ay45c4-page24 - stars But> 50 of all stars are in...

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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 e ects 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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