A handy rule of thumb to remember for creating any images containing stars (astrophotography, shooting the Milky Way, shooting star trails, etc) is this one which will help you calculate the maximum time exposure you can use before any stars start to record movement blur.
On a full-frame sensor camera the accepted formula for calculating star movement is: 600/the focal length of your lens. So if you’re shooting on a full-frame Canon EOS 5D with a 17mm lens then 600/17 = about 35 seconds. If you want to capture an image of the Milky Way with the stars kept all very sharp-looking then you cannot expose any longer than this period of time. If you’re shooting star trails and purposely want to introduce star movement then you will need an exposure longer than 35 seconds otherwise the stars will just record as a series of noticeable dots when you blend the captures together.
On a cropped-frame sensor camera use the formula 300/the focal length of your lens. So if you’re shooting on a cropped-frame Canon EOS 600D with a 24mm lens (or at 24mm on any zoom lens) then 300/24 = 12.5 seconds. Shooting sharp star captures like the Milky Way will be tough as any exposure longer than 12.5 seconds will start to show movement blur on the stars. If you’re purposely trying to shoot star trails then anything over 12.5 seconds will start to give you the movement blur you’re after.
On my Canon EOS 5D camera I find that anything much longer than a 60 second exposure introduces shocking levels of digital noise (from the excessive heat the sensor starts to create) so shooting architecture with star trails in the background, like this capture below through my 17mm tilt-and-shift architectural lens, I therefore need to find an exposure time balance that works best somewhere between 35 seconds and 60 seconds.
Depending on the light, the architecture and the creative effect I am trying to achieve I can either lower the aperture setting from f11 down to f4 or raise the ISO setting from 100 to 800. Raising the ISO too high will ultimately introduce grain and cause a deterioration in image quality so it’s all just a technical matter of juggling the parameters (ISO, aperture & time exposure) to find a solution that gives a sensational enough result.
Come on, get out there and give it a try, you know you want to…!