Multiple star systemsEdit

Systems with more than two stars are termed multiple stars. It is estimated that approximately 1/3 of the star systems in the Milky Way are binary or multiple, with the remaining 2/3 consisting of single stars. The brighter star is called the primary and the other is its companion star. 

Some orbital ranges are impossible for dynamical reasons (the planet would be expelled from its orbit relatively quickly, being either ejected from the system altogether or transferred to a more inner or outer orbital range), whilst other orbits present serious challenges for eventual biospheres because of likely extreme variations in surface temperature during different parts of the orbit. 

Multiple star systems are called triple, trinary or ternary if they contain three stars; quadruple or quaternary if they contain four stars; quintuple with five stars; sextuple with six stars; septuple with seven stars; and so on. 

Binary starsEdit

A binary star is a star system consisting of two stars orbiting around their common center of mass. It is estimated that 50–60% of binary stars are capable of supporting habitable terrestrial planets within stable orbital ranges.

The components of a binary star system may be designated by their relative temperatures as the hot companion and cool companion.

In pairs where the two stars are of equal brightness, they are also of the same spectral type. In systems where the brightnesses are different, the fainter star is bluer if the brighter star is a giant star, and redder if the brighter star belongs to the main sequence

If components in binary star systems are close enough they can gravitationally distort their mutual outer stellar atmospheres. In some cases, these close binary systems can exchange mass, which may bring their evolution to stages that single stars cannot attain. 


An eclipsing binary star is a binary star in which the orbit plane of the two stars lies so nearly in the line of sight of the observer that the components undergo mutual eclipses. If one of the stars is larger than the other, one will be obscured by a total eclipse while the other will be obscured by an annular eclipse.

Orbital periods can be less than an hour, or a few days, but also hundreds of thousands of years.


While it is not impossible that some binaries might be created through gravitational capture between two single stars, given the very low likelihood of such an event and the high number of binaries, this cannot be the primary formation process. It is most probable that binaries are already formed during star formation.

Runaway starsEdit

It is also possible for widely separated binaries to lose gravitational contact with each other during their lifetime, as a result of external perturbations. The components will then move on to evolve as single stars. A

close encounter between two binary systems can also result in the gravitational disruption of both systems, with some of the stars being ejected at high velocities, leading to runaway stars.

In extreme cases this event can cause the white dwarf to exceed the Chandrasekhar limit and trigger a supernova that destroys the entire star, and is another possible cause for runaways.

Astrometric binariesEdit

Astronomers have discovered some stars that seemingly orbit around an empty space. Astrometric binaries are relatively nearby stars which can be seen to wobble around a point in space, with no visible companion. The companion could be very dim, so that it is currently undetectable or masked by the glare of its primary, or it could be an object that emits little or no electromagnetic radiation, for example a neutron star.

Configuration of the systemEdit

Detached BinariesEdit

Detached binaries are binary stars where each component is within the area where the gravitational pull of the star itself is larger than that of the other component. The stars have no major effect on each other, and essentially evolve separately. Most binaries belong to this class.


Semidetached binary stars are binary stars where one of the components fills the binary star's area of gravitational pull and the other does not. Gas from the surface of the gravitational area component (donor) is transferred to the other, accreting star.

Contact binaryEdit

A contact binary is a type of binary star in which both components of the binary fill their area of gravitational pull. The uppermost part of the stellar atmospheres forms a common envelope that surrounds both stars. As the friction of the envelope brakes the orbital motion, the stars may eventually merge.

Cataclysmic variablesEdit

When a binary system contains a compact object such as a white dwarf, neutron star or black hole, gas from the other star can accrete onto the compact object. This releases gravitational potential energy, causing the gas to become hotter and emit radiation.

X-ray binariesEdit

In X-ray binaries, the compact object can be either a neutron star or a black hole. These binaries are classified as low-mass or high-mass according to the mass of the donor star. 

High-mass X-ray binaries contain a young, early type, high-mass donor star which transfers mass by its stellar wind, while low-mass X-ray binaries are semidetached binaries in which gas from a late-type donor star overflows the area gravtiational pull and falls towards the neutron star or black hole.