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Binary Stars

Binary Stars. Over half of the stars in the sky have stellar companions, bound together by gravity and in orbit around each other. Types of Binaries. Visual Binaries.

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Binary Stars

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  1. Binary Stars Over half of the stars in the sky have stellar companions, bound together by gravity and in orbit around each other.

  2. Types of Binaries Visual Binaries • Optical Binaries-are chance superpositions, where two stars appear close together but do not actually orbit one another. (Like Mizar & Alcor) Physical Binaries- where one star orbits another, and each star can be seen in the telescope.

  3. OPTICAL DOUBLES • Not a true binary system • Stars only appear close together in the sky • Mizar & Alcor in the Big Dipper While Alcor and Mizar are Optical Double stars and only appear to be near each other, Mizar is actually a Physical Binary star.

  4. Orbits and Masses of Visual Binaries The primary importance of binaries is that they allow us to measure stellar parameters (especially mass). The center of mass is the location where a fulcrum would be placed to balance the stars on a seesaw.

  5. Masses of Binary stars Newton’s Modification of Kepler’s Law P must be in years, a in AU M in solar mass, where Sun = 1

  6. Types of Physical Binaries Eclipsing Binary –(If the angle is good ) two stars that regularly eclipse one another causing a periodic variation in brightness. Spectroscopic Binary - two stars that are found to orbit one another through observations of the Doppler effect in their spectral lines . At least half of the stars in the sky are binaries. Eclipsing Binary stars are also referred to as Extrinsic Variable Stars.

  7. Eclipsing Binaries Sometimes the orbital plane is lined up so that the stars pass in front of each other as seen from the Earth. Each eclipse will cause the total light from the system to decrease. The amount of the decrease will depend on how much of each star is covered up. The period is from one large dip to the next and with the period you can get the mass of both.

  8. Spectroscopic Binaries Some binaries are too close together to be resolved, you may still be able to detect the binary through the Doppler shift (in one or both stars). They must be relatively close to each other (short orbital period). If you can see both stars’ spectrums, you may be able to use Doppler shifts to measure the radial velocities of both stars. This gives you the mass ratio, regardless of the viewing angle (e.g. nearly face-on, nearly edge-on, etc.). This is usually useful information.

  9. The End

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