Many objects in space are very dim, so they require a lot of light to be collected to make good images. You can achieve this either by doing long exposures or by stacking a lot of short exposure images. Longer exposures require you to track the object as it moves across the sky, and you run the risk of streaks from meteors and satellites. Stacking shorter exposures eliminates these problems, but adds a lot of noise to the final image. A lot of astrophotography is about striking balance between these two.
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