Observing and Measuring Visual Double Stars

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A number of new double star systems have been identified. The total number of double star observations published in this series including CCD and photographic measures is about Copies of the Circulars are sent to main centres of interest in professional double star astronomy i. In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

We will try to keep this updated as new double stars are added. The Section publishes these Circulars which contain measures of visual double stars using micrometers, CCD cameras and on-line astrometry as well as a discussion of the nature of possible visual binary system. If you have any comments or problems with this page or any other Webb Deep-Sky webpage then please contact the Website Administrator.

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Conversely, spectroscopic binary stars move fast in their orbits because they are close together, usually too close to be detected as visual binaries. Binaries that are found to be both visual and spectroscopic thus must be relatively close to Earth. 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.

Algol , a triple star system in the constellation Perseus , contains the best-known example of an eclipsing binary. Eclipsing binaries are variable stars, not because the light of the individual components vary but because of the eclipses. 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. An eclipsing binaries' period of orbit may be determined from a study of its light curve , and the relative sizes of the individual stars can be determined in terms of the radius of the orbit, by observing how quickly the brightness changes as the disc of the nearest star slides over the disc of the other star.

Since about , measurement of extragalactic eclipsing binaries' fundamental parameters has become possible with 8-meter class telescopes. This makes it feasible to use them to directly measure the distances to external galaxies, a process that is more accurate than using standard candles.

Nearby non-eclipsing binaries can also be photometrically detected by observing how the stars affect each other in three ways. 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.

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. 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 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 radiation , for example a neutron star.

The visible star's position is carefully measured and detected to vary, due to the gravitational influence from its counterpart. The position of the star is repeatedly measured relative to more distant stars, and then checked for periodic shifts in position. Nearby stars often have a relatively high proper motion , so 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. This method of detecting binaries is also used to locate extrasolar planets orbiting a star. 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 telescopes can avoid the blurring effect of Earth's atmosphere , resulting in more precise resolution. Another classification is based on the distance between the stars, relative to their sizes: [30]. Detached binaries are binary stars where each component is within its Roche lobe , i. 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.

Double star

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 motion , the stars may eventually merge. When a binary system contains a compact object such as a white dwarf , neutron star or black hole , gas from the other donor star can accrete onto the compact object.

This releases gravitational potential energy , causing the gas to become hotter and emit radiation. Cataclysmic variable stars , where the compact object is a white dwarf, are examples of such systems.


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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 or a white dwarf overflows the Roche lobe and falls towards the neutron star or black hole. It is therefore believed to be a black hole; it was the first object for which this was widely believed. 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.

Table of Contents: Observing and measuring visual double stars

Another phenomenon observed in some Algol binaries has been monotonic period increases. This is quite distinct from the far more common observations of alternating period increases and decreases explained by the Applegate mechanism. Monotonic period increases have been attributed to mass transfer, usually but not always from the less massive to the more massive star [37]. 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. Double stars are also designated by an abbreviation giving the discoverer together with an index number. The components of a binary star system may be designated by their relative temperatures as the hot companion and cool companion. 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 three objects being actually required, as conservation of energy rules out a single gravitating body capturing another and the high number of binaries currently in existence, this cannot be the primary formation process.

The observation of binaries consisting of stars not yet on the main sequence supports the theory that binaries develop during star formation. Fragmentation of the molecular cloud during the formation of protostars is an acceptable explanation for the formation of a binary or multiple star system.

The outcome of the three-body problem , in which the three stars are of comparable mass, is that eventually one of the three stars will be ejected from the system and, assuming no significant further perturbations, the remaining two will form a stable binary system. As a main-sequence star increases in size during its evolution , it may at some point exceed its Roche lobe , meaning that some of its matter ventures into a region where the gravitational pull of its companion star is larger than its own.

The mathematical point through which this transfer happens is called the first Lagrangian point. If a star grows outside of its Roche lobe too fast for all abundant matter to be transferred to the other component, it is also possible that matter will leave the system through other Lagrange points or as stellar wind , thus being effectively lost to both components. Studies of the eclipsing ternary Algol led to the Algol paradox in the theory of stellar evolution : although components of a binary star form at the same time, and massive stars evolve much faster than the less massive ones, it was observed that the more massive component Algol A is still in the main sequence , while the less massive Algol B is a subgiant at a later evolutionary stage.

The paradox can be solved by mass transfer : when the more massive star became a subgiant, it filled its Roche lobe , and most of the mass was transferred to the other star, which is still in the main sequence. In some binaries similar to Algol, a gas flow can actually be seen. 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.

If a white dwarf has a close companion star that overflows its Roche lobe , the white dwarf will steadily accrete gases from the star's outer atmosphere. These are compacted on the white dwarf's surface by its intense gravity, compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists of degenerate matter and so is largely unresponsive to heat, while the accreted hydrogen is not.

Hydrogen fusion can occur in a stable manner on the surface through the CNO cycle , causing the enormous amount of energy liberated by this process to blow the remaining gases away from the white dwarf's surface. The result is an extremely bright outburst of light, known as a nova. In extreme cases this event can cause the white dwarf to exceed the Chandrasekhar limit and trigger a supernova that destroys the entire star, another possible cause for runaways.

The Hubble Space Telescope recently took a picture of the remnants of this event. Binaries provide the best method for astronomers to determine the mass of a distant star. The gravitational pull between them causes them to orbit around their common center of mass. From the orbital pattern of a visual binary, or the time variation of the spectrum of a spectroscopic binary, the mass of its stars can be determined, for example with the binary mass function.

In this way, the relation between a star's appearance temperature and radius and its mass can be found, which allows for the determination of the mass of non-binaries.


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Because a large proportion of stars exist in binary systems, binaries are particularly important to our understanding of the processes by which stars form. In particular, the period and masses of the binary tell us about the amount of angular momentum in the system. Because this is a conserved quantity in physics, binaries give us important clues about the conditions under which the stars were formed.

In a simple binary case, r 1 , the distance from the center of the first star to the center of mass or barycenter , is given by:. When the center of mass is located within the more massive body, that body will appear to wobble rather than following a discernible orbit.

Images are representative, not simulated. The position of the red cross indicates the center of mass of the system. It is estimated that approximately one third of the star systems in the Milky Way are binary or multiple, with the remaining two thirds being single stars. That is, the likelihood of being in a binary or a multi-star system steadily increases as the mass of the components increase. There is a direct correlation between the period of revolution of a binary star and the eccentricity of its orbit, with systems of short period having smaller eccentricity.

Binary stars may be found with any conceivable separation, from pairs orbiting so closely that they are practically in contact with each other, to pairs so distantly separated that their connection is indicated only by their common proper motion through space. Among gravitationally bound binary star systems, there exists a so-called log normal distribution of periods, with the majority of these systems orbiting with a period of about years. This is supporting evidence for the theory that binary systems are formed during star formation.

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. The mass of a star can be directly determined only from its gravitational attraction. Apart from the Sun and stars which act as gravitational lenses , this can be done only in binary and multiple star systems, making the binary stars an important class of stars. In the case of a visual binary star, after the orbit and the stellar parallax of the system has been determined, the combined mass of the two stars may be obtained by a direct application of the Keplerian harmonic law.

Unfortunately, it is impossible to obtain the complete orbit of a spectroscopic binary unless it is also a visual or an eclipsing binary, so from these objects only a determination of the joint product of mass and the sine of the angle of inclination relative to the line of sight is possible. In the case of eclipsing binaries which are also spectroscopic binaries, it is possible to find a complete solution for the specifications mass, density , size, luminosity , and approximate shape of both members of the system.

While a number of binary star systems have been found to harbor extrasolar planets , such systems are comparatively rare compared to single star systems. Observations by the Kepler space telescope have shown that most single stars of the same type as the Sun have plenty of planets, but only one-third of binary stars do. According to theoretical simulations, [68] even widely separated binary stars often disrupt the discs of rocky grains from which protoplanets form.

On the other hand, other simulations suggest that the presence of a binary companion can actually improve the rate of planet formation within stable orbital zones by "stirring up" the protoplanetary disk, increasing the accretion rate of the protoplanets within. Detecting planets in multiple star systems introduces additional technical difficulties, which may be why they are only rarely found. A study of fourteen previously known planetary systems found three of these systems to be binary systems.

All planets were found to be in S-type orbits around the primary star. In these three cases the secondary star was much dimmer than the primary and so was not previously detected. This discovery resulted in a recalculation of parameters for both the planet and the primary star. Science fiction has often featured planets of binary or ternary stars as a setting, for example George Lucas' Tatooine from Star Wars , and one notable story, " Nightfall ", even takes this to a six-star system.

In reality, 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.

Planets that orbit just one star in a binary system are said to have "S-type" orbits, whereas those that orbit around both stars have "P-type" or " circumbinary " orbits. The large distance between the components, as well as their difference in color, make Albireo one of the easiest observable visual binaries. The brightest member, which is the third-brightest star in the constellation Cygnus , is actually a close binary itself.

Also in the Cygnus constellation is Cygnus X-1 , an X-ray source considered to be a black hole. It is a high-mass X-ray binary , with the optical counterpart being a variable star. It is located in the constellation Canis Major. In Friedrich Bessel deduced that Sirius was a binary.

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In astronomers at the Mount Wilson Observatory determined that Sirius B was a white dwarf , the first to be discovered. An example of an eclipsing binary is Epsilon Aurigae in the constellation Auriga. The visible component belongs to the spectral class F0, the other eclipsing component is not visible. The last such eclipse occurred from —, and it is hoped that the extensive observations that will likely be carried out may yield further insights into the nature of this system. Another eclipsing binary is Beta Lyrae , which is a semidetached binary star system in the constellation of Lyra.

Systems with more than two stars are termed multiple stars. Algol is the most noted ternary long thought to be a binary , located in the constellation Perseus. Two components of the system eclipse each other, the variation in the intensity of Algol first being recorded in by Geminiano Montanari.

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This system also underscores the fact that no search for habitable planets is complete if binaries are discounted. There are also examples of systems beyond ternaries: Castor is a sextuple star system, which is the second-brightest star in the constellation Gemini and one of the brightest stars in the nighttime sky. Astronomically, Castor was discovered to be a visual binary in Each of the components of Castor is itself a spectroscopic binary. Castor also has a faint and widely separated companion, which is also a spectroscopic binary.


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  8. The Alcor—Mizar visual binary in Ursa Majoris also consists of six stars, four comprising Mizar and two comprising Alcor. From Wikipedia, the free encyclopedia. Not to be confused with Double star. For the hip hop group, see Binary Star band. Main article: Visual binary. This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources.

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    Play media. Configurations of a binary star system with a mass ratio of 3. The black lines represent the inner critical Roche equipotentials, the Roche lobes. Main article: Applegate mechanism. Main article: Barycenter. Main article: Habitability of binary star systems. Space portal Star portal. Philosophical Transactions of the Royal Society of London. Bibcode : RSPT Double Stars.

    Dordrecht: D. Reidel Publishing Company. University of Tennessee. Reidel Publishing Company, Dordrecht. John Michell, B. Philosophical Transactions. Kam-Ching Leung, pp. Mason, Gary L. Wycoff, and William I. Hartkopf and Brian D. Hartkopf, United States Naval Observatory.

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