A common rule of thumb to figure out your maximum shutter speed for sharp stars at night is to divide 500 by your focal length. Sometimes it’s called the 600 Rule or the 400 Rule or several other numbers that can be used depending on your sensor size. Unfortunately, it’s a a very inaccurate rule today.
The so-called 500 Rule was designed for 35mm film grain at higher ISOs; but current digital sensors far out-resolve grainy film, especially with high-megapixel count,... more (link is external)