# Baade-Wesselink method

The Baade-Wesselink method is a way of calculating the sizes and other properties of certain kinds of pulsating star, including Cepheid
variables. Measurements of a star's color and light output at two different
times, *t*_{1} and *t*_{2}, in the pulsation cycle
are used to find the ratio of the star's radii, *R*(*t*_{2})/*R*(*t*_{1}),
at these times. Spectra of the star throughout its pulsation period are
then used to find the radial velocity of its surface. Knowing how fast the star's surface is moving, the difference *R*(*t*_{2}) - *R*(*t*_{1}) is found
by adding the products of velocity and time during the interval *t*_{1} to *t*_{2}. Given the values of both *R*(*t*_{2})/*R*(*t*_{1})
and *R*(*t*_{2}) - *R*(*t*_{1}), it
is easy to solve for the radii. The method is named after Walter Baade and the Dutch astronomer Adriaan Jan Wesselink (1909–1995).