Chunyuan (26) vroeg:
How many stars are in the sky?
This is actually quite a relevant question for the city of Leiden in the upcoming months. We will explain why in a bit.
How many stars there are in the sky depends on what you call "the sky"! If by "the sky" you mean "the observable universe" the number is unimaginably enormous. It depends a bit on who you ask, and how they calculate it. Stars are clustered in galaxies, and the number of galaxies that is estimated to exist recently grew by about 10 fold.
Based on the observations made by the Hubble telescope the number of galaxies was estimate to be roughly 120 billion (120.000.000). Nowadays it is estimated that there may be as many as 2 trillion galaxies 2.000.000.000. An average galaxy contains about one hundred billion stars 100.000.000.000.
If you multiply those numbers you get an perplexing 1 septillion in the American numbering system; 1 quadrillion in the European system. That is a 1 with 24 zeroes! 1.000.000.000.000.000.000.000.000.
Something that makes it actually a bit more complex is: many of the galaxies that are observed are so far away, that their light took billions of years to reach us. That means that many of those stars do not actually exist anymore, although others may have formed in the mean time. Should they be counted?
Further complicating the matter is that the universe may actually be much bigger than the part we observe, since we can see only the part of which the light has had enough time to reach us.
If by "the sky" you mean "that what we see if we look up at night without a telescope" the answer depends on your eyes for starters, but mainly it depends on how dark it is where you are.
There are about 5000 stars that are bright and close enough to be theoretically seen by the average human eye when there is perfect darkness. The earth gets in the way of at least half of these when you stand on the surface, so in perfect conditions you would see about 2500. In reality though, there is often a lot of stray light from artificial light sources. Light pollution, you could call that. That means that on many places on earth, such as cities and other urbanized places, you can actually see none.
To get attention to this subject, on the evening of the 25th of September 2022, the city of Leiden will turn of as many lights as possible in the city center. That way, its inhabitants will finally be able to see at least some stars from their roofs and gardens. Be sure to take part in this magical event!