Secondary Calibrators

Secondary Calibrators:
Secondary calibrators of the distance scale depend on statistical measures of stellar properties,
such as the mean brightness of a class of stars. It has been known since the 1800's that stars
follow a particular color-luminosity relation known as the Hertzsprung-Russell Diagram.

The existence of the main sequence for stars, a relationship between luminosity and color due to
the stable, hydrogen-burning part of a star's life, allows for the use of spectroscopic parallax. A
stars temperature is determined by its spectrum (some elements become ions at certain
temperatures). With a known temperature, then an absolute luminosity can be read off the HR
diagram.

The distance to a star is simply the ratio of its apparent brightness and its true brightness
(imagine car headlights at a distance). The method allows us to measure the distances to
thousands of local stars and, in particular, to nearby star clusters which harbor variable stars.
A variable star is a star where the brightness of the star changes over time (usually a small
amount). This is traced by a light curve, a plot of brightness and time.

Particular variable stars, such as Cepheids, have a period-luminosity relationship. Meaning that
for a particular period of oscillation, they have a unique absolute brightness.

The result is that it is possible to measure the light curve of Cepheids in other galaxies and
determine their distances.