• The key assumption is that chemistry and physics work the same everywhere in the universe. Hydrogen, for example, behaves the same -- emitting or absorbing photons at precisely the same quantum wavelengths -- as what's been observed for over a century in spectroscopy labs on Earth. Spectroscopy works like a hi-res prism to precisely measure wavelengths, while quantum mechanics insures that photons are emitted at a specific wavelengths that show up as lines. So we analyze the light from far distant stars, mostly hydrogen and helium -- or light from the vast, hot clouds of gas and fine dust lying between the stars -- matching observed lines to specific chemical composition and temperature. This method also measures redshift to estimate an object's distance using Hubble's Law (yeah, same guy). Precision measurements of orbital motion can reveal things about mass or density of an object; subtle perturbations may imply the presence of a nearby unseen gravitational body; etc. Astrophysicists are very clever at deducing properties of distant objects using mathematical models applied to very careful observations.

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