Binary stars can appear to us in many different forms. The different types explained below are called visual, astrometric, eclipsing, and spectroscopic. A single binary system can be labeled with more than one of these names: for example, the two stars could be seen to be both eclipsing and spectroscopic.

Remember, the key concepts relevant to binaries are Kepler's Laws.  The orbits are ellipses or circular (Kepler's First Law)and the stars orbit around their common center of mass (Kepler's Third Law after Newton's modification), which is closer to the more massive star.  But both stars must orbit because they are pulling gravitationally on each other.  Neither can remain stationary.  Finally, a star orbits faster when it is closer to the center of mass (Kepler's Second Law).

There are also optical doubles, which are not binaries. They are also described below.

Visual Binary Stars
 
 
 
If two stars are orbiting each other and if we can we can see both of the stars then we call the system a "visual binary." Those of you who have seen Albireo or gamma Andromedae from the campus observatory have seen good examples of visual binary systems. The positions of the two stars that make up gamma Andromedae are shown above.

Astrometric Binary Stars (Invisible Dancers)

As the two stars in the binary system orbit each other they move back and forth on the sky. Sometimes we can only see one of the two stars (because the second is too dim). When we see one star moving back and forth on the sky (as if it were in an orbit) then we assume the presence of a binary partner star. We call such systems "astrometric" because it is our measurements of the position (the astrometry) of the moving star that lead us to the conclusion that it is a binary system. The binary partner of the bright star Sirius was discovered in this way.



Eclipsing Binary Stars

For some binary systems, but not all of them, Earth is looking at the orbit of the two stars so that we see one star pass in front of the other star as they orbit each other. Earth does not have this perspective on all binary systems, but those systems from which we can observe eclipses give us valuable information.



Spectroscopic Binary Stars

 Sometimes when astronomers observe a point of light on the sky, they may not be able to classify the star as any of the other binary types. It may be impossible to see both stars in the telescope, or the only visible star may not move enough to be noticeable, or it may not change brightness. However, a spectrum may reveal that we are in fact looking at a binary system.

The most important feature of such a spectrum is that the two stars' spectra may be "Doppler shifted". Continue on to the Doppler Shift page of the lab to learn about this.

A "single-line" spectroscopic binary is a binary system where the spectrum reveals only one star and there is a detectable Doppler Shift that changes in a periodic way, while a "double-line" spectroscopic binary is one where the spectrum is the combined spectrum of two stars, each of which shows a periodic Doppler Shift.


Optical Double Stars

Not all stars that appear next to each other in the sky are orbiting each other. Some stars just appear next to each other because they lie along the same line of sight. These are called "optical doubles." How many of the stars in this picture of the sky do you think are actually next to each other? Unfortunately, since optical double stars are not actually close and, therefore, are not actually orbiting each other, such "systems" cannot help us learn about the individual stars. They just look cool.