How Many Stars Are There in the Universe? | Unveiled

How Many Stars Are There in the Universe?

Only 5% of the universe is made up of physical, observable matter. But it’s so impossibly vast that even that 5% is extremely difficult to grasp, let alone to categorise. But that doesn’t mean that we aren’t trying to work out exactly what lies in the far-off depths of outer space.

This is Unveiled, and today we’re answering the extraordinary question; how many stars are there in the universe?

It’s sometimes said that the number of stars in the sky is the same as the number of grains of sand on planet Earth; a comparison which truly underlines how difficult the task at hand is. Because how would anyone even begin to count precisely how many grains of sand there are? Counting stars is arguably even harder still, though, because we can’t possibly travel to stars to physically count them; at least, with the grains of sand, we actually could - in theory - visit all of the beaches! In fact, star-counting is such a tricky task that we struggle with even the specific figures for just our own galaxy, the Milky Way. We can say with 100% accuracy that there’s one star in the solar system, our sun, but as we move outwards, the numbers get steadily bigger and more uncertain.

We are trying to make sense of them, though… Professional astronomers and amateur stargazers alike have been attempting to come up with a valid figure for the number of stars for years, so we do have a lot of estimates. But, perhaps unsurprisingly, they tend to vary wildly! Figures can range from 10 to the power of 21 to 10 to the power of 24 (or even higher) and, when you’re dealing with numbers this vast, that difference of three zeroes on the end is almost unfathomable in itself!

The Milky Way itself is thought to contain somewhere between 100 billion and 400 billion stars, so there’s a big margin for error! But, since we tend to think of the Milky Way as average in size, this massive ballpark figure can be applied elsewhere. While estimates on the number of galaxies also vary a lot, according to data from the Hubble telescope it’s thought there could be at least 200 billion galaxies in the universe. So, even at the lowest end of the calculation, 100 billion stars per galaxy multiplied by 200 billion total galaxies lands you at a ridiculously big number… Twenty sextillion stars, and that’s still quite a conservative guess… because according to other estimates, the total number of galaxies is actually more like two trillion. Multiply that by the upper limit 400 billion stars per galaxy, then, and the eyewatering scale of the universe really comes into focus!

Perhaps the most incredible thing of all, though, is that even the higher estimates are usually based on the observable universe, only – that is, the parts of it that we can see. It’s not at all straightforward physics, but we can see light that’s travelled 13.8 billion lightyears to reach us… but, the actual size of the observable universe is more like 92 billion lightyears across. But, what about beyond that? It’s thought that further away than it’s possible for us to even know, the same type of universe filled with the same types of structure could exist… so we now don’t even know the parameters we’re working with! Add into the problem that, even with the observable universe, we also have cosmological inflation to contend with as the universe expands… and, for some, it’s why “stars in the universe” is simply something that we’ll never be able to truly comprehend!

And we haven’t even breached another fundamental fact of the cosmos, yet; that the number of stars is constantly changing. New stars, or protostars, are forever being formed in space, all while old stars are slowly dying. But because even the speed of light is so slow in the grand, cosmic scheme of things (travelling at only 186,000 miles per second), were we to try to make exact observations of all of the stars that are alive at any one time, by the time we’d finished adding them up (after many thousands of years) those observations will have already become outdated. Charting the stars is simply too big a job!

So, let’s take a big step into the future. Say, in a hypothetical world, we managed to put humans onto every habitable world there is, spreading across the expanse of the universe and providing us with thousands of vantage points to determine “what’s out there”. In such a world, all of the human settlements could perhaps make much more accurate observations of their immediate surroundings… but even they would fall short of a total figure. They’d all have to confer with other outposts or colonies to ever combine their findings and reach one true “number of stars”. And seeing as communications as we currently understand them can only ever travel at the speed of light, we’ll have reached the same problem we can predict happening now - by the time all of the information is pooled together most of the data would be redundant. Even in a far-off, farfetched future reality, we could only say with any precision how many stars there were in the universe a long, long time ago; we could never say how many there are now.

So, counting stars is arguably the ultimate in statistical impossibilities! And, because of a variety of factors like star life cycles, universal expansion and the limitations of the speed of light, we simply don’t know how many there are in the universe, and we probably won’t ever know for sure. But it’s definitely a lot - with even the lower estimates stretching to 20+ digits. It sounds like a simple question, but it’ll never have a simple answer. And that’s how many stars there are in the universe.