Eli5: how do we know the properties of things in space like stars and planets if we’ve never been on them?
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We measure mostly the light we receive. From position of the peak intensity in the spectrum we can derive the temperature, combined with the color can give us the current state the star is in.
From specific lines in the spectrum we can derive what element is present and have further hint ln the stars current state. For observing periodically changing brightness we can see if it is a special type of star or if it has a planet in orbit.
From this period we can estimate the orbital speed of the planet. If we can see the star move, we can derive the mass of the planet.
All of this is based on physics that habe been around for a long time. Gravity, Thermodynamics, Electrodynamics, Plasmatheory. Even Quantummechanics is needed for the explanation of why a star shines.
Most of this is theoretically know and verified by experiments in labs. And because physics is universal, laws that work here in labs also work anywhere in the universe and can tell us all the wonderful things.
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