Binary stars

So my first blog is gonna be about binary stars. Binary stars are quite common throughout the Universe. But what is a binary star ? A pair of stars located at nearly the same position in the night sky is called a double star. 

A pair of binaries "very close"

Double stars however could be either optical doubles or true binaries. Optical doubles appear together in the night sky from Earth but could be hundreds of light years apart. On the other hand true binaries orbit each other and are quite close together. The distance separation between two binary stars could be on the order of few AUs.

Binary stars orbit around their common centre of mass, often called their barycentre.

True binaries, or just binaries, are useful to astrophysicists because they are among the few ways to find the mass of stars. Binaries can be detected by few particular ways.  

Binaries can be differentiated from single stars by looking at the spectral lines of the star. Single stars have only one characteristic pattern of spectral lines. On the other hand binaries have different chemical composition, hence have different spectra and can be differentiated from the loners.

After determining that the star is binary, astronomers use Doppler shifting to find the orbital velocities of the revolving stars. The semi major axis of the orbit which the stars revolve can also be found. Using these data astronomers find the masses of the binary stars using Kepler's laws.

Binary stars can sometimes be monstrous like the one shown in the above image and may swallow matter from their companion stars. The older star pulls mass from the younger one and grows and swells up. This mass is mainly hydrogen, so the old stars uses it for renewed fusion and regains its youth. It becomes bluer as it swallows matter from its companion. These type of stars are called vampire stars.Once it reaches a limit, it blows up in a supernova and the victim star loses its companion. 

Binary stars whose orbits lie in a plane which is edge on to the observer on Earth appear to eclipse each other at specific intervals of time. These binaries are termed as eclipsing binaries and are useful in determining luminosities of the individual stars. 

In some binaries the gravitational pull of one star over the other could be so high that when the weaker star orbits around it, it gets spun around so hard that it could be flung away with a very high velocity that it escapes its gravity. Such a star wanders around the Universe alone and could have been the history of the single stars we see today.

Some binary stars maybe such that one star is very heavy than the other. So the common centre of mass, the barycentre lies inside the heavy star. So the heavy star appears to rotate around a point inside itself, ie , it appears to wobble.

Binary stars are designated as A and B, the brighter one being A and the dimmer one B. An example is the binary system Sirius which consists of two stars Sirius A and Sirius B (In adjoining image, the bigger and brighter one is Sirius A and the small blue one is Sirius B ).

Stars can also exist as groups of more than two. These systems are called multiple star systems. An example is the star Castor in the constellation Gemini which consists of six stars. 

To sum it up binary stars are interesting and fun to observe.