What magnification do I need for stargazing versus hiking?

These are essentially different tools. For hiking, 8x or 10x in a 32–42mm objective is the sweet spot for handheld use. For astronomy, reviewers and astronomy communities generally recommend 15x70 or 20x80 binoculars with a tripod, the larger objectives gather more light for deep-sky targets but are too heavy and shaky for handheld trail use.