A binary star is a stellar system consisting of two stars that orbit each other under their mutual gravitational attraction. A binary star system can be either an optical double, in which the two stars appear to be close together in the sky as seen from Earth, or a true binary, in which they are actually bound together by their mutual gravity.
The term “binary star” is generally used when both stars are similar in mass and size, and orbit each other at a relatively close distance. If one of the two stars is much more massive than the other, it is called a “stellar companion”. Binary star systems are very common in our Milky Way galaxy, with about half of all stars being members of such systems.
One of the most famous binary star systems is Beta Centauri (also known as Mimosa), which consists of two bright orange-hued giant stars. These stars are about 180 light-years away from Earth and have a orbital period of about 5 years. Another well-known binary system is Sirius, consisting of a white main sequence star (Sirius A) and a faint white dwarf companion (Sirius B). Sirius A is about twice as massive as our Sun and 25 times brighter, while Sirius B has only 1/8th the mass of our Sun but shines 1000 times brighter due to its extremely high surface temperature.
There are several different types of binary star systems, depending on how the two component stars interact with each other. In some cases, both stars are similar in size and mass and orbit each other at relatively close distances (such as Sirius), while in others one star may be much larger than the other (such as Beta Centauri). The type of binary system also affects how long it takes for the two stars to orbit each other; for example, Sirius has a very short orbital period because both stars are so massive that their mutual gravitational attraction is very strong. On the other hand, binaries with widely separated component stars can have orbital periods lasting for millions or even billions of years!
Binary systems can also be classified according to how their component stars interact with each other; for example, some binaries consist of twostars that simply pass by eachother without any noticeable interaction (these are called “wide binaries”), while others may have such tight orbits that tidal forces cause them to gradually merge into one single object over time (these are called “contact binaries”). There also exist what’s known as “eclipsing binaries”, where one star periodically passes in frontoftheother from our perspective here on Earth; this causes periodic changesin brightness as seen from us that can be used to studythe individual componentstars’ properties in great detail. Finally,”spectroscopic binaries” occur when we cannot directly seebothcomponentstars but instead detect their presence via Doppler shifts intheir spectral lines caused by their orbital motion aroundeachother – these make up around halfof all knownbinarysystems!