Measuring the distance to galaxies
		• For pulsating variable stars (CEPHEIDS), observations of period  luminosity • Apparent brightness  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