For stars that are too distant to have measurable parallaxes, their distances can be estimated from the star's spectral type, luminosity class, and apparent magnitude with a procedure called spectroscopic parallax.

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Spectroscopic parallax is a real method used to estimate the distance of stars that are too far away to have their parallax measured. It uses the star's spectral type, luminosity class, and apparent magnitude to estimate its distance.

For Stars That Are Too Distant to Have Measurable Parallaxes