Double optics

A star doubles optical is an apparently close star couple but from which the two components are actually separated by a long distance. They seem visual Double because of their apparent proximity in projection on the sky and they are thus not true binary stars gravitationally dependant.

Historically, the debate to know if all the double stars observed were or not double optics is primarily unrolled starting from the medium of the 18^ème century (Lambert, Michel) and will be closed by William Herschel in 1803 with the description of the binary first stars.

To prove that two stars form really a physical couple and not a optical couple , it is necessary to have highlighted them Orbite, which can take centuries, or to show that they are close because one has an indication of their distance or speed. Conversely, if one observes a relative displacement which is uniform and linear between the two components, it is that it is probably about an optical couple which one sees the own Mouvement of each component.

For lack of observational indications of this type making it possible to take a decision as for the nature of the couple, it remains the possibility of a statistical test, with the naturally associated risk of error. One can evaluate thus which is the probability of having an object with angular separation ρ (in seconds of arc) of star considered, his presence having randomly only. It is clear that the probability of being in the presence of a double optics increases with the stellar density on the plan of the sky (e.g. in direction of the very dense center of our Galaxy), with the distance (e.g. in direction of a Galaxie solved), when the instrument used has a good To be able of resolution and that the observations are deep in apparent Magnitude.

With the simplifying assumption that the stars have in a small zone of the sky a uniform surface density D (of many stars a second arc square until the Magnitude connect m ), the probability of being at least a star magnitude m in the element of surface π ρ ² around a given star is obtained with the Loi of Poisson by:

P (ρ, m) =1-exp (- π ρ ² D)

For example, with the resolution of an instrument like the Space telescope Hubble, and in direction of the galactic center in the very dense Window of Baade, one can discount to observe approximately 3 million stars per square degree in the visible one with magnitude 20, that is to say a chance on 2 to have at least a star more brilliant than 20 at less than one second of arc of a given star; on the other hand, in direction of the galactic poles, this probability falls down to a chance on thousand. Therefore, if there is a couple separated by one second from arc, it is very probable that it is an optical couple in the first case, but much less in the second.

See too

Internal bonds

Double visual

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