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