Measuring the distance to galaxies
•For pulsating variable stars (CEPHEIDS), observations of period ® luminosity
•Apparent brightness µ luminosity/(distance)2
•\ Distance measure  - works for  1 kpc to 30 Mpc
m – M = distance modulus
           = 5logd – 5 (d in pc)
\d = 10 (m-M+5)/5  pc
Measure P = 42 d, m = +22.0
\L = 33,000 L¤ & M = -6.5
\d = 10 (28.5 + 5)/5 = 10 6.7pc
       = 5 x 106 pc = 5 Mpc