= Astrophysics
{wiki}
Astrophysics uses <physics> to explain <stars>, <planets>, <galaxies>, and their <electromagnetic radiation>; <cosmology> extends the questions to the universe as a whole. Much of its evidence arrives as <light>. The challenge is to infer the source's physical properties from what reaches an observer: a bright <star> might be nearby, unusually luminous, or both. <Stellar parallax> and <observed radiative flux> make this distinction quantitative.
This is an <inverse problem>. A <forward problem> predicts observations from a source model and the intervening <radiative transfer>; the <inverse problem> works backwards from observations to possible models. A <spectral line> can constrain <chemical elements>, <temperature>, and motion through the <Doppler effect>, but its strength also depends on the <stellar atmosphere>. Combining different measurements is therefore more informative than interpreting brightness alone. The attraction is that a tiny change in the direction of <light> can supply a distance, turning received <radiative flux> into intrinsic <luminosity>. https://www.esa.int/Science_Exploration/Space_Science/Gaia/How_does_Gaia_study_the_Milky_Way[ESA: Parallax, spectra, and physical properties]{ref}.
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