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Using the fact that Cyg X-1 is in a Binary System to find its Mass

In general, it is not possible to directly determine the mass of a single star from its light alone (though this may soon change as instruments are becoming sensitive enough to measure the second order effect in the relativistic Doppler shift, which can provide the mass). But in the case of a binary system, the mass of the stars can usually be determined, if certain assumptions are made.

Cygnus X-1 is part of a binary, or double star system. Its companion star, HDE 226868, is a BO Supergiant.

Info Click here for more information on Cyg X-1 and its companion star.
Info Click here to go back to your physics textbooks and find how to find the mass of stars in a binary system.

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