What You Need to Know About Double Stars

What are Double Stars?

D O U B L E   S T A R S, sometimes abbreviated as just doubles, can be considered as the name of the broader category of gravitationally bound star systems that have two or more components visible using optical aid. Under this broad term, all triple stars and other multiple systems are also be included. Double stars are fairly common type of stellar objects, and at the last count, some 86 000 pairs are known (2002).

The simplest scenario for a double star is where two stars lie close together in the sky. At a glance, an observer would be uncertain if these two stars were orbiting each other or were merely arranged by chance alignment. Such doubles  are usually termed visual pairs or just pairs whose usage is often loosely given to doubles while casually observing the night sky. Over time enough evidence maybe obtained to prove that these objects do orbit each other. These pairs are more specifically referred to as true binary (binaries). Other double stars sometimes do not show evidence of orbital motion and simply instead move in straightened paths through space. These are called optical double stars.

One of the primary aims, which becomes the major task for any serious observer, is to find the true physical association of double stars (whether real or not), including the discovery or refinement of true connection. By the measurement of the scalar quantities of position angle and separation. When combined with other measures of the past, mathematical reductions can be applied to determine either connection by celestial perspective or of physical union.

Although measurements maybe important, double stars can also be observed for fun. Certainly there is nothing quite so attractive than two close and colourful stars lying in a starry field. Some of the brightest of them show significant colour differences, including the northern star Alberio / Beta Persei, which has a golden yellow star with a fainter sapphire blue companion. Yet the southern sky does has its fair share of interesting doubles, including the nearest star, Alpha Centauri, Alpha Crucis, Theta Eridani and p Eridani - all being “must see” for any new southern observer. Much pleasure can be gained simply by seeing their differences between the countless number of pairs. All pairs 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 a combination of skill and good seeing conditions - if not just to test out you telescope’s optics.

Basics of Nomenclature

In the naming of components for double stars is more traditional than it is 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 discoverer’s distinction is used. This primary to secondary then applies until the true masses are determined. The difference in brightness between the two stars is termed the Delta-m, written as Δm and usually quoted to one decimal place.

Each of the brightest component can also be nominated as ‘A’, the faintest ‘B’. For 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 binary stars.  In 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 objects is the separation and is normally measured in second of arc.

These are also combined with the visual co-ordinate terms which influenced by the Earth’s 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 telescope’s optical configuration becomes irrelevant.

Another system of orientation is the quadrant position 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 double’s orientation an position by the two quantities ; Position Angle (PA or θ) and Separation (Sep. or ρ)

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 0o position angle is celestial north, the 90o position angle is EAST, 180o is SOUTH, 270o is WEST through to 360o that is again NORTH. The position angle is influenced by the Precession of the Equinoxes, so all values must refer to a certain epoch. Ie. Epoch 1950.0 or 2000.0. 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 and roughly a tiny 1/1800th the lunar diameter.

Once learning if a pair is binary, finds that motion increasing in position angle is said to orbit in a direct motion, while a decreasing one is referred as retrograde motion. This has to be distinguished, because unlike the planets, it indicates the likely orbital movements of the two starswhich can be in eaither direction. In the binary’s true orbit, closest orbital approach is called apastron, while the furthest distance is periastron. Binary motions in this latter circumstance will rapidly change - especially if the orbit is highly eccentric, near apastron. During periastron the observed change will be slower.

Multiple Stars

Multiple stars are small stellar groups that contain between three and twenty components. They importantly have other properties that do not apply to binary systems as they are arranged in so-called hierarchal arrangements.

Triple systems are often have the combination of a close pair with another fainter and more distant companion. A 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.

Another type of multiple system is the trapezia, named after the brightest object in the Orion Nebulae - the Trapezium. These objects can contain four or more stars all roughly of equal mass. Most systems of this type are very young and are thought to be unstable. These systems may revert to triple systems of a close pair, with a solitary star in orbit. The other star is rejected from the system altogether at a high velocity, and may make 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 being visible in small telescopes.  

General Observing Techniques

Double star or binary star is sometimes not a very simple task, and often require 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 be gained by observational experience. Any newcomer to observational astronomy could learn much about observing techniques and finding objects - even if the stargazer has no real interest in this topic.

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

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.

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Last Update : 20 April 2005

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