Measuring a and P of binaries
	� Two types of binary stars � Visual binaries:�� See separate stars � a large, P long � Can�t directly measure component of a along line of sight � Spectroscopic binaries:� See Doppler shifts in spectra � a small, P short � Can�t directly measure component of a in plane of sky � If star is visual and spectroscopic binary get get full set of information and then get M