The weather is getting warmer, summer holidays are approaching, and breach trips are being planned.
We often hear that there are more stars in the sky than grains of sand on Earth. But is that true? Well, it depends.
In a dark site, with our naked eye, humans can see about 5000 stars in the night sky. However, that is at one location at night.
There are stars in the Northern Hemisphere that we can't see here in the Southern Hemisphere as the Earth is blocking them.
The Yale Bright Star Catalog counted every star bright enough across the world, visible to the human eye. The total is 9096 stars. Surely there are more grains of sand than this.
However, there are a lot more stars in our Milky Way Galaxy than just the ones we can see.
We can't sit there and count the stars individually. If we could count five stars every second, it would take more than 10,000 years to count them all.
If we could count five stars every second, it would take more than 10,000 years to count them all.
It is tricky to figure out how many stars there are in our galaxy.
Things like dust block light and make some stars hard to see.
One of the best ways is to not count the stars, but weigh them.
In fact, weigh our whole Milky Way. Now we can't get out a giant scale, but we can do it a few different ways.
One way is to look at how stars move around the galaxy.
We can measure how fast they move, and use that to figure out how much gravitational force there is to cause those speeds, and from that, get a mass. We can also measure how bright our Milky Way is. By figuring out how bright our Milky Way is, we can then figure out how much power (like a light bulb) it has, and then how many stars we need to generate that power.
With both ways, we use our sun as the average weight and brightness.
We know these measurements really well, and why some stars are smaller or bigger, it averages out (over the large numbers we are talking about).
Using different techniques gives us on average 300 billion stars in our Milky Way. That is a lot more than 5000 or 9000, but still, probably not as much as grains of sand on the Earth.
However, what if we add up all the stars in all the galaxies? Galaxies, like stars, are bigger and some are smaller. On average, we expect that the average galaxy has about 100 billion stars - slightly less than our Milky Way.
Figuring out how many galaxies there are in the universe is also tricky. Using a variety of telescopes, we've counted how many galaxies are in a small patch of the sky, and then multiple that across the whole sky to see how many there are.
We get anywhere from 300 billion to 2 trillion galaxies. Considering that, 300 billion galaxies with 100 billion stars each gives us 30,000,000,000,000,000,000,000, or 30 followed by 21 zeroes worth of stars in the universe.
Now that is more than grains of sand on the Earth.
- Brad Tucker is an astrophysicist and cosmologist at Mount Stromlo Observatory, and the National Centre for the Public Awareness of Science at ANU.
