Unformatted text preview: stars. But > 50% of all stars are in multiple star systems. So we can use the
relative motions of binary stars to get stellar masses.
The periods of binary stars vary typically from a few hours (e.g., 6h
for Mirzam, alias Ursa Majoris) to hundreds of years (e.g., 700 yr for 61
Cygni). Some are even shorter, but for these it is likely that the stars are
highly distorted by tidal eects and their masses will be subject to systematic
errors (e.g., \semidetached" or \contact" binaries).
A variety of data may be available for a binary star. We may see the
motion of one star around the other on the sky (which is rare), or we may
see only variations in the velocities of the stars (as seen from Doppler shifts
in their spectra | this is more common). Binary stars may be single-lined
(see light from only one star) or double-lined.
Consider, for example, the second case, a \double-lined spectroscopic
binary star." Here, we see the two stars' spectra superimposed, and we
can measure the radial velocities for both stars, but the stars are too close
together for us to see their orbits by measuring the changing positions of the
stars relative to one another. Then what we see are star velocities like
τ velocity, v K1
system velocity not,
in general, zero K2 0 .5 time, t /τ 1 1.5 83 ...
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- Fall '09
- Binary Stars, Ursa Majoris, multiple star systems