It’s true that you can’t simply place a planet or galaxy on a scale to measure how heavy it is. Luckily, astronomers have a few tricks up their sleeves for weighing celestial objects.
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Observing and Understanding DEEP SPACE - Volume 3: The Ultimate Guide to Stargazing: Thompson, Gregg D: 9780645466959: : Books
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Ask Astro: How do scientists weigh celestial objects?