If you want to shoot the night sky and get sharp dots for the stars, you can only leave your shutter open for so long before the earth’s rotation causes blurring. A little blurring looks bad (and I have made some bad mistakes there). A lot of blurring (i.e. several minutes) is a star-trail and can be nice. (I will save that for another day).
So how long can you leave your shutter open to get relatively sharp looking stars? There is something called the Rule of 500, which essentially is that if you divide the focal length of your lens into 500, it gives you your shutter speed in seconds. The Rule of 500 is controversial (some say it should be Rule of 600, others that it is just not that simple), but it at least gives you something to start with. So for a 16mm lens, I can leave the shutter open for 31.2 seconds before the stars start to blur. (500/16=31.2). This assumes you have a full frame sensor or are making the sensor conversion.
So I often will shoot my 16-35mm lens at 16mm and leave it open for 25-30 seconds. That is what I did here.