Binary star

A double star (also binary system ) is composed of two stars, are apparently or actually close together in the sky. "Real" (physical ) binary stars are gravitationally bound together and orbit periodically a common center of gravity.

In this mutual encircling each star has its own elliptical path, the major axis is inversely proportional to its mass in relation to the second star. The web speed and the distance between the two star changes in rhythm with the rotation time as the second of the animation illustrated. The first animation shows the smooth movement in the rare case of two circular orbits.

Similarly, there is a multiple star (also multiple system or multiple star system ) of three or more stars.

About 60-70 % of all stars in our Milky Way galaxy are part of a double or multiple star systems, which is related to the physical conditions of star formation. In tight clusters however, they are rare because of mutual perturbations.

Types of double stars

We distinguish the following types of binary stars:

  • Optical double star (apparent double star ): two stars that appear from Earth in almost the same direction in the sky, but not gravitationally affect each other. It is known to very flashy star pair α / β Centauri (distance 4.3 and 500 light years), the the southern sky around the "Southern Cross " makes it so appealing. This type of apparent double star - of which there are also much closer are together standing - indeed is of little interest for astrophysics, probably, however, for other areas of astronomy as the astrometry (very different self-motion! ), The sky photography or simply the freiäugige observing the night sky.
  • Physical double stars or binary systems: two stars that are gravitationally bound because of their proximity and move according to Kepler's laws around a common center of gravity. Most physical binary systems have already formed during star formation. Others have later united by trapping under the influence of at least one other star to a bound binary system. Captured double stars have usually due to their independent development of different ages and metallicities.
  • Geometric binaries ( spatial binary stars ): stars that are spatially close to each other, but are not bound to each other due to their high relative velocities and describe a common hyperbolic orbit around their common center of gravity. It is to the unique event of a star encounter, so the two stars form only for a limited time a (geometric) double star and never meet then possibly again.

The following are the physical double stars are treated.


About half of all stars in the universe are part of a binary system. Depending on the distance of the stars are from each other, the orbital periods of binary systems from a few hours (for very nearby stars ) or many thousands of years. The distance may be so low that the Roche limit is exceeded, so that the two stars are in material contact or matter can flow from one to another star. The importance of double stars for astronomy is that it is in their case, the chance to determine reliably with the help of Kepler's laws, the mass, the diameter and the density of stars.

The brighter of the two stars of a binary system is called the main star (or principal component ) and designated by the letter A, the fainter companion is and is denoted by B.

Multiple stars

A physical system of more than two stars is called a multiple star. Usually you discovered multiple stars initially as a double star. The often invisible companions make themselves noticeable as disturbances of the other components of the system. Multiple stars are composed of subsystems, which are always arranged in pairs. The subsystems consist in turn of a single or double stars.

Examples of multiple stars are:

  • η Orionis: a spectroscopic binary with a distant companion, orbital period of the binary star 8 days, the companion to the Double Star 3470 days.
  • ξ Ursae Majoris: appears as a double star with an orbital period of 59.6 years, but each component further includes a binary star system (with orbital periods of 4 and 699 days ).
  • AB Doradus: appears as a double star with an orbital period of 1600 years, but each component further includes a binary star system (with orbital periods of one and a half years ).
  • Mizar: It is unclear whether the binary star system Alcor is gravitationally bound to the four -star system, which is could be a six -star system in the case.
  • HD 98800 is a quadruple system consisting of 2 double systems. It contains dust disks and possibly planet.
  • ε Hydrae
  • α Geminorum (Castor ): three spectroscopic binaries with an eclipsing binary.

View the possibility of observation

We can separate double stars by the possibility of observation:

  • Visual Double Stars
  • Spectroscopic Binary Stars
  • Photometric binaries ( they form a subset of the spectroscopic binary stars)
  • Astrometric binary stars (stars with invisible companions )

Visual Double Stars

Visual double stars are well suited to determine the resolving power of a telescope. You simply select a number of double stars, each with about the same bright stars whose angular distance decreases. After observing a given device can be stated from what angle distance the stars can not be separately distinguished.

Spectroscopic Binary Stars

Spectroscopic binaries are no longer visually separate and be detected by abnormalities of the spectrum as such. In similar brightness overlap the spectra of both stars and form due to different spectral type, a composite spectrum. Is the difference in brightness of the two stars but larger than a size class, so the spectrum of the main star outshines the spectrum of the companion. In both cases show periodic shifts of spectral lines due to the periodic changes in the radial velocity of the star around the common center of mass at (Doppler effect) that it is a double star spectrum.

Photometric Binary Stars

You are eclipsing binary star and reveal their character through periodic change in brightness. The orbital plane of the components thus falls into the line of sight to the observer, so that cover both stars periodically. This change in brightness can be measured using photometric methods.

Astrometric binary stars

The astrometric binary stars reveal their nature as a result of periodically changed positions relative to other stars in the line of sight. This position changes are superimposed on the observed proper motion of the star and are caused by the circulation around a common center of gravity with an unseen companion. This method can also be searched for extrasolar planets.


The angular momentum of a gravitationally collapsing interstellar cloud, the probability for the formation of a binary system increases rather than a single star. Today it is assumed that stars form in groups in larger clouds ( " breeding grounds "). It is doing a great probability that such a star located in close proximity combine to form a system.

In addition, there is the possibility that in the context of three-body encounters, in which a star undergoes an increase in kinetic energy, leaving only the other two gravitationally bound.


Since ancient times, double stars were suspected. The star catalog of Ptolemy ( around 150 AD ) recorded the double star ν1 and ν2 Sagittarii: " The star in the eye [ the shooter ], which is foggy and twice ". This is not a physical double star.

The invention of the telescope then made the resolution of double stars possible. For the first time Johann Baptist Cysat 1619 describes a corresponding observation.

The Mannheim court astronomer Christian Mayer describes from 1775 double stars as physically related objects, which he calls " fixed star satellites ", and published in 1779 the first double star catalog. In the following years is also the name " double star" in use. William Herschel confirms the existence of physical double stars in 1800 and introduces the commonly used in astronomy technical term binary star. For the star 61 Cygni pair Friedrich Wilhelm Bessel in 1812 calculated the first time a stellar parallax.

Planets in binary systems

Also in double-star systems might exoplanets. There are two types of planetary orbits: planets of the " S- type" circle only one of the two stars and are practically not influenced by the other star, as the other star too far away and / or too low in mass. Planet from the "P- type" orbiting both stars far to the outside as if they were a single star. Depending on the constellation of the stars there are zones for S- and P- types of planets. There have already been discovered in recent years, some exoplanets in binary systems, and our next double star, Alpha Centauri, is even considered a potential candidate for planets that could harbor life in theory. The Kepler space telescope has discovered even equal to a binary star system in the year 2012 at the binary star system Kepler -47 two exoplanets on stable orbits.