What is the best exposure time to shoot stars at night?

Star Trails

You may know that, if you expose stars long enough, they slowly turn into trails, lines, instead of dots. How can we figure out the longest exposure time to avoid turning star dots into trails?

Star Dots

For star dots...

For More Information

If you are interested in more information about infinity focus, here are some links:

Footnotes:

1 EV numbers are a way to express the brightness of a scene in a scale that combines the shutter speed and aperture settings into one number -- letting the photographer choose what combination of shutter speed and aperture settings to use. Each EV number equals one stop of brightness, so a scene with an EV of 6 is one stop brighter than a scene with an EV of 5. EV 0 corresponds to an exposure time of 1 s and a relative aperture of f/1.0. See Wikipedia

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