Binary Stars: Mizar and Alcor

Did you know that some of the brighter stars in the sky aren't just stars alone? Many of the bright stars you can see in the night sky are actually binary stars. Binary stars are stars that orbit around the same center of mass and are therefore gravitationally bound because of this orbit. 

There are many types of binary stars based on the way we are able to detect the binary system. Some of these types including visual, spectroscopic and ecliptic binaries. Visual binaries are binary star systems that can be resolved into the multiple star components with a telescope. Spectroscopic binaries are much more distant making them harder to resolve with telescopes. Instead, they are detected by Doppler shifts in their spectral lines.  Ecliptic binaries are also unable to be resolved by telescopes, but can be detected by a change in magnitude when the stars eclipse one another. 

One fascinating binary star system is Mizar and Alcor. This binary star is seen in the handle of the constellation Ursa Major, more commonly known as the Big Dipper. Mizar and Alcor sit at a distance of 85.8 and 81.5 lights years from Earth and have apparent magnitudes of 2.2 and 4.1. Interestingly, Mizar and Alcor are one of the few binary stars that are able to be seen split as two stars with just your eyes alone. Mizar is the brighter star with Alcor sitting faintly beside it. 

Mizar and Alcor are a visual binary, but there is more to this star system than just these two stars. Mizar has a much closer binary star companion known as Mizar B, making Mizar known as Mizar A. This binary pair is also a visual binary star. However, there is still more to this system. Mizar A, Mizar B and Alcor also each have a spectroscopic binary companion. 

References

https://www.atnf.csiro.au/outreach//education/senior/astrophysics/binary_intro.html

https://astronomy.swin.edu.au/cosmos/c/Centre+of+Mass

https://www.atnf.csiro.au/outreach//education/senior/astrophysics/binary_types.html

https://skyandtelescope.org/observing/celestial-objects-to-watch/meet-mizar-and-alcor-the-horse-and-rider/