The Binary Star Systems

Just like we have siblings, do you know that even the stars in our universe can have siblings! Rather, a solar system like ours, with its single sun, is actually quite rare to find. Most star systems contain their solar twins, and the 16th article of our series Basics of Astrophysics, aims to explore the binary star systems in detail.

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What are Binary Stars?

A binary star system is the one in which two stars are gravitationally bound to each. In these kind of star systems, either one star revolves around the other or both of them revolve around a common centre of mass. Most stars in our universe are in binary systems. The brighter star is officially classified as primary star (A). The dimmer of the two is secondary (B). In cases where the stars are of equal brightness, the designation given by their discoverer is taken into consideration . According to some estimates, up to 85% of stars are in our night sky are binary systems. Some of them are in triple or even higher-multiple systems.

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Classification of Binary Star Systems

Binary stars are basically classified as either wide or close binaries. As the name suggests, in wide binaries, the orbits of the two stars keep them far apart from each other. Throughout their life, these stars have little or no effect on one another.
On the contrary, in close binaries, the companions are close enough to interact physically. The gravitational pull of one star can deform and sometimes devour the other star. Sometimes, the transfer of matter from one star to another also takes place.


An example of close binaries

Apart from this basic classification, binary systems are also categorised on the basis of their detection as follow :

Visual Binaries

Visual binaries are two stars which are far enough from each other to be easily resolved by a telescope. Long-term observations are made to plot the relative positions of the members of the system and to calculate the orbits of the stars. About 5 to 10 percent of visible stars are visual binaries.

Spectroscopic Binaries

The companions in this system appear close even when viewed through a telescope. So, to observe these and study their individual behaviors, the behavior of their spectral lines is noticed. The majority of binary systems have been detected by Doppler shifts in their spectral lines. If a binary system is unresolved into its components then its spectrum will actually be a combination of the spectra from each of the component stars. Analysis of the spectral line shifts versus time helps to reveal information about the radial velocities of the component stars. Several other factors are also involved in the spectroscopic detection of binaries.

Eclipsing Binaries

Binary Star Systems
Eclipsing Binary stars

These are two stars whose orbits are at an angle with respect to one another. So, as observed from Earth, when one passes in front of the other, it appears to cause an eclipse. This feature is based mainly on the line of sight rather than any particular feature of the pair.

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Astrometric Binaries

Some stars, if observed repeatedly over time, show some remarkable changes in their proper motion. If this is a periodic occurrence, we can infer that these perturbations are occurring due to the gravitational influence of an unseen companion. In simple words, the primary star appears to dance around empty space, as the other sibling is too dim to be observed. Relatively few binaries have been detected astrometrically. That is because this method requires long-term observations and the uncertainty in position and proper motion measurements. 

Importance of Binary Star Systems

Binaries provide the best method for astronomers to determine the mass of a distant star. The gravitational pull between the partners causes them to orbit around their common center of mass. From the orbital pattern of a visual binary, or the time variation of the spectrum of a spectroscopic binary, the mass of its stars can be determined. Using this data, the relation between a star's appearance and its mass can be found. This allows for the determination of the mass of non-binaries. Some of the binary systems can also act as amazing test bed for General Relativity and search for gravity waves. Binaries are also expected to produce the conditions which are suitable for evolution of life.

Author's message

I hope this article gave you a basic information about the binaries. With new advancements in technology, new discoveries in the field of multiple star systems are taking place on a regular basis. Binaries have become a hot topic of research these days. One of the recent debate going on in this field is about our Sun being a part of a binary system in the past. Though a lot is known by now, still a lot more needs to be explored. So, if you are looking for some fresh research opportunities in field of Astronomy and Astrophysics, the study of binaries can be one option.

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