A double star is the term used to describe two stars that are so close together in the sky that they have a small angular distance when viewed from the earth or possibly appear as a single star with the best optics and are at least very likely gravitationally bound. In this case, the stars form a physical unit, i.e. they circle around the common center of gravity. The exact mass of the two stars can be determined from this movement , which is of great importance as a "calibration method" for astrophysics . Double stars that can no longer be separated in the telescope can often be recognized spectroscopically or by their movement. Binary stars are a common phenomenon and for example the closest system to the sun , Alpha Centauri , is a multiple system consisting of the double star Alpha Centauri A and B and the distant companion Proxima Centauri .
In a "real" (physical) double star (also binary system, English binary star ), the single star gravitationally bonded to each other and encircle periodically the common center of gravity with orbital periods between a fraction of a day, many thousands of years . In the so-called telescopic double stars (appearing as a pair of stars in a telescope), the periods are predominantly in the range from 50 to 500 years. When circling each other, each star has its own elliptical orbit , the major semi-axis of which is inversely proportional to its mass in relation to the second star . The orbital speeds and the distance between the two stars change with the rhythm of the orbital period, as the second animation shows. The third animation shows the even movement in the rare case of two circular paths .
In the case of the apparent, only optical double stars, on the other hand, the single stars are not physically related and have very different distances to the earth. Optical double stars as a coincidence are only of interest to amateur observers; In specialist publications, the term “double star” is therefore used throughout to mean “physical double star”. The term “foreground” or “background star” is used here for the stars that are only in the field of view.
Similarly, a multiple star (also multiple system or multiple star system ) consists of three or more stars. In triple systems, a double star is circled by a distant companion, in quadruple stars two close double stars usually orbit each other (e.g. Epsilon Lyrae ). For even larger systems, there are several ways in which the system can be constructed. The most component-rich known systems are with seven stars Jabbah and AR Cassiopeiae in the constellation Kassiopeia .
The first double stars were already known in antiquity, although no difference between visual and physical was to be expected in the idea of a fixed star sphere at that time. The star catalog of Ptolemy (.. 150 AD) records the (optical) binary ν 1 and ν 2 Sagittarii : "The star on the eye [the shooter], the foggy and twice is". The star couple Mizar / Alkor in the Big Dipper was also known and the subject of myths.
The invention of the telescope then made the discovery of many binary stars possible. The first such observation was made by Johann Baptist Cysat in 1619 . In 1651 Giovanni Riccioli published the knowledge that the above-mentioned Mizar itself consists of two components (now called Mizar A and B). The Mannheim court astronomer Christian Mayer described double stars as physically related objects from 1777 onwards. Other astronomers, however, doubted its so-called “fixed star satellites”. In 1779, Mayer published the first double star catalog with 72 objects including their distances and celestial coordinates.
In 1782 John Goodricke suspected that the " eclipsing " Algol, known since 1667, could also be a double star . He observed the period of light change very closely (2.87 days) and suspected it was obscured by a large body or an uneven surface with spots similar to those on the sun. In a letter from John Michell to Henry Cavendish in July 1783, the phenomenon was explained with two different stars.
Up to Christian Mayer (1777) and Wilhelm Herschel's stellar statistics (from 1780), double stars were only considered to be perspective effects. Wilhelm Herschel confirmed the existence of physical double stars around 1800 when he determined the orbital movement of four of them in 20 years. He can therefore be regarded as the real “discoverer of the double stars” - although Johann Heinrich Lambert, John Mitchell or Christian Mayer had similar thoughts before. Herschel introduced the technical term binary star , which is used in English-speaking astronomy (in German the term “double t star” was also used for a time). His first double star catalog (1782) contained 269 objects, which he increased to 850 by 1803. From now on more and more astronomers dealt with them and were thus able to prove the validity of Newton's law of gravitation up to great distances.
Friedrich Wilhelm Struve took micrometric measurements on 2,714 double stars from 1824 to 1837. In 1827 he published the Catalogus novus stellarum duplicium, expanded in 1837 to include Stellarum duplicium et multiplicium. For the star pair 61 Cygni , Friedrich Wilhelm Bessel calculated a star parallax for the first time in 1838 , with two well-located background stars enabling a particularly precise series of measurements.
Until 1880, only systems with an angular difference of 0.5 "could be measured well, but with the new giant telescopes from Vienna and Pulkowo this limit could be halved. Sherburne Burnham even lowered it to 0.16 "on the Lick Observatory's 91 cm refractor in 1890 . A great advance in 1889 was the detection of close pairs of stars through their periodic shift of spectral lines due to the Doppler effect . Today they are known as spectroscopic binary stars . Such line shifts were initially only observable in bright stars such as Mizar , Spica , Algol and Beta Aurigae .
By 1895, 11,000 double stars were already known, 800 of them with precisely measured orbits. About 50 turned out to be fourfold to sixfold stars, some with very eccentric orbits. Thomas See modified the Kant-Laplace hypothesis in 1893 in order to explain the formation of double and multiple star systems from a primordial nebula and rotating equilibrium figures. At that time, several stars were also published as multiple systems that were later not confirmed - e.g. B. Gemma , α Delphini or o Orionis . At the turn of the century, spectroscopy was not yet fully developed, so that single- spectrum shifts could not be reliably differentiated from other anomalies.
Towards the end of the 19th century, the proportion of orbiting double stars was estimated to be just under 20% of all fixed stars. According to current knowledge, however, 60 to 70% of all stars in the Milky Way are part of double or multiple star systems, which is related to the physical conditions during star formation . They are only rarer in tight star clusters because of mutual orbital disturbances .
Types of binary stars
A distinction is made between the following types of double stars:
Physical binary stars or binary star systems
Two stars that are gravitationally bound due to their spatial proximity and move around a common center of gravity according to Kepler's laws . Most physical binary star systems have already formed during star formation . Others only later united to form a bound binary star system through capture under the influence of at least one other star. Captured double stars usually have different ages and metallicities due to their independent formation .
Optical double stars (apparent double stars)
Optical double stars are stars that stand in almost the same direction in the sky from the earth, but which are so far away from the earth that they do not influence each other gravitationally . Well known is the very conspicuous star pair α / β Centauri at only 4 ° angular distance, which makes the southern sky around the "Southern Cross" so attractive. The true distances are, however, 4.3 and 530 light years, respectively .
This type of apparent double stars - of which there are also much closer ones - is hardly of interest for astrophysics , but it is for other areas of astronomy such as astrometry (very different proper movement !), Celestial photography or simply for free-eyed observation of the starry sky .
Another, but not yet fully clarified example is the “ eye tester ” in the constellation Big Dipper , consisting of two stars at an angle of 11 ′: the lighter Mizar (ζ¹ Ursae majoris, distance 78 light years) and the “little rider” Alkor (ζ² UMa , 81 light years) in the middle of the "wagon drawbar". The two stars are about 3 light years apart, which is far beyond the size of the solar system (6 light hours to Pluto ) and is more comparable to the distance to our neighboring stars Proxima and α Centauri .
It is not yet completely clear whether the two stars Mizar and Alkor really circle each other because of the large distance and the resulting small orbital curvature. Alkor is approaching the larger Mizar star system (which is itself a close quadruple system), but the relative speed could be too great for permanent proximity (→ hyperbolic orbit ). In the positive case, the mutual orbital period would be about 1 million years . The double star Mizar / Alkor can be easily separated with the naked eye with normal eyesight - the "little rider" (2 levels of brightness less) sits Mizar 0.19 ° north.
Geometric double stars (spatial double stars)
Stars that are spatially close to one another, but are not bound to one another due to their high relative speeds and describe a common hyperbolic orbit around their common center of gravity. It is a one-off event of a star encounter , the two stars only form a (geometric) double star for a limited time and then never meet again.
Proxima Centauri was previously thought to be a possible geometric companion to Alpha Centauri . However, since 2016 it has been clarified that Proxima Centauri is gravitationally bound to the other two stars of Alpha Centauri and that it is therefore not a geometric double star. See also: Affiliation of Proxima Centauri to Alpha Centauri .
Classification according to observation method
You can classify double stars according to the possibility of observation:
- Visual double stars (mostly only with the telescope )
- Photometric double stars (variable brightness through mutual coverage)
- Spectroscopic binary stars (Doppler shift in the spectrum)
- Astrometric binary stars (change of position)
- X-ray binary stars (invisible component is a neutron star)
Visual double stars
... can be observed optically (sometimes even with the naked eye). Double star systems that can be separated with the naked eye are also called eye checkers . Mostly these are near-Earth stars (r <200 ly) or stars with an apparent brightness smaller than magnitude 6, which circle around the common center of gravity, but are relatively far apart, such as Mizar (ζ UMa) and Alkor . (Mizar itself is a quadruple system that cannot be resolved either with the naked eye or with a telescope.)
Telescopic double stars
... can only be seen separately in the telescope (angular distances from 0.1 ″ to about 100 ″). The periods of rotation are mostly a few decades to centuries. The path elements can be determined from the movement . The star pairs are also suitable for determining the resolving power of a telescope. A number of binary stars with similarly bright components but decreasing angular spacing are selected for this. The observation series on the telescope shows the distance from which the stars can no longer be separated. An example of a binary star system whose components can be separated with a very simple telescope is α Centauri .
Spectroscopic binary stars
... circle so closely that they can no longer be separated in the telescope. They reveal themselves through anomalies in their line spectrum or a periodic Doppler effect : When one star moves towards us, the other moves away. In the common spectrum the spectral lines split up into blue and red and you can even measure their radial velocity.
If the brightness is similar, the two color bands overlap to form a mixed spectral type . However, if the difference in brightness between the two is greater than one magnitude , the spectrum of the main star outshines that of the companion and the line shift can only be determined to one side.
The turnaround times of these close pairs are a few hours to weeks. Periodic line shifts were first detected in 1889 for the stars Mizar, Spica, Algol and Beta Aurigae.
Photometric (eclipsing) binary stars
... are eclipsed and reveal their double star character through periodic changes in brightness. The orbital plane of the components falls approximately in the line of sight of the observer, so that both suns periodically obscure each other. This change in brightness can be measured using photometry . In addition to the luminosity, the diameters of both stars can usually also be determined from the special features of the light curve .
Astrometric binary stars
... reveal their nature through periodic changes in position relative to other stars in the background . These changes are superimposed with the proper motion of the observed star to form a kind of serpentine line and are caused by the orbit around a common center of gravity with an invisible companion. This method is also used to search for extrasolar planets .
X-ray binary stars
Properties of physical binary stars
Over half of all stars in our Milky Way (possibly even 70%) are part of a binary star system. Up to a distance of 20 light years it is around 60%.
The brighter of the two stars is the main component or main star named with the letter A designates the fainter's companion and with B , respectively.
All of the spectral classes are represented, with a slight overhang from A to G, with spectroscopic pairs from B to F. There are no differences to individual stars in terms of luminosity, spatial distribution and movement. Most of the main sequence stars are like these , but systems with 1 or 2 giants are also represented.
The orbital times depend on the distance between the stars
- with close, spectroscopic pairs usually between a few hours and weeks,
- with telescopic mainly for years to centuries,
- with extreme cases of millennia (e.g. with the eye tester in the big car), but where the paths and affiliations are not always clear.
The significance of the double stars for astronomy is that in their case there is a chance of reliably determining the mass , diameter and density of stars with the help of Kepler's laws . This is particularly accurate with a precisely measurable radial velocity and with photometric double stars.
Interaction of the components
A distinction is made between the type of double stars depending on the mutual influence.
- Detached binaries (separate double stars) orbit each other at a greater distance and therefore interact little.
- Semidetached binaries (half-separated double stars) circle each other at a much smaller distance and thus already interact with each other. This means that at least one of the stars exceeds the Roche limit . This often leads to a mass transfer to the other star.
- Contact binaries (double stars in contact) orbit each other at a minimal distance and thus interact very strongly. In extreme cases, they can merge or merge in a common shell .
The distance in a binary star system can be so small that the two suns almost touch and matter can flow from one component to the other. Stars that show this phenomenon are known as interacting binary stars . Particularly in close binary star systems, the components can interact so strongly that they reach states that would not be possible with a single star. This special situation means that many phenomena in variable stars can be traced back to the binary star nature of a system.
Formation of physical double stars
With the angular momentum of a gravitationally collapsing interstellar cloud , the probability of the formation of a binary star system instead of a single star also increases. Today it is assumed that stars are formed in groups in larger clouds (“breeding areas”). There is a high probability that such close together stars will combine to form a system.
In addition, there is the possibility that in the context of three-body encounters, in which one star experiences an increase in kinetic energy , the other two remain gravitationally bound.
Development of the components
In general, both stars in a binary system are of the same age. Due to the fact that the components can influence each other under certain circumstances, however, stages of development are sometimes observed that do not occur with individual stars. A possible mass transfer between the components has the greatest influence. Often times the evolution begins to deviate when the more massive component comes to the end of the main sequence phase and evolves into a giant star . The expansion of the star's shell increases the likelihood that matter will flow off onto the smaller companion, which can massively change the development of both components. An example is the eclipsing star Algol , in which the more massive component still lingers in the main sequence, while the lighter component is already in the subgiant phase . This observation is explained by the fact that the now lighter component was originally the massive one and has lost so much mass to the companion that it is now the lighter one. Another extreme example of mutual influence are the EL Canum Venaticorum stars . In these systems the white dwarf has such a low mass that the original star should actually still be in the main sequence phase at the current age of the universe. These white dwarfs can only be explained by an extreme loss of mass to the other component.
The mutual influence can also turn around. For example, when the more massive component has its giant phase behind and has evolved to the white dwarf, it can be played back material from the remaining star accrete . Such stars can in part be observed as cataclysmic variables . Cataclysmic systems are considered to be the source of dwarf novae , novae as well as the supernova of type Ia .
It is believed that most of the hyperspeed and runaway stars observed stem from a double or multiple star system. If the system is disturbed by passing stars, the stars in the system can be torn apart. This accelerates them enormously and in extreme cases can even reach the necessary escape speed to leave a galaxy.
A physical system of more than two stars is called a multiple star or multiple star system . Most of the time, multiple stars are first discovered as double stars. The companions that have not been observed until then or have not been recognized as such then become noticeable as disturbances in the other components of the system. Multiple stars consist of subsystems that are always arranged in pairs. The subsystems in turn consist of single or double stars. The graphic opposite shows possible combinations of a double star system (b) up to a five-fold system (f).
For example, triple star systems are always made up of a double star system and another companion. Companion and double system circle around a common focus, which results from the focus of the double system and the individual companion.
An older statistic about the frequency of multiple star systems estimates that about 50% of all stars belong to a double star system, 20% of all stars are part of a triple system and 10% belong to multiple systems with more than three stars. Accordingly, only 20% of all stars would be single.
Examples of multiple stars are:
- EZ Aquarii , a spectroscopic binary system with a period of 3.8 days, shares a common orbit with EZ Aquarii B, with a duration of 823 days.
- η Orionis , a spectroscopic double star with a distant companion, orbit period of the double star 8 days, the companion around the double star 3470 days.
- ξ Ursae Majoris appears as a binary star with an orbital period of 59.6 years, but each component contains another binary star system (with orbital periods of 4 and 699 days).
- AB Doradus appears as a binary star with an orbital period of 1600 years, but each component contains another binary star system (with orbital periods of 1 and 2.5 years).
- Mizar : It is unclear whether this four-star system is gravitationally bound to the double star system Alkor . If this were the case, it would be a six-star system.
- HD 98800 is a four-fold system that consists of two double systems. It contains disks of dust and possibly planets .
- ε Hydrae
- 1SWASP J093010.78 + 533859.5, consisting of two close pairs approx. 140 AU apart, both of which are variable coverage ; one of these pairs is orbited by another star.
- α Geminorum ( Castor ), three spectroscopic binary stars with an eclipse variable .
- β Scorpii ( Akrab )
- θ 1 Orionis B , as part of the trapezoid in the Orion Nebula, in turn, part of an optical multiple star, with each component itself being a physical multiple system
Planets in binary star systems
There can also be exoplanets in binary star systems . There are two types of planetary orbits: Planets of the "S-type" orbit only one of the two stars and are practically not influenced by the other star, since it is too far away and / or too low in mass. A planet of the "P-type" ( circumbinary planet ), on the other hand, orbits both stars far outwards, as if they were a single star. Depending on the constellation of the stars, there are zones for S and P types of planets. A number of exoplanets have been discovered in binary star systems in recent years, and our next binary star, Alpha Centauri , is even considered a potential candidate for planets that could theoretically harbor life. The Kepler space telescope has in 2012 two exoplanets on stable orbits around the binary star system Kepler-47 discovered.
According to a study published by NOAO in 2014, binary star systems have exoplanets as often as single stars.
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Catalog of photometric binary stars
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- Double stars
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