(1-1) Definition

The term ‘double star’ is really a misnomer, as it only infers two stars near each other in the sky. Under this broad base triple stars and other multiple systems would also be included. The general definition was first stated by Sir William Herschel in his paper of 1802, the ‘Construction of the Universe

...a real double star (binary) - the union of two stars, that are
formed together on one system, by the laws of attraction.

It was the ancient Greeks named the term ‘διπλονρ’ for double star, by the observation of the visual pair if ν1 and ν2 Sagittarii. These stars are about 14'min arc or 840"sec arc. apart. Since the invention of the telescope, true double stars are considered to be just below naked eye resolution. The modern definition states a double star is;

Two or more stars, having a maximum distance of 5'arcmin or 300"arcsec apart.

Under this definition, the globular and open star clusters would also be included. Ideally this becomes impractical because the multiplicity is an array of nearly countless independent duplication of stars. All star clusters also show quite different characteristics from the multiple stars. Although bound by the universal force of gravitation, the dynamical features that hold the stars together do not apply with star clusters. For instance, orbits are undefined, due to the variability of the gravitational attraction between the individual stars, and therefore produce unstable motions. It is interesting to note that the theories on how the star clusters actually hold together is normally controlled by a central core binary. This contains most of the kinetic energy of the cluster, stopping individual components escaping away from the system. Another reason why open star clusters and globular star clusters are not included is also based on their great distances from the Sun. Most open clusters lie in the order of several kiloparsecs, with the globulars generally lying tens of kiloparsecs away. Most of the observed visual pairs that we see through the telescope are contained within a radius of some five-hundred (500) parsecs or 1 600 light years.

(1-2) Basic Terminology

Of the true double stars, the term pair is generally applied to stars in which the physical connection is unknown. The term is also loosely applied, especially when casually observing the sky. Double Stars is used as a term applied normally to the subject in question. If enough evidence is obtained, with sufficient observational data in which an orbit may be deduced, the term binary or binary system is used. They are distinct from a simple double star, as physically association has been determined.

(1-3) Nomenclature

In the naming of components, the following trends are applied;

The brighter or major star is called the primary, while the second star is called the secondary or the companion. If both stars are of equal brightness or magnitude then the discoverer's initial distinction is used. This distinction between the primary and secondary then applies until the masses are determined.

Difference in magnitude is termed 'Delta-m', written as Δm, usually to one decimal place.

The brightest component can also be nominated as ‘A', the faintest ‘B'. A multiple system has the components listed in decreasing magnitude, and are referred to as the companions ‘B', ‘C', ‘D' etc. Pairs that are nominated 'AB' are binary stars. Within a multiple, the binary is ascertained as 'AB' or ‘AxB' is noted as the controlling binary. The components from this binary are noted from it's mid position. So a star ‘D' would be listed as ‘AB-D'. In multiple systems this can be quite complicated. A system ‘AB-CD' or ‘ABxCD', identifies two binaries, with internal orbits in the system. These pairs together may or may not be associated together in the multiple. If they are associated a 'x' is used between the two pairs, otherwise ‘-' is used if not known. Some systems have a close knit of stars, then one away from the group. This star is identified by the nominated letter, however the centre of the knitted group is identified as ‘P',  so ‘AP-Q' would be component ‘Q' some distance away form the group ‘AP', probably containing the brightest component. (Probably because sometimes it is not the case.)

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. The distance between the two objects is normally measured in second of arc. Combined with the motion of the Earth, the compass directions of ‘n' north and ‘s' south are applied, with the terms ‘p' preceding and ‘f' following. The preceding star would be before the object, if the drive was disconnected and the star allowed to drift through the field, and have a smaller value for right ascension. The following star is behind the object, having a smaller value for right ascension. This is very useful, because the telescope’s optical configuration is irrelevant.

In the telescopic field the star's position away from the object in question can be referred by its Quadrant position. The quadrant position can be used to check the discover's nominated star designation. This is especially important if the pair has not been observed for sometime because it may have moved or you simply want to measure it. Quadrant 1 is termed 'nf'; North-Following, Quadrant 2 is termed 'sf' South-Following, Quadrant 3 is 'sp' South Preceding and Quadrant 4 becomes 'np' North-Preceding. Figure 1 shows this particular system more clearly.

The most useful measure of position is by the two scalar quantities of position angle or θ (PA) and separation (Sep.) or the Greek letter ρ.

Position Angle (P.A.)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, 270o for west, and through to 360o and back to north again. The observed position angle is also influence by the precession of the equinoxes, so all values must refer to a certain epoch. Ie. Epoch 2000.0. These quoted values are easily converted

Separation is simply defined as the distance between the centre of the two stars measured in seconds of arc. or abbreviated as sec.arc. or just as " .

If the pair is found to be a true binary, a motion of increasing position angle is said to orbit in a direct motion, while the decreasing one is called retrograde motion. This has to be distinguished, unlike the planets, as it indicates the movements of the orbit so the observer can determine motion. A maximum distance in a binary orbit is referred to as the maximum elongation, the closest is minimum elongation. In the true orbit, eliminating the apparent random direction of the inclined orbital path against the Earth’s orbit, the closest approach in the orbit is called apastron, while the furthest point is called periastron. The observed motions in the binary will rapidly change, especially if the orbit is highly eccentric, near periastron, while at apastron the change will be very slow.

(1-4) Classification

Double Stars can be divided into four distinct categories. Class 1 are the main topic of this Handbook. The classification is based on the method of observation, and providing the system contains two or more stars.

Class / Type

1. Optical
i. Optical Doubles
ii. Visual Pairs
iii. Visual Binaries
iv. Unresolved Binaries

2. Photographic
i. Spectroscopic Binaries
ii. Astrometric Binaries

3. Photometric / Dynamical
i. Eclipsing Binaries
ii. X-Ray Binaries
iii. Variables
iv. Symbiotic Stars
v. Novae and Supernovae

4. Other
i. Multiple Stars
ii. Open Star Clusters
iii. Globular Star Clusters
iv. Association

Brief Summary of Each Class

Class 1 : Optical Double Stars

Stars that appear close together in the sky but are aligned by chance are referred to as Optical Double Stars. By measuring accurately the motions of each of the stars, or their common proper motions (cpm.), can lead over time to a real or apparent connection.  True binary systems will have similar common proper motions, while optical pairs may be in any direction. optical doubles are generally easy to detect, as most are wide pairs.

Examples include; δ1,2 Apodis, α1,2 Librae, β1,2 Tucanae, and α1,2 Capricorni.

Visual Pairs

General field doubles that have known field connections are all known as visual pairs. Time as it elapses, they generally sub-divided into visual binaries or optical doubles. Visual pairs make up the majority of all systems.

Examples include; Δ4 (Eri), Σ337 or x Velorum.

Visual Binaries

Visual Binaries are systems where the evidence points to the stars that are joined in a orbit, either based on visual or astrometric measurements. If the orbital elements that have been calculated with reasonable precision are called Absolute Binaries. To be an absolute binary, at least one orbit has to be completed before it can be included in this category. The ‘Fourth Catalogue of Visual Binaries’, published by the U.S. Naval Observatory by the authors Worley and Heintz, contains 847 binary systems with some 180 (21%) considered as absolute binaries. The confidence in the accuracy of the binary star's orbits are placed in a scale from 1 to 5, where 1 is considered as reliable. 5 is classed as indeterminate. The majority of all binaries are in the three category. Examples of these categories include; 1 for Alpha Centauri, 3 for p Eridani and 5 for the widest pair of Alpha Crucis.

Temporary Visual Binaries are a sub-class of this group. They are considered stars having a great separation of about 4 000 Astronomical Units (A.U.) or 0.2 parsecs. The limits in the stability of a binary is estimated to be about 500 A.U., so these stars are thought to have hyperbolic or parabolic orbits. The probability for the capture of these types of system is small, especially due to the high independent velocities of each of the components. Interestingly, the dissolution of the stars are more rapid due to either perturbations as influenced by nearby stars. Advanced computer simulations have shown that multiples and the star clusters can loose members quite easily. If the total energy of the stars in the system exceeded a certain quantity, an individual star if distant enough with a high enough velocity, the system will reject it. Examples of such stars in the process of rejection are not really known. The time in which we have been observing is far to short compared to the time the dissolution will occur. Theory proposes that most of the field stars visible in the night time sky have been temporary binaries. This is certainly true during the period of star formation, beginning probably during the time the star becomes free of the nebula.

Unresolved Binaries

One of the methods of detecting binaries is by the use of High Speed Photometry during Lunar Occultations. If the separations are between 0.3 and 0.01 arc seconds., this is a highly effective method. Discoveries are often made by the observation of  ‘fades’ in magnitude, with the star losing it brightness over a period averaging about 0.6 secs. Large stars like Antares in Scorpius, shows a single gradual fading in brightness. Binaries will reveal two peaks in the fade. The distance between the two peaks is proportional to the rate of the Moon’s motion, against the distance that the stars are apart. In the southern hemisphere observers are best to view or contact the Royal Astronomical Society of New Zealand (RASNZ Occultation Page. Here the experienced occultation observer Graham Blow has made extensive studies of the techniques required to do such observations.

Examples; Alycone in the Pleiades.

Class 2 : Spectroscopic Binaries

Two stars less than 0.1 arc seconds cannot be resolved by visual techniques because of the atmosphere and the conditions of seeing. The observer must then rely on other techniques that are  less direct. Such stars are commonly called Spectroscopic Binaries. These stars vary slightly in their individual radial velocities. From the changes in the orbits, one moving away from us, the other away from us, the spectra has a duplication of lines, caused by Doppler shifts. The degree of motion, or radial velocity, is then measured at a rate of a maximum of 30 kilometres per second. Each of the corresponding or shared spectral lines in turn will cross each other over a period of one to a thousand days. Gaining a sufficient amount of data will determine the orbit of the two stars.

The first object discovered this way was made by the observer E.C. Pickering in 1889. He discovered that the primary of the wide pair of Zeta (ζ) Ursa Majoris, was in fact a binary itself.
If the orbit can be determined with reasonable precision, the system can be predicted well into the future. If the brightness of the two stars exceed more than 1.5 magnitudes, the bright spectral lines will start to merge, forming an apparent set of single lines.

Some systems are transitions of between visual and spectroscopic binaries. All spectroscopic systems must be determined photographically or by CCD using a spectrograph. The moderately smaller professional telescope, around 1.0 metres are commonly used for such studies. The South African Observatory makes observation regularly of southern objects spectroscopic binaries.

Examples include; α Virginis (Spica), α Crucis and σ Puppis.

Astrometric Binaries

These are also a type of unresolved binaries. They can only be detected from the varying proper motions which they exhibit. The best examples of unseen companion of Sirius was first discovered by Bessel in 1844. The star Procyon was added to this list in 1903. Most of these types of objects are surrounded by white dwarf components, where the companion is eight to twelve magnitudes fainter than the primary. A white dwarf, because of it's solar like mass, causes the primary to ‘wobble' in the overall common proper motion. Detection of the companion is sometimes made visually, like the discovery of Sirius companion by Alvin Clarke made in February 1862. Others remain still undetected. Stars that have Brown Dwarfs or even planetary objects are suspected in a great number of stars. The difficulty is to come up with precise observations to confirm the existence of such objects. The errors, based on observational techniques, base the detection of such companions around 0.02 arc seconds. A telescope orbiting in space may be the only way to confirm or deny their existence.

It has been proposed in the mid-1980's, that the Sun maybe itself an astrometric binary, in which some brown dwarf is orbiting is orbiting in a period of eighteen million year. Appropriately named Nemesis, and may be the object responsible for the demise of the dinosaurs sixty-five million years ago. It orbits to a maximum of 100 000 A.U. from the Sun in a highly elliptical orbit. Aphelion, nearest the Sun, is thought to disrupt the Inner Planets, before returning to the depths of nearby interstellar space. It's absolute magnitude has been suggested to be at a magnitude of +30, well beyond the technology of present telescopes. Magnitude of the companion brown dwarfs is currently estimated somewhere between +20 and +30.

Examples of astrometric binaries include; Sirius, Procyon, ζ (Zeta) Cancri, Zeta Ursa Majoris, Lal 21185 and Ross 614.

Class 3: Photometric or Eclipsing Binaries

These objects are in fact in the realm of Variable Star. Photometric or Eclipsing Binaries are named after the periodic changes in brightness. The interaction of the disks, by one component to another, either by eclipse or transits makes way for their detection. By mathematical analysis, the true nature of the stars maybe deduced. By obtaining the intrinsic light curves, obtained by a photoelectric photometer, the nature of the objects can be revealed.

Orbital periods of these type of stars can vary between several hours to several days. A number have periods of several years.

Photometric Binaries can be considered like spectroscopic binaries except that the inclination of the orbit is at 90º. Some systems have been known to exhibit both properties.

Over 3 000 photometric systems are known. Each are classified by the gravitational forces between the components, which is commonly referred to as degree of detachment. Some of these stars are so grossly distorted, that they appear like dear-drops or distorted ovals. Consequently the light curves that are observed have different properties, some having smooth changes in brightness, some are more jaggered.

The stars with the closest orbits, called contact systems, can transfer material from one star to another. This causes deviations in their evolution. The example of the W UMa class of variables are so close that an eventual total merger is possible. These objects are categorised as FK Coma Berenices type stars which show fast rotational velocities with extreme magnetic properties. Their eventually outcome is the R Corona type variables. theoretically merging a star with a white dwarf, causing a core less than 1.4Msolmass solar masses.

Examples include; W Crucis, β Lyrae, ζ (zeta) Aurigae (Period 27 years) and RS Canum Venticorum.

Eruptive Variables, X-Ray Binaries and Novae

Generally binary systems can suffer brightness variations that are often caused by a single unseen companion such as a white dwarf or low luminosity star. These can transfer material from one star to another. It can cause a violent reaction by producing an outburst of energies in very short periods such as in the dwarf novae like U Geminorum. Some are not violent outbursts but may show slow periodic changes, indicating either atmospheric instabilities or changes in both stars. Most of these type have at least one star as a white dwarf. Examples include; Y Cygni, TX UMa and DQ Herculis.

X-Ray binaries can be claimed to be similar, except the peak of the X-Ray portion of the electromagnetic spectrum. Only high temperatures can only be produced from hot gases. The material is said to be material falling into the receiving star, either as an accretion disk or ‘hot-spot’ above the surface of the star. Examples of these include Cygnus X-1, Musca X-1 or Circinus X-1.

In some examples, once material drawn towards the star can accumulate on the companion white dwarf’ stellar surface. As the gas continues to flow it forms a thick, high pressure atmosphere. Here the star’s internal core temperature is confined, so causing a ‘pressure cooker’. Once the star reaches at temperature that is hot enough, the nuclear fuel comprised mainly of hydrogen and helium, can spontaneously ignite, causing quick and dramatic increase in luminosity - perhaps rising between ten to fifteen magnitudes. These eruptions appear as the Novae
Sometimes, after the outburst, radiating shells of gas can be seen. For example, Novae Persei in 1901, is a classic example.
The process of novae production is also not necessarily a one off event. These are the Recurrent Novae that have been observed to recur over tens of years. Presumably, the source of the material still continues to be supplied to the dense companion, reigniting when the temperature reaches the critical level.

Even more dramatic are the Supernovae Type I which is one of the most violent events astronomers can observe. This is also caused by mass transfer of material from one star to a white dwarf companion weighing just under 1.4Msolmass Solar Masses. At this mass the white dwarf is dangerously balancing to a point of instability. The matter existing on the surface is suffering enormous gravitational forces. The forces in the atom are normally much greater compared the gravity produced by stellar objects, preventing the electrons physically merging with the protons in the atom. This balance is properly called the degenerate pressure. If any material is added to the system, the star must eventually go over the edge. It reaches this stage, and in the blink of an eye, the star collapses. The nuclei of all atoms are squashed into the protons, with the end product of neutrons and a huge amount of energy. An implosion occurs which shrinks the star from an object the size of the Earth, to one about five kilometres in diameter in just 0.4 seconds! The star cannot cope with the incredible changes, and the energy produced throughout the core of the star, explodes the entire mass back into space. Normally this should produces a neutron star but the violence produced from the collapse is far greater than the gravity holding it together.

Type 4: Larger Optical or Dynamical Stellar Systems

Multiple Stars

These are groups of stars that contain between three and twenty components, all under a mutual attraction of gravity. Multiples have unusual properties that do not apply to binary systems. Multiples arrange themselves in hierarchal arrangements, that have their own complexities. Triple systems normally are a close pair, orbited by a smaller companion in a much longer orbital period. Alpha Centauri, is an example. The close pair AxB orbit in 79.8 years, while the ‘C’ component, Proxima Centauri orbits once every 100 000 years or so. Another type of system is the Trapezia, named after the brightest in the Orion Nebulae called the Trapezium. These objects contain four (or more) stars, usually 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.
Examples include the; Alpha (α) Centauri system, the Trapezium in Orion θ1, α Geminorum, Sigma (σ) Orionis and α Crucis.

Open Star Clusters

Open Star Clusters (OSC’s) are relatively young objects compared to the Sun. They each can contain anywhere between 50 and 1000 stars. Over 80 000 systems are known (2005) in our sector of the galaxy, normally associated with nebulae and the Milky Way’ spiral arms.

All clusters really should not be considered as binaries star, but perhaps more as extended multiple stars. There are two basic similarities to the Multiples; The orbital motions observed in the cluster is controlled by Soft and Hard Binaries. These binary stars control the orbital energies of the cluster, literally holding them together. The relative strengths of this binding force depends on the number of components and the size of the group. This in turn, determines how long the cluster will stay together, without losing membership. The ages in which a star cluster will hold together is perhaps somewhere between one million or one billion years. Examples; NGC 4755 ‘The Jewel Box’, M41 in Canis Major, the Pleiades and Hyades in Taurus, the Southern Pleiades IC2602 and NGC 3572 in the ηCarinae Nebula.

NOTE : A more extensive written article on Open Star Clusters and examples pf them appear in the “Catalogue of the 100 brightest Open Clusters” appears here in Southern Astronomical Delights.

Globular Star Clusters

Globular Star Clusters (GSC’s) compared to the open star clusters and the Sun, are much older. They contain between 500 to 10 000 times more stars than their open cousins. The largest contain several million stars. The two brightest are the southern gems of ω Centauri and 47 Tucanae. Over two-hundred globulars star clusters are known to be associated with our galaxy, all orbiting around the galactic centre of the galaxy. A number of the nearby galaxies also have GSC’s.

The stars in cluster are also controlled by the hard binaries, though several of these could possibly exist in the core of the cluster. These controlling pairs are also known as durable binaries, as they can hold the cluster together for billions of years. Core binaries in globular systems would have to be massive objects to act as a controlling force, suggesting either stellar remnants like neutron stars or even black holes. Some globulars are known as strong X-ray sources, eluding to these types of objects.
Examples; M22 in Sagittarius, M13 in Hercules, M30 in Capricornus, M4 and M80 in Scorpio, ω Centauri and 47 Tucanae.

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