>>3216467Because for different parts of the stacks you use different exposure. High ISO for the stars (more like optimum, 800 for Canon APS-C, 1600 for Nikon/Pentax/Sony APS-C, 400-800 for MFT) and wide aperture to keep the exposure time below trailing, focusing on the stars. Low ISO, closed down aperture (f/4-8) for the foreground, focusing on the foreground, exposure time allows trailing. You make 10-20 frames for each, stack each to reduce noise then combine into a single image. Most milky way landscapes use this method.
For deep sky wide field (~100-400mm focal length) you use single stacking, except for larger bright nebulae like the Orion nebula or the Andromeda galaxy where you make a lower exposure time for the bright core and combine the two stacks in Photoshop.
To answer your question, in astrophotography we don't care too much about sensor noise because we have stacking. We want the most signal from the most photons and that is at the optimum ISO setting which I listed above.
