# Binary Stars

A surprisingly large fraction of the stars are in binary or multiple star systems. Sometimes the binary stars are obviously separated, like 61-Cygni in the close neighborhood of the Sun. Binary stars which can be resolved by a telescope are called "visual binaries".Others can be detected from periodic variations in luminosity associated with the eclipsing of one star by the other ("eclipsing binaries"). Still other can be detected from the overlapping of different kinds of spectra ("spectral binaries"). Modern interferometric measurements have added to our ability to distinguish and study binary stars.

Binary orbits can contribute to the measurement of the masses of different kinds of stars that appear in such systems.

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Star Concepts

Star Classification

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# Visual Binary Stars

Binary stars that can be visually resolved with the use of a telescope are called visual binaries. Clearly this designation depends upon the resolving power of the telescope used, but provides a useful classification. This class includes the dramatic example of 61-Cygni in the close neighborhood of the Sun.

Binary orbits can contribute to the measurement of the masses of different kinds of stars that appear in such systems. From the measurement of the period and semi-major axis of the binary stars' orbit, the sum of the masses of the stars can be obtained if the distance to the pair is known. In addition, if the orbit of each of the stars can be measured, the individual star masses can be deduced.

If a binary star's orbital period is observed to be
T = years = days,
and its semimajor axis a = astronomical units,
then the sum of the masses of the stars is m1 + m2 = solar masses. If the individual semi-major axes are found to be
a1 = AU
a2 = AU
Then the individual star masses are
m1 = solar masses
m2 = solar masses.
 1951 The case of 61-Cygni can be used as an example of the above calculation. 61-Cygni was one of the first stars for which the distance was determined by parallax. It is about 11.4 light years (3.48 pc) from the solar system. Its period is 722 years and with the use of the distance and the changes in angular position, its semi-major axis was determined to be 86.4 AU. The above calculation gives a mass sum for the two stars to be 1.24 solar masses. The reported masses for the two stars are 0.63 and 0.70 solar masses, which doesn't quite agree with the above. To get 1.33 solar masses with a period of 722 years would require a semi-major axis of 88.5 AU.
Index

Star Concepts

Star Classification

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