Binary Star or Double Star | Why are Binaries Important? | ScienceMonk


What is a Binary Star?

Generally, a binary star system (before naming used to be called as double stars) is one in which two stars which are gravitationally bound, orbit around a common center of mass, obeying Kepler’s laws of motion.

Of which, the brighter one among the binary is called as the primary star and other as a secondary star. Sir William Herschel coined the word ‘Binary’ in 1802.

Binary Stars


Classification of Binary Star:

Based on orbital motion, they are classified as wide pair and close pair. If two stars orbit around each other at larger separation, then they are called a wide pair binaries. In wide pair, both the stars evolve independently with very little impact from its respective companion.

If two stars orbit around each other very closely such that mass transfer may happen due to tidal forces then they are called as close or contact pair binaries. In close or contact binaries the mass transfer may occur between primary to secondary or vice versa.

In some scenarios, the primary star exerts strong gravitational force and may pull secondary entirely on to it. Also, Binary systems are classified according to their means of detection. They are:

  • Visual Binary System
  • Astrometric Binary System
  • Spectroscopic Binary System
  • Eclipsing Binary System

Visual Binary System:

A visual binary is a binary system in which the components are gravitationally bound and can be individually resolved through a telescope. The components of this system are very widely separated from tens to a few hundred A.U.s.

In 1780, William Herschel measured the separation and orientation of over 700 double stars and found that among them only 50 pairs changed orientation over two-decades of observations. F. J. W. Struve standardized the method of visual binary observation.

Around 10% of visible stars are visual binaries. Less than 1,000 visual binary systems have been detected.
Examples of visual binaries that can be resolvable through small telescopes include α Crucis, β Crucis, and γ Cen and Castor in Gemini Constellation.
There are some stars that appear very close to each other but are not gravitationally bounded and actually can be hundreds of parsecs apart. They are called optical pairs, which are not true binaries.

Astrometric binary System:

A binary inferred from the perturbation, i.e., the wobble of its motion by an unseen companion where one of the components is very brighter and another very faint is called an astrometric binary.

Precise measurement of stellar motion is required to find such small wobbles is a part of the field called astrometry. Such measurements can only be performed on stars near-by 10 parsecs because they have a relatively very high proper motion, so these binaries appear to follow a sinusoidal path across the sky. This method is also helpful in finding extra-solar planets around such stars.
Friedrich Bessel was the first to detect the line of Sirius path is slightly wavy rather than straight. Sirius A&B system is the best example of Astrometric binary.

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Spectroscopic Binary System:

The binary system in which the stars orbit very close to each other or are very far away, that they can’t be resolved through a telescope, but their orbital motion can be detected in their spectra are called spectroscopic binaries.

There are two types of spectroscopic binaries if the taken spectra show only one set of lines, then they are called as single-line spectroscopic binaries, and if it shows the lines from both the components, then they are called as double-line spectroscopic binaries.

In spectroscopic binaries, if the components are very close, then there may be an exchange of mass between them due to tidal interactions. In many cases, orbital periods may range from a few hours to months, with separations of much less than an A.U. and for double-line spectroscopic binaries, it may vary from hours to decades. An example of this class is Mizar in Ursa Majoris.

Eclipsing Binary System:

The binary star system in which the orbit plane of the two stars lies in the line of sight of the observer that they undergo eclipses alternately. These binaries offer a straightforward way to measure the distances to galaxies to a 5 percent level of accuracy.
Eclipsing binaries are variable stars, not because the luminosity of the individual components differ but because of the eclipses. The light curve of an eclipsing binary is characterized by periods of almost constant light, with periodic drops in intensity.

If the components are of not similar sizes, then one will be obscured by a total eclipse while the other by an annular eclipse. The orbital period of an eclipsing binary may be determined from the study of the light curve, and the comparative sizes of the individual stars can be determined in terms of the orbital radius by observing the brightness changes.
The first detected eclipsing binary was Algol, β Perseus, also known as the Demon Star. Others include β Lyrae, W Ursae Majoris, etc.
The study of eclipsing binaries is very important as they provide details of the physical behavior of stars, their complete orientation in space, limb darkening, oblateness, reflection, gravitation, etc.

Roche Lobe-Classification of Binary Star system:

Roche lobe is a tear-drop shaped equipotential region around the stars in the binary system, where the matter is bound to the star. If anyone of the stars in the binary system, in its evolutionary phase, expands to fill its equipotential surface till inner Lagrangian point L1, then its atmospheric gases escape and drawn towards its companion.

The component which has filled its Roche lobe and is losing mass is usually stated as the second component in the system where its accreting companion is the primary star, note that it can be either more or less massive than the secondary star.
Based on this configuration, the binary systems are classified as detached, semi-detached and contact. When both the binaries fail to fill this region, then such systems are called as detached binary systems.

In this case, the primary star evolution is not affected as long as the binary does not come into contact. When one of the binaries have expanded beyond this Roche lobe, mass transfer to its companion takes place. Such systems are called semi-detached systems.

There is also a case where both stars expand to (over) fill their Roche lobes, and the two stars share a common atmosphere circumscribed by a dumbbell-shaped equipotential surface. Such systems are referred to as contact binaries.

This means that the stars can be observed to be either in geometrical contact or thermal contact or both along with the mass transfer. The Roche lobe overflow which is triggered either by a star filling its Roche lobe or by angular momentum losses causing contraction of the orbit can be responsible for numerous astronomical/astrophysical activities such as novae, supernovae (Type Ia), etc.

Why are Binaries Important?

Binary stars are very enthralling astronomical objects which play a key role in understanding fundamental properties of stars, their physical behavior, and orientation. They are very common, i.e., nearly 70-80% of the stars we observe in the night sky are binaries.

Also, 85% of stars in our Milky way are multi-star systems, i.e., binaries or triple or quadruple star systems. Since determining the stellar mass of a single star is very difficult, binary stars are of immense importance to astronomers as they allow the masses of stars to be determined by analyzing their orbits. Binary star evolution plays a vital role in the formation of many interesting astronomical objects.

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