Binary and Multi-Star Systems

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         Five to ten percent of the stars visible to us are visual binary stars.A binary star system is the system consists of two star which orbit a common center of mass following Kepler's Laws.The system is divided into 3 type: Visual binaries;spectroscopic binaries; eclipsing binaries.

   

More details about binary system are shown in Structure of Binary and Multi-Star Systems and  Formation of Binary Star Systems

   

          A multiple star consists of three or more stars which appear from the Earth to be close to one another in the sky. There are actually two type. One is that the star are physically close and are bounded by gravity. The other type is just stars that are close optically. Only the first type is called multiple star systems.

         Most multiple star systems has three stars, so they are often called trinary or ternary. Larger systems, like quadruple stars (4 stars), Alcyone (5 stars), Castor (6 stars), and so on are less likely to occur.Examples are shown in Binary and Multi Star Systems in the Universe

         Multiple stars have sizes intermediate between binary systems and open star clusters, which is dynamicly more complex and typically contains 100 to 1,000 stars. They can be divided into two classes corresponding dynamically to these two extremes.

         The most common one is the one organized in a hierarchical manner,in which smaller orbits nest inside larger orbits.Shown blow is an example.

   

         Other multiple stars, termed trapezia, are usually very young, unstable systems. These are thought to form in stellar nurseries, and quickly fragment into stable multiple stars, which in the process may eject components as galactic high velocity stars. An example of such a system is the Trapezium in the heart of the Orion nebula.

   

More details are shown in Structure of Binary and Multi-Star Systems


Images from: 

 http://www.astropix.com/HTML/B_WINTER/TRAPEZ.HTM [2]

 http://www.universetoday.com/24203/what-is-a-binary-star/ [10]

 http://www.nature.com/nature/journal/v481/n7382/full/nature10818.html [12]

 http://www.mpg.de/594617/pressRelease200904021 [13]