

Be a citizen scientist

How many stars are in the universe, anyway?

American astronomer Carl Sagan once stated that “there are more stars in our Universe than there are grains of sand on all the beaches on Earth”. The question left people surprised at how big the universe is considering that the sand along the earth’s coastlines amounts to trillions of tones. There is no definite way to determine the exact number of stars in the universe and the number of sand grains along the shores but mathematical estimations can be made.

How Many Stars Are There in the Universe? 

Finding the answer to the number of stars in our universe involves generating a mathematical problem of - dare we say it - cosmic proportions. The Milky Way Galaxy has between 100 and 400 billion stars. There are more than 100 billion galaxies in the universe with some researchers placing the figure at about 500 billion. The lowest number of stars that can be found in the universe is ten sextillion (10 billion billion) and 200 sextillions at the higher end. These are huge numbers that are incomparable to anything on earth.

How Many Grains of Sand Are There on Earth? 

A single grain of sand found on the beach is half a millimeter in diameter. Twenty grains make up about a centimeter, and 8,000 make up one cubic centimeter. To calculate the volume of sand, you need to determine the amount of coastline that consists of sandy beaches. Dr. Jason Marshall “The Math Dude” estimates the volume of the beaches to be 700 trillion cubic meters. Mathematically, the figure amounts to five sextillion grains of sand. The mathematician suggests that this is just an estimate and the number could change by a factor of two to a low of 2.5 and a high of 10 sextillions.

Conclusion

A mathematical conclusion can be made that the least number of stars is equal to the highest number of sand grains. However, it is likely that there are five to ten times most stars than sand on the beaches. In 2016 researchers, observing images from the Hubble Space Telescope stated that there could be more than 2 trillion galaxies in the observable universe, which is ten times more than the highest number expected. This is in addition to the fact that the entire universe cannot be observed by any telescope on the earth.

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