What shutter speed should be used to avoid star-trails in astro-photography?

Q: What shutter speed should be used to avoid star-trails in astro-photography?

A: In order to avoid star-trails please divide 500 by the focal length used e.g. 500/50mm = 10 seconds. This will allow you to calculate the longest possible exposure time ensuring that there is no star-trail (a blurry line effect) and that the objects are pin-point in sharpness.

If you are photographing using a higher ISO calculate using 400 instead. For further details and a handy chart based on focal length and sensor size please click through to an article from Petapixel published in January 2015: http://petapixel.com/2015/01/06/avoid-star-trails-following-500-rule/