What is the difference between binary and double stars?

I am REALLY REALLY confused I'm trying to research double stars, binary stars and variable stars. But after reading all the information I've gathered it seems like double stars and binary stars are exactly the same things!! Whats the difference!?|||They are not the same thing. Binary stars are actual physical systems, where the two stars revolve around each other. Double stars include binaries, but also include visual doubles, where two stars happen to lie along the same line of sight, but are not physically close to each other and don't form a physical system.





Variable stars vary in brightness. Some are close binary stars where one star eclipses the other, but most are intrinsic variables, where the star itself varies in brightness because of astrophysical processes going on in its atmosphere.|||Binaries and doubles are almost the same.





All binaries are doubles.





Most doubles are binaries.





The difference, can be between a true double and a visual double.





A "true double" binary star system is two stars that are orbiting a common ceneter of gravity.





A "visual double" star, is one where two stars appear along the same line of sight as viewed from earth. But they are not gravitationally linked. They appear as a double star because the angle we see them at. Often times, in "visual double" stars, one is much, much closer to us than the other, so they "look like" they are two stars that are close together.





True binaries really are close together.








Variable Star:


A star that "varies" in brightness.





Basically, there are two kinds (there are more, but most are sub categories, and I wish not to confuse you more).





Eclipsing Binaries:


A true double star, where the two stars are orbiting each other. From earth, the fainter one, in its orbit, will pass in front of the brighter one, blocking out some of it's light, creating a "variance" in the the brightness of the "star system" as a whole.





Cephied Variables:


Stars that have specific luminosity variances. These are actually used to detect the distance of many galaxies, if a Cephied can be detected. Because we know their true magnitude, and that light diminshes by the square of the distance, we can calculate the distance based on the brightness of one of these (research Edwin Hubble!!!) ... Wikipedia can explain these stars very well and accurately





http://en.wikipedia.org/wiki/Cepheid_var鈥?/a>