Discovery[ edit ] The term binary was first used in this context by Sir William Herschel in when he wrote:  If, on the contrary, two stars should really be situated very near each other, and at the same time so far insulated as not to be materially affected by the attractions of neighbouring stars, they will then compose a separate system, and remain united by the bond of their own mutual gravitation towards each other.
This should be called a real double star; and any two stars that are thus mutually connected, form the binary sidereal system which we are now to consider. By the modern definition, the term binary star is generally restricted perechi în binar pairs of stars which revolve around a common center of mass.
Binary stars which can be resolved with a telescope or interferometric methods are known as visual binaries.
The latter are termed optical doubles or optical pairs. Early examples include Mizar and Acrux. The Washington Double Star Cataloga database of visual double stars compiled by the United States Naval Observatorycontains overpairs of double stars,  including optical doubles as well as binary stars.
Orbits are known for only a few thousand of these double stars,  and most have not been ascertained to be either true binaries or optical double stars. If the motion is part of an orbit, or if the stars have similar radial velocities and the difference in their proper motions is small compared to perechi în binar common proper motion, the pair is probably physical.
Edge-on disc of gas and dust present around the binary star system HD Methods of observation[ edit ] Binary stars are classified into four types according to the way in which they are observed: visually, by observation; spectroscopicallyby periodic changes in spectral lines ; photometricallyby changes in brightness caused by an eclipse; or astrometricallyby measuring a deviation in a star's position caused by an unseen companion.
Main article: Visual binary A visual binary star is a binary star for which the angular separation between the two components is great enough to permit them to be observed as a double star in a telescopeor even high-powered binoculars. The angular resolution of the telescope is an important factor in the detection of visual binaries, and as better angular resolutions are applied to binary star observations, an increasing number of visual perechi în binar will be detected.
The relative brightness of the two stars is also perechi în binar important factor, as glare from a bright star may make it difficult to detect the presence of a fainter component. The brighter star of a visual binary is the primary star, and the dimmer is considered the secondary.
In some publications especially older onesa faint secondary is called the comes plural comites; companion. If the stars are the same brightness, the discoverer designation for the primary perechi în binar perechi în binar accepted.
The time of observation is also recorded. After a sufficient number of observations are recorded over a period of time, they are plotted in polar coordinates with the primary star at the origin, and the most probable ellipse is drawn through these points such that the Keplerian law of areas is satisfied.
This ellipse is known perechi în binar the apparent ellipse, and is the projection of the actual elliptical orbit of the secondary with respect to the primary on the perechi în binar of the sky. From this projected ellipse the complete elements of the orbit may be computed, where the semi-major axis can only be expressed in angular units unless the stellar parallaxand hence the distance, of the system is known.
Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. This animation was assembled from 55 images of the CHARA interferometer in the near-infrared H-band, sorted according to orbital phase. Sometimes, the only evidence of a binary star comes from the Doppler effect on its emitted light.
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In these cases, the binary consists of a pair of stars where the spectral lines in the light emitted from each star shifts first towards the blue, then towards the red, as each moves first towards us, and then away from us, during its motion about their common center of masswith the period of their common orbit. In these systems, the separation between the stars is usually very small, and the orbital velocity very high.
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Unless the plane of the orbit happens to be perpendicular to the line of sight, the orbital velocities will have components in the line of sight and the observed radial velocity of the system will vary periodically. Since radial velocity can be measured with a spectrometer by observing the Doppler shift of the stars' spectral linesthe binaries detected in this manner are known as spectroscopic binaries.
Most of these cannot be resolved as a visual binary, even with telescopes of the perechi în binar existing resolving power.
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In some spectroscopic binaries, spectral lines from both stars are visible and the lines are alternately double and single. Such a system is known as a double-lined spectroscopic binary often denoted "SB2". In other systems, the spectrum of only one of the stars is seen and the lines in the spectrum shift periodically towards the blue, then towards red and back again. Such stars are known as single-lined spectroscopic binaries "SB1".
The orbit of a spectroscopic binary is determined by making a long series of observations of the radial velocity of one or both components of the system.
The observations are plotted against time, and from the resulting curve a period is determined.
If the orbit is circular then the curve will be a sine curve. If the orbit is ellipticalthe shape of the curve will depend on the eccentricity of the ellipse and the orientation of the major axis with reference to the line of sight. It is impossible to determine individually the semi-major axis a and the inclination of the orbit plane i. However, the product of the semi-major axis and the perechi în binar of the inclination i.
If either a or i can be determined by other means, as perechi în binar the case of eclipsing binaries, a complete solution for the orbit can be found. About 40 are known. Visual binary stars often have large true separations, with periods measured in decades to centuries; consequently, they usually have orbital speeds too small to be measured spectroscopically.
Conversely, spectroscopic binary stars move fast in their orbits because they are close together, usually too close to be detected as visual binaries.
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Binaries that are found to be both visual and spectroscopic thus must be relatively close to Earth. Eclipsing binaries[ edit ] An eclipsing binary star is a binary star system 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. Algola triple star system in the constellation Perseuscontains the best-known example of an eclipsing binary.
Play media This video shows an artist's impression of perechi în binar eclipsing binary star system. As the two stars orbit each other they pass in front of one another and their combined brightness, seen from a distance, decreases. Eclipsing binaries are variable stars, not because the light of the individual components perechi în binar but because of the eclipses.
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The light curve of an eclipsing binary is characterized by periods of practically constant light, with periodic drops in intensity when one star passes in front of the other.
The brightness may drop twice during the orbit, once when the secondary passes in front of the primary and once when the primary passes in front of the secondary. The deeper of the two eclipses is called the primary regardless of which star is being occulted, and if a shallow second eclipse also occurs it is called the secondary eclipse.
The size of the brightness drops depends on the relative brightness of the two stars, the proportion of the occulted star that is hidden, and the surface brightness i. Typically the occultation of the hotter star causes the primary eclipse. This makes it feasible to use them to directly measure the distances to external galaxies, a process that is more accurate than perechi în binar standard candles.
The first is by observing extra light which the stars reflect from their companion. Second is by observing ellipsoidal light variations which are caused by deformation of the star's shape by their companions.
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The third method is by looking at how relativistic beaming affects the apparent magnitude of the stars. Detecting binaries with these methods requires accurate photometry. Astrometric perechi în binar are relatively nearby stars which can be seen to wobble around a point in space, with no visible companion.
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The same mathematics used for ordinary binaries can be applied to infer the mass of the missing 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 radiationfor example a neutron star.
The position of the star is repeatedly measured relative to more distant stars, and then checked for periodic shifts in position. Typically this type of measurement can only be performed on nearby stars, such as those within 10 parsecs.
Nearby stars often have a relatively high proper motionso astrometric binaries will appear to follow a wobbly path across the sky. If the companion is sufficiently massive to cause an observable shift in position of the star, then its presence can be deduced. From precise astrometric measurements of the movement of the visible star over a sufficiently long period of time, information about the mass of the companion and its orbital period can be determined.
However, the requirements to perform this measurement are very exacting, due to the great difference in the mass ratio, and the typically long period of the planet's orbit.
Detection of position shifts of a star is a very exacting science, and it is difficult to achieve the necessary precision. Space perechi în binar can avoid the blurring effect of Earth's atmosphereresulting in more precise resolution. Configuration of the system[ edit ] Detached Semidetached Contact Configurations of a binary star system with a mass ratio of perechi în binar. The black lines represent the inner critical Roche equipotentials, the Roche lobes. Another classification is based on the distance between the stars, relative to their sizes:  Detached binaries are binary stars where each component is within its Roche lobei.
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 Roche lobe and the other does not.
Gas from the surface of the Roche-lobe-filling component donor is transferred to the other, accreting star. The mass transfer dominates the evolution of the system. In many cases, the inflowing gas forms an accretion disc around the accretor. A contact binary is a type of binary star in which both components of the binary fill their Roche lobes.
The uppermost part of the stellar atmospheres forms a common envelope that surrounds both stars. As the friction of the envelope brakes the orbital motionthe stars perechi în binar eventually merge. Cataclysmic variables and X-ray binaries[ edit ] Artist's conception of a cataclysmic variable system When a binary system contains a compact object such as a white dwarfneutron star or perechi în binar holegas from the other donor star can accrete onto the compact object.
This releases gravitational potential energycausing the gas to become hotter and emit radiation.
Cataclysmic variable starswhere the compact object is a white dwarf, are examples of such systems. 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-typehigh-mass donor star which transfers mass by its stellar windwhile low-mass X-ray binaries are semidetached binaries in which gas from a late-type donor star or a white dwarf overflows the Roche lobe and falls towards the neutron star or black hole.
In Cygnus X-1, the mass of the unseen companion is estimated to be about nine times that of the Sun,  far exceeding the Tolman—Oppenheimer—Volkoff limit for the maximum theoretical mass of a perechi în binar star. It is therefore believed to be a black hole; it was the first object for which this was widely believed.
Variations in period[ perechi în binar ] Main article: Applegate mechanism The Applegate mechanism explains long term orbital period variations seen in certain eclipsing binaries. As a main-sequence star goes through an activity cycle, the outer layers of the star are subject to a magnetic torque changing the distribution of angular momentum, resulting in a change in the star's oblateness. This is quite distinct from the far more common observations of alternating period increases perechi în binar decreases explained by perechi în binar Applegate mechanism.
Monotonic period increases have been attributed to mass transfer, usually but not always from the less massive to the more massive star  A and B[ edit ] Artist's impression of the binary star system AR Scorpii The components of binary stars are denoted by the suffixes A and B appended to the system's designation, A denoting the primary and B the secondary.
Additional letters, such as C, D, etc. Examples: Antares Alpha Scorpii is a red supergiant star in a binary system with a hotter blue main-sequence star Antares B. Antares B can therefore be termed a hot companion of the cool supergiant. Since the nature of the companion is not well-established in all cases, it may be termed a perechi în binar companion".
The secondary appears to have a higher temperature than the primary and has therefore been described as being the "hot companion" star. It may be a Wolf—Rayet star.
This combination is the result of a cool red supergiant accompanied by a smaller, hotter companion. Matter flows from the supergiant gustul vieții cum să obțineți libertatea financiară the smaller, denser companion.