Astronomers use the inverse square law to estimate brightness but also rely on spectral analysis. Why is spectral analysis preferred for measuring distances beyond a few thousand light-years?

A star’s spectrum reveals its spectral type, which helps astronomers estimate its intrinsic, or absolute, brightness. They can compare that with how bright the star appears to estimate its distance using the inverse square law. This is useful beyond a few thousand light-years, where parallax becomes difficult to measure accurately; spectral analysis does not directly measure distance or replace every other method.

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