BINARY STARS

One could think that stars are isolated celestial bodies, just like the Sun. Actually, two thirds of the stars are gathered into multiple systems. Most of them are binary systems (that is formed by two stars alone, like Sirius), but there exist systems formed by three, four and even six stars.
These stars rotate around a common point, which is the baricenter of the system, each one describing an elliptical orbit. One focus of the ellipse coincides with the baricenter.

One should not mistake physical binaries with optical doubles, that is those stars that are close to each other due to just a perspective effect (for example, Mizar and Alcor, the stars of the "tail" in Ursa Major).
There are several different types of binary systems.
Visual binaries are those systems where both components can be recognized with a telescope.
The resolving power of telescopes is limited, so not all binary systems can be visually distinguished. However, the presence of two rotating stars can be revealed also in an indirect way. This is the case of eclipsing binaries, or photometric binaries. The stars move one around the other, and if their orbital plane is along our line of sight, then one will periodically hide the other, producing an occultation or eclipse.
In this case, we do not observe a real eclipse, rather a luminosity change of the system. An example is Algol, in the Perseus constellation, whose total magnitude changes from 2.3 to 3.5, in a period of 69 hours.
In other cases, the binary systems can be detected by means of the Doppler shift that can be seen in their spectral lines. Moving around the baricenter, the two stars alternatively approach and recede from the observer, so their spectrum is shifted to the blue or red, respectively. An example of these spectroscopic binaries is Mizar in Ursa Major.
 
 
 

Binary stars are particularly important, since sometimes they allow to measure the mass of the components, by analyzing the relative orbital parameters and using the Kepler laws. This is a fundamental operation, in order to assign a characteristic mass to the stars of each spectral type , and to provide some constraint to the stellar evolutionary models.
 

The red dwarf Wolf 359 and the binary system Wolf 424 A and B. Wolf 424 B, the smaller star of the binary system, probably has a mass lower than 0.08 solar masses, which is the minimum mass for the nuclear fusion reactions to ignite in its core. (HST)

 
 
 

The binary system M40, in Ursa Major. (SEDS)