In a clear night sky, far away from the city, a few thousand stars can be seen with the naked eye. Using a simple amateur telescope, millions of stars appear in front of us. Counting all the stars in the sky isn’t viable. So, how can scientists know the number of stars that exist in the universe?
The Universe was created about 14 billion years ago, when the Big Bang took place. In consequence, this is the maximum distance at which we can see an object in the universe. It is impossible for us to see an object that is found beyond 14 billion light-years away from us (if such an object really exists), because its light hasn’t had enough time to reach us. Therefore, when we talk about the universe in this article, we only refer to our observable universe.
Counting the galaxies
Scattered throughout the universe are galaxies, enormous clusters of stars, planets, gas and dust. The Earth is found in a spiral galaxy: the Milky Way. Its stars group together in spiral arms that swirl around the galaxy’s centre.
Before calculating the number of stars in the universe, astronomers have, first of all, to estimate the number of galaxies that exist.
To do this, astronomers make very detailed photos of small parts of the sky and count all the stars they can see. Then, they multiply this number by the number of pictures required to photograph the entire sky.
The most recent estimations indicate that the universe contains approximately 10,000,000,000,000 (10 trillion) galaxies.
Counting the stars
Astronomers can’t precisely know how many stars contains each galaxy. Some of them are so far away that there isn’t any way to observe them with sufficient detail.
Thanks to the luminosity of the galaxies, it is possible to obtain some approximations of the quantity of stars they contain. However, typically the number of stars in the Milky Way is used as a reference.
It is estimated that our galaxy contains about 100 billion stars.
A gigantic number
Knowing how many galaxies exist, and how many stars contain each one of them, the next step is to multiply these two numbers.
The result is incredibly high: 1,000,000,000,000,000,000,000,000 stars, or a 1 followed by 24 zeros (one septillion).
In spite of this, it is thought that this number is in fact much bigger as, thanks to future improvements in telescopes, more galaxies will surely be discovered in the universe.