What are Double Stars ?

D O U B L E   S T A R S, sometimes abbreviated just as doubles, can be considered as the group of the broader category of gravitationally bound star systems that have two or more visible components. Under this sweeping definition, the triple stars and other multiple systems all fall under the same umbrella. Double stars in fact are fairly common types of stellar objects, and at the last count, we presently recognise more than 102,000 (2006)!

The very simplest definition for any double star is when two stars just happen to lie close together in the sky. At a casual glance, an observer would be uncertain if these two stars were orbiting each other or were merely joined by chance alignment. Such doubles are called pairs or visual pairs, whose familiar usage often loosely suggests optically observing of numbers of double stars in the night sky. Assessing their motions in respect to each star, may in time, provide enough evidence to prove that they do indeed orbit about each other. Such systems are more specifically referred to binaries or true binaries. Other double stars sometimes do not show any real evidence of motion and appear fixed, or simply appear to move in straightened paths through space. The latter of these are called optical double stars. Most known binary stars have not complete on whole revolution, but are distinguished from the pairs only by having a known curved path or some partial arc.

One of the primary aims, which becomes the major task for any serious observer, is to find the true physical association of double stars, and whether it is real or not. It may also include the discovery and refinement of this connection, achieved through careful measurement of the position angle and separation at known observation dates. When these positions are combined with other measures, mathematical reductions can be applied to determine the probability of some celestial perspective or physical union.

Although measurements maybe important, doubles can also be observed just for fun. Certainly there is nothing quite so attractive as seeing two close bright stars placed within a populated starry field. Some of the brightest also familiar show significant colour differences. Probably the best example is the very beautiful northern system of Beta Persei (β Per) or Alberio, which has a very lovely golden-yellow primary star nicey contrasted with its fainter gorgeous sapphire blue companion. Another classic in the southern skies is x Velorum (Dunlop 95) — dubbed appropriately Alberio Australis, which has the contrasting colours of rich orange-yellow and pale blue. Both β Per and x Vel can be seen readily in small telescopes.

Our southern skies does have its fair share of interesting doubles. These include. the nearest star, Alpha Centauri (α Cen), Alpha (1,2) Crucis (α1,2 Cru), Theta Eridani / Acamar (θ Eri) and p Eridani (Dunlop 5) — all being the must see” pairs for any new southern observer or northern hemisphere visitor.

Much pleasure can be gained by simply seeing the many different configurations between the almost countless numbers of pairs. They make reasonable distractions during the time of Full Moon when deep-sky observing of faint objects are severely affected by bright moonlight. Numerous other pairs offer the further challenge of being difficult to resolve. In these circumstances separation of the two often requires combinations of acquired observational skills and excellent seeing conditions. For some, double stars are just an excellent opportunity to test out the optical quality of their telescopes.

Multiple Stars

Multiple stars or multiples are small stellar groups that contain between three and twenty components. They have properties that do not apply in the same way as binary systems. Consequentially multiples have different kind of sub-types, being organised into different groups or so-called hierarchal arrangements.

The largest groups of multiples are the triple stars, which often have the combination of a close pair with another more distant companion. One classic example of this is the nearest star system of Alpha Centauri. Here the close pair A and B stars orbit in 79.8 years while the C star is Proxima Centauri that orbits once every 100 000 years or so. Some triples may have this distant companion as the brightest star, and the both stars of the main double appearing fainter. Although this might seem paradoxical, it is usually resolved when the masses have been determined, as the combined mass of the close pair prove larger than the solitary mass of its smaller companion.

Another type of common multiple systems are the trapezia. These are named after the brightest stellar object in the Orion Nebulae - the famous Trapezium. These multiples can contain four or more stars that are all roughly the same mass and size, however unlike other multiples, they are very young and are gravitationally unstable. Some of these systems may eventually revert to stable triple systems — forming close pairs, with the solitary companion star orbiting at some distance. The other star(s) are violently rejected from the system altogether, and these may explain the high-velocity runaway stars observed in the Milky Way.

Other examples of general multiples include; α Crucis and Sigma (σ) Orionis, both of which are visible in small telescopes.

Basics of Nomenclature

In the naming of components for double stars is more traditional than necessarily accurate, however, in more recent times this has slowly becoming more standardised. Common sources in the literature sometimes vary from time to time. However, the following trends are applied;

The brighter or major star is called the primary and the companion or second star is called the secondary. Sometimes the term comes, or plural comites, is also applied, but has been outdated for nearly ninety years. If both stars are of equal brightness or magnitude, the discoverers distinction is used. This primary to secondary then applies until the true masses are determined. Differences in brightness between the two stars is termed the Delta-m, written normally as Δm, and is usually quoted to one decimal place.

Each of the brightest component can also be nominated as A, the faintest B. For all multiple systems, the components are listed in decreasing magnitude, and are referred to as the companions B, C, D, etc. Pairs that are nominated AB are certain binary stars. In the multiple systems, which star is which can get quite complicated, often with combinations of the these component letters. For identification or descriptive purposes the system, or any telescopic astronomical object, the diurnal motion of the Earth can be used to show the position of the surrounding objects. Distances between the two stars is the separation and is normally measured in second of arc or arcsec or abbreviated as ′.

These are also combined with the visual co-ordinate terms which influenced by the Earths motion in space. These apply to the compass directions of n north and s south, sometimes with the terms p preceding and f following. Each preceding star is before the object; known if the star is allowed to partly drift through the field, while the following star tracks behind the object. This system is very useful because the telescopic optical configuration becomes irrelevant.

Another system of orientation is the quadrant position or just quadrant which is sometimes used. No only can this be applied to double stars, but may be usefully employed with telescopic deep-sky objects and their various observed features.

Quadrant 1
nf North-Following
Quadrant 2
sf South-Following
Quadrant 3
sp South Preceding
Quadrant 4
np North-Preceding

Far more useful and specific are the descriptions of any doubles orientation an position by the two quantities ; Position Angle (PA or θ) and Separation (Sep. or ρ)

Representation of Position Angle and Separation.
Fig. 1. Orientation of Double Stars :
Representation of Position Angle and Separation.

POSITION ANGLE is defined as the angle of the primary through the secondary, as measured in the angle deviating from NORTH increasing towards the EAST.

A 0° position angle is celestial north, the 90° position angle is EAST, 180° is SOUTH, 270° is WEST through to 360° that is again NORTH. The position angle is influenced by the precession of the equinoxes, so all values must refer to some certain epoch. I.e. Epoch 1900.0, 1950.0, 2000.0 or the future 2050.0. It is important to note that these quoted values are easily converted, if necessary, which often applies when using older star or double star catalogues.

SEPARATION is simply defined as the distance between the centre of the two stars measured in seconds of arc or abbreviated as arcsec or ″. A distance of 1 arcsec corresponds to the very tiny angle subtending roughly 1/1800th of the lunar diameter. Most telescopic doubles are usually defined as stars which are less than 300 arcsec — being well below naked-eye resolution. For the widest pairs, the separation is sometimes given minutes of arc or arcmin, abbreviated sometimes as; ′, However, this practice is usually discouraged with double star observers in favour of units in arcsec. I.e. 120 arcsec instead of 2 arcmin, or 120″ instead of 2′.

Once learning if some visual pair is binary, then the motion that increases in position angle is said to orbit in a direct motion, while decreasing ones are referred to as retrograde motion. This has to be importantly distinguished, because unlike the planets, it indicates the likely orbital movements of the two stars, which maybe in either direction.

In the binarys true orbit, closest orbital approach is called periastron, while the furthest distance is apastron. Binary motions in this latter circumstance will rapidly change — especially if the orbit is highly eccentric and near its periastron. During apastron the observed change in position will be slower.

Classes of Double Stars

Based on the separation of various pairs it is sometimes useful to categorise them into sub-groups. This helps both in writing up descriptions or estimating the apertures or magnification required to see them. The Table below is a useful recommended guide of separations for amateurs, but assumes perfect seeing.

Difficult <0.5″ <23cm
Very Close 0.5″ to 2.0″ 23cm to 5.8cm
Close 2.0″ to 5.0″ 5.8cm to 2.3cm
Standard 5.0″ to 10″ 2.3cm to 1.2cm
Wide 10″ to 30″ 1.2cm to 3.9mm
Open 30″ to 60″ 3.9mm to 1.9mm
Very Wide >60″ >1.9mm
Naked-eye >300″ --

Importance of Double Stars

Astronomers for sometime have known that knowledge about double stars does contribute to some of the most important aspects in the development of stellar evolution and the general nature of stars. As stars are fundamental building blocks of most objects, such properties as mass, density, stellar atmospheric phenomena, shape etc. becomes desirable quantities for any type of theoretical analysis.

General Observing Techniques

Observing many of the wider bright double stars can be easily achieved using binoculars or small telescopes. Examples can be easy pairs like Alpha and Gamma Crucis. Most can be seen regardless of the quality of the observing conditions. There are several dozen pairs in this category to satisfy the curious observe at any time of the night or year.

Closer double stars or binary stars can sometimes more problematic, and can be sometimes be not a very simple task — even for experienced observers. Often they require knowledge and considerations such as observation conditions, telescope resolution, differences in brightness to simple recognition of the system. Each of these factors must be taken into account, and can ONLY be gained by some observational experience. Any newcomer to observational astronomy could learn much about observing techniques and in finding objects — even if the stargazer has no real interest in this topic.

If you wish to start looking at doubles it is often importantly suggested to have access to several common references listing pairs and one or two decent star atlases. I.e. Sky Atlas 2000.0. Uranometria 2000.0, etc. or electronic astronomical programs, like Megastar or Sky Map Pro.

It is probably best to start with the brighter pairs that are fairly wide apart and are easily located near known bright stars. Such examples are Alpha Crucis at the base of the Southern Crux, followed by Gamma Crucis at the top of the Cross. Next you could follow say, Alpha Centauri, which might prove a little difficult over the next few years (2009-2012). After this you might like to try some more wide pairs with large differences in magnitude. This could be followed by some more challenging examples that are below 5 arcsec, attempting those of equal magnitudes followed by ones with larger differences in brightness.

If you have the aperture above 25cm to 30cm, observers might like to attempt far more difficult pairs like Beta Centauri and Beta Muscae. These can be difficult to split under poor seeing conditions, and observing them over several nights will probably given an appreciation of the difficulties faced under differing seeing conditions.


Last Update : 23rd April 2011

Southern Astronomical Delights © (2011)

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