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Double, Triple, Multiple Star Systems In The Milky Way Galaxy

Artist's impression of a triple star system.

When you look up at a clear, dark night sky without any light pollution, you should be able to see as many as 2,000 or more stars.  That is a small portion of the 200 billion stars that make up our Milky Way Galaxy.  

Most of the stars are single stars like our sun, but about a third of the stars are double, triple or multiple star systems.  These multiple star systems, depending upon their masses, either orbit a common point known as the barycenter or appear as if one star orbits the other like a planet orbiting its star.  

Looking at the night sky, binaries look like a single star to our eyes, but a pair of binoculars or a telescope will split the star into its binary components.  Some stars are just in the same line of sight, and too far to be a binary.  These are optical doubles.  

If you saw the movie Star Wars, there is an iconic scene showing Luke Skywalker near his home on the planet Tatooine.  In the distance, two suns setting around which his planet orbits.  Astronomers now believe that some binary and multiple star systems can have planets.  

One of the favorite double stars for amateur astronomers is Albireo, in the summer constellation Cygnus the Swan.  Albireo forms the head of the swan.  It appears as a single star, but in a small telescope, the observer sees two close stars with contrasting colors of gold and blue.  The colors of the star give us a clue as to their temperatures, with the blue star being the hottest, and thousands of degrees hotter than our sun.  

Astronomers are unclear if this pair is a true binary or an optical.  If they are true binary, their orbital period may be at least 100,000 years.