Main Sequence Fitting

Remember the Hertzprung Russell diagram, aka a color magnitude plot (Hipparcos/Gliese catalogs, courtesy Richard Powell):

How can we use this for distances?

Spectroscopic parallax: know the spectral type of a star, infer its absolute magnitude. Get distance from distance modulus m-M.

Color-magnitude diagram (CMD) fitting: match CMD of star clusters to the HR diagram to get shift in m-M, and therefore distance.

Cluster CMDs

Open
Clusters

Globular Clusters

Stellar Evolution and cluster ages

(Mike Guidry, U Tennessee)