Double visual

A star visual Double is a close star couple on the sky whose two components can be observed separately with an instrument like a Télescope, and this definition thus depends in particular on the To be able of resolution of the instrument. Among those, the visual binary are those whose property of binary star is revealed in general by the orbital movement relative of the secondary around the primary education.

History

Doubles visual

Although Mizar and its rider Alcor, with their separation of 12 ', was probably used as test of vision since Antiquity, the first use of the term “star double” which reached us comes from Ptolémée in the Almageste (Flight VII-VIII, ~137): it is about ν¹ and ν² Sagittarii, of separation 14 ', indicated as “ Quæ in oculo Sagittarius is nebulosa and bina ” (cf Peters & Knobel 1915).

The second act will be played with the development of the instrumentation: the invention of the Telescope. The history of double stars retains that the first double star “telescopic” is Mizar, but diverges as for the discoverer: was the father Jesuit Giovanni Baptista Riccioli in 1650 preceded by Benedetto Castelli, describing with his former Master Galileo, the delle on January 7th, 1617 “ una beautiful thimble che siano in cielo ”? At all events, the discoveries of first double stars are shelled starting from the middle of the 17th century:

They are at the beginning discoveries made by chance, which one probably thought that it was about double optics, and no systematic study was undertaken until the realization of the first double star Catalog by Christian Mayer with Mannheim in 1777-1778 (Bode 1781).

The third act corresponds to the description of binary visual with the whole beginning of the 19th century, which was going to give a very strong impulse to the collection of double stars, because the orbital movement could make it possible “to weigh” stars - and to be the only direct method besides to do it.

The number of discoverers is much more considerable than the list suggests it above; fault of being able to quote them all, one can for example mention wire of Herschel (John) or of Struve (Otto). On the other hand, the Catalogs mentioned contain on the one hand double stars (or multiples) which can be as well double optics as the true binary ones and they are in addition partly redundant: in term of pure discovery it seems that one must grant approximately 2640 of them to Wilhelm Struve, 1260 in Burnham, 4500 in Aitken and Hussey, at least 2996 for Hipparcos, 13250 for Tycho.

The binary visual ones

The first confirmation of the existence of binary visual was made by the astronomer-musician William Herschel the 1^er July 1802 in front of the Royal Society. It is also on this occasion which it clarified the difference between what Ptolémée had indicated like “double star” and what it then called a binary star: “ yew has certain star should Be situated At any, perhaps immense, distance behind another, and goal little deviating from the line in which we see the first, we should cuts the appearance off has double star. Goal thesis stars being totally unconnected would not form has binary system. Yew, one the contrary, two stars should really Be situated very near each other; and At the same time so far insulated ace not to Be materially affected by neighboring stars, remain united by the jump off to their mutual gravitation toward each other ” (Herschel 1802).

The idea itself was not new, because Christian Mayer in 1779 had considered the possibility of small suns orbiting of larger, but Herschel (1782) had taken its distances at the time, by considering premature this assumption. In this debate, it should be noted that Lambert had argued in 1761 qu ' binary should present an orbital movement, that this one had not been observed, and that in general the doubles were to be double optics. Meanwhile, John Michel (1767) had used a more subtle statistical argument (though not completely correct) proving that on the contrary the probability was too weak to randomly find two stars so close catches in a sample limited in magnitude of independent stars: “ it is highly probable in particular, and next to has off certainty in general, that such double stars … C really consist stars placed together, and under the inflence off general nap law ”.

In the contrary case, the observation of the couples of different luminosity (most brilliant being able to be nearest a priori ) could make it possible to measure own movements and Bode, in its comments on the catalog of Mayer, thus suggested the relative motion study of double stars. By using a similar argument, Herschel (1782) started méticuleusement to measure a great number of couples, by beginning on November 11th, 1776 with θ¹ Orionis. He sought to measure a Parallaxe annual differential, following in that the made suggestion one century and half earlier by the famous defender of the Héliocentrisme: “… which great progress for astronomy? Because in this manner, in addition to establishing the annual movement, we could manage to know the size and the distance from star. ” (Galileo 1632).

Paradoxically, Herschel was not going to contribute on this last point: it will be necessary to await Bessel 35 years to later acquire the first measurement of parallax. On the other hand, in an article founder of the study of binary (1803), it was going to provide a list of orbital couples, Castor at the head. Not only the controversy on the physical nature of a certain number of double systems was regulated, but the way was open to prove at the same time that the law of the gravitation of Newton was really universal because valid out of our Solar system; and finally that the stars could have a absolute Magnitude different, since objects of the same couple, therefore at the same distance, often had a difference in significant magnitude (Herschel continued nevertheless to consider during years that the brightness was an indicator of distance).

As for the first point, it was still necessary to await a quarter century to prove it, when Felix Savary (1827) calculated how to reconstitute the orbit of the couple, noncommonplace problem because the Orbite observed is projection on the tangent level of the sky of the true orbit. The first “numerical application” of Savary was for ξ Ursae Majoris, of period 60 years, this double having been discovered by Herschel on May 2nd, 1780, his/her son John Herschel recomputing the orbit in 1831.

The research and the determination of the orbits were going then to continue throughout S. the Catalog of double stars and multiples moving relative some of Camille Flammarion contained 819 couples in 1878. With the 1^er March 2005, the sixth catalog of the visual binary star orbits contained 1832 orbits of 1745 systems.

Classification

It is clear that the visual double stars are divided into:

optical couples
physical couples

This last category being able to be also astrometrical, if the positions on the sky of each component could be measured precisely with the Astrométrie (for example Hipparcos or HST).

In what follows, one will be interested only in those whose Keplerian orbit can be highlighted.

Theory and application

Equations of the movement

The trajectory of secondary star relative to the primary education is a homothetic Orbite of that of each component around the center of mass. One can refer to the astrometrical binary where the equations of the movement in equatorial Coordonnées are described, with the difference close the angle between the line of the nodes and the main roads in the plan of the true orbit refers here to the secondary, with ω₂ = ω₁ + π , and that the equatorial radius has is that of the relative orbit.

However, and since work of Herschel (William), the relative positions of the secondary are usually located in polar coordinates: separation ρ (in second of arc) between the components, and the angle of position θ counted positively starting from North in direction of the East, since Herschel (John). The data observed give ρ sin θ = - Δα cos δ and ρ cos θ = - Δδ , where Δα and Δδ is the variations of the equatorial coordinates due to the orbital movement only, and which one knows the expression according to the orbital parameters.

Masses and luminosities

The third law of Képler modified by Newton is written a³/P² = G M/ (4 π² ) where M is the total mass of the system, G the gravitational Constante, P the period, and has the equatorial radius of the orbit.

In physical units adapted to the problem of double stars, one thus has:

(M_1 + M_2) = \ frac {(\ varpi has) ^3} {P^2}

where:

* M₁ = mass of primary star in solar Mass.

* M₂ = mass of the secondary object in solar Mass.

* a₁ = Equatorial radius of the orbit of the primary education around the barycentre in Second of arc.

* a₂ = Equatorial radius of the orbit of the secondary around the barycentre in Second of arc.

* has = a₁ + a₂ = Equatorial radius of the relative orbit in Seconde of arc.

\ varpi

= annual Parallax in Second of arc

* P = orbital period in Year S.

The knowledge of the orbit (member of right-hand side) thus makes it possible to have access to the sum of the masses, the problem being to know the parallax of star. To obtain the individual masses, it is necessary that the binary visual one is also:

is astrometrical, because the orbit was obtained by Astrométrie absolutely or compared to other basic stars. The orbit of each component around the barycentre is then known, giving the report/ratio of the masses, since a₁ M₁ = a₂ M₂ . The parallax is generally known in this case.
is spectroscopic Binaire. If it is binary with a spectrum (BS1), the known slope gives the equatorial radius of the primary education in astronomical Unité and it is necessary to make additional assumptions. If it is binary with two spectra (BS2), the report/ratio of mass is obtained, as well as the equatorial radii in astronomical unit. By the relationship between the equatorial radius of the couple in angular units and this same equatorial radius in linear units, one thus obtains precedes the orbital parallax what is called of it .

For want of anything better, one can make use of a Relation mass-luminosity, but the masses obtained are not then purely any more orbital.

The individual Magnitudes are available since the system is solved, and the intrinsic luminosities are obtained if the parallax is known.

Instruments of observation

Many techniques of observation were developed to observe and measure double stars. At dated November 20th, 2005, 588822 measurements of double stars of Catalog WDS and average separation in second of arc between components were divided into:

The other observation aids are the visual Interférométrie (~0.17"), the Space telescope Hubble (~1"), the Heliometer (~48"), the adaptive Optical (~2.4"), the Interferometry at long base (~0.12"), measurements of Screening (~4.5"), etc

Random links: