The term double star is really a misnomer, as it only infers two stars being near each other in the sky. Under this broad classification, then triple stars and other multiple systems would also have to be included. A general and useful definition was first stated by Sir William Herschel in his 1802 paper — 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.

Ancient Greeks first named the term διπλονρ for double star, by the naked-eye observation of the visual pair if ν1 and ν2 Sagittarii. These stars are about 14 arcmin or 840 arcsec apart. Since the invention of the telescope, true double stars are considered to be just below naked eye resolution. The modern definition states that double stars are;

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

Under this definition, all 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 multiple stars. Although bound by the universal force of gravitation, the dynamical features holding the stars together do not apply with star clusters. For instance, orbits are undefined due to the variability of gravitational attraction between individual stars, and therefore produce unstable orbital motions. It is interesting that many modern theories on how the star clusters hold together is now normally thought to be controlled by one, or perhaps several, core binaries. This central binary contains most of the bound-up kinetic energy of the cluster, preventing individual components escaping. 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 within roughly the order of several kiloparsecs away, with the globulars generally lying tens of kiloparsecs away. Many observed telescopic visual pairs are nearly all within 500 parsecs (1,600 light years).


Of the true double stars, the term pair is generally applied to stars where the physical connection is unknown. This same 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 so an orbit may be deduced, the term binary or binary system is used. They are distinct from simple double stars, as real physical association has been determined.


When naming components, the following simple trends are often applied to double stars;

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 discoverers initial distinction is used. This distinction between the primary and secondary then applies until the masses are accurately determined.

Difference in magnitude is termed Delta-m, written as Δm, usually to one or two decimal places.

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 any multiple, the binary is ascertained as A or A×B is noted as the controlling binary. The components from this binary are noted from its 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 AB×CD, identifies two binaries, with internal orbits in the system.

These pairs together may or may not be associated together in some multiple star. If they are associated ×’ is used between the two pairs, otherwise B is used if not known.

Some older 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. (Though sometimes this is not the case.)

For identification or descriptive purposes the system, or any telescopic astronomical object, the diurnal motion of Earth can be used to show the position of the surrounding objects. Distances between the two close objects is normally measured in second of arc. Combined with the motions of the Earth, the compass directions of n north and s south are applied, with the terms p preceding and f following. A 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. A following star is behind the object, having a smaller value for right ascension. This is very useful because the telescopic optical configuration is irrelevant.

In the telescopic field the stars position away from the object in question can be referred by its Quadrant position. The quadrant position can be used to check the discovers 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
Quadrant 4 becomes np North-Preceding.

Figure 1 on the earlier page shows this particular system far more clearly.

One more 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 or 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 0° position angle is celestial north, the 90° position angle is east, 270° for west, and through to 360° and back to north again. Observed position angle is also influence by the slow precession of the equinoxes, so all values must refer to certain standard epochs. I.e. 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 arcsec. or just as ″.

If some pair is found as a true binary, the increasing motion of position angle is said to be direct motion, while the decreasing one is called retrograde motion. This has to be distinguished, unlike the planets, as it indicates the direction so the observer can determine future or past orbital motion. Maximum distance in binary orbits are referred as maximum elongation, the closest is minimum elongation. In the true orbit, eliminating the apparent random direction of the inclined orbital path against the Earths orbit. Hence, the closest orbital approach becomes periastron, while the furthest point is called apastron. Observed binary motions will rapidly change, especially if the orbit is highly eccentric and near periastron. At apastron the change will be much slow and pedestrian. Differences in the rate of change in orbital motion is dependant on the main systems eccentricity or e.


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


1. Optical
i. Optical Doubles
ii. Visual Pairs
iii. Visual Binaries
iv. Unresolved Binaries
[v. Wide Pairs]

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



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 knowing real or apparent connection. True binary systems will have similar common proper motions, while optical pairs could 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 commonly known as visual pairs. As time elapses, they will all generally sub-divided into visual binaries or as optical doubles. It is the task of the double star observer to use the historical measures and current positions to assess the true gravitational connection between the two components. However, due to the extensive range of feasible orbital periods, those roughly between several hundred years and several tens of thousands of years, most of the observed positions have changed little; or if at all. Added to the eccentricities of the orbits or the distances of the systems, etc., find our knowledge of optical systems or binary stars among the visual doubles are not certain. Perhaps as much as 70% of all systems are theoretically gravitationally attached, the reality finds, perhaps only 5% to 10% to date has been verified observationally. Therefore, of all catalogued double star systems, we find that visual pairs make up the majority of all systems; and whose numbers only slowly dwindle as the attachment or not is discovered.

Examples are; Δ4 (Eri), Σ337 / STF 337 or wide system of x Velorum (Δ95).

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 late-Charles Worley and Wulff Heintz, contains 847 binary systems with some 180 (21%) considered as absolute binaries.

Confidence in binary star orbits accuracies are originally placed on the simple scale from 1 to 5, where 1 is considered as reliable. 5 is classed as indeterminate. A majority of all binaries are in the three category. Examples of these categories include; 1 for Alpha Centauri, 3 for the southern system of p Eridani and 5 for the widest pair of Alpha Crucis.

Temporary Visual Binaries are the sub-class of this group. They are considered stars having greater separation of about 0.2 parsecs or 4,000 Astronomical Units (AU). Estimated stability limits for binaries are about 500 AU, so these stars are thought to have either hyperbolic or parabolic orbits. Probability for these system types to be captured is small, especially due to components having high independent velocities. Interestingly, dissolution of these star systems are more rapid due to perturbations as influenced by the nearby stars. Some advanced computer simulations have shown that both multiples and star clusters can loose members quite easily. If the total stellar system energy exceeded certain quantities, then it is possible for some small or distant individual star to achieve a high enough velocity, and escape free into interstellar space.

Examples of such stars in the process of rejection are not exactly known but are expected. As the time which we have been observing such systems is far to short compared to the time the dissolution occurs. Advanced theory proposes that most of the field stars visible in the night time sky may have been once have been temporary binaries. This is certainly true during the star formation period, probably even beginning when the star becomes free of its own life-bearing nebulosity.

Unresolved Binaries

One of the methods of detecting binaries is by the use of high speed photometry during lunar occultations. If separations are between 0.3 and 0.01 arcsec, this nay be 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 seconds. Large stars like Antares in Scorpius, shows single gradual fading in brightness. Binaries will reveal two peaks during the fade. The distance between the two peaks is proportional to the rate of the Moons motion, against the distance that the stars are apart. Observers in the southern hemisphere are likely 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.


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 have individual radial velocities that vary slightly. From the orbital changes, one star is moving away from us, the other away from us, so the single stellar spectra has the duplications of two sets of lines, caused by Doppler shifts. The degree of motion or radial velocity, is then measured at the maximum rate of around 30 kilometres per second. Each of the corresponding or shared spectral lines in turn will cross each other over the periods of between one and one thousand days. If the observer gains sufficient data, then the mutual orbit of the two stars can be determined.

One of the first object discovered this way was made by the observer E.C. Pickering in 1889, who 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 systems positions can be predicted well into the future. If the two stellar brightnesses exceed more than about 1.5 magnitudes, then the bright spectral lines will start to merge and forming apparent single lines.

Some systems are transitions of between visual and spectroscopic binaries. All spectroscopic systems must be determined photographically or by CCD using the spectrograph. 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 special type of unresolved binaries. They are only detected by the varying proper motions which they exhibit. One of the best examples is the unseen companion of Sirius as first discovered by Bessel in 1844. Procyon was similarly added to this list in 1903. Most of these types of objects have white dwarf companions, often being eight to twelve magnitudes fainter than the primary. White dwarves, because of their solar like masses, causes the primary to slightly wobble in the overall common proper motion. Companion detection is sometimes made visually. This is like the discovery of Sirius companion, Sirius B, by Alvin Clarke made in February 1862. Others, however, still remain undetected. Stars that have Brown Dwarfs or even planetary objects are suspected in a great n umber of stars. One difficulty is to come up with precise observations to confirm the existence of such objects. Errors, based on observational techniques, have detection of such companions to 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 since the mid-1980s, that the Sun maybe itself an astrometric binary. This might be some small brown dwarf or large planet that is orbiting in its highly elliptical orbit once every eighteen million years or so. Appropriately named Nemesis, this hypothetical object has the maximum solar distance of about 100,000 AU, but when it reaches perihelion, when nearest the Sun, is thought to possibly disrupt or perturb the inner planets orbits, before returning to the depths of nearby interstellar space. Nemesis as an idea was proposed as the possible cause that was responsible for the demise of the dinosaurs sixty-five million years ago. Later investigations have since proven almost beyond doubt that this extinction was due to an asteroidal collision in Central America; immediately discarding the need for such a rouge object.

Suggested absolute magnitudes has been made at +30, being well beyond the present telescopic technology, though other techniques may reveal. Theoretical companion brown dwarfs are currently estimated between +25 and +30 magnitude. Actual existence of a solar dark companion has been met with much scepticism. It is now mostly rejected by the astronomical community.

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


Photometric or Eclipsing Binaries

These objects are in the realm of variable stars than the ordinary double stars. Orbital periods of these type of stars can vary between several hours to several days. A few have periods of several years. All photometric or eclipsing binaries are mostly named after the periodic changes in brightness. Interaction of the disks are observed by one component to another, either detected during eclipses or transits. Using established mathematical analysis, the true natures of each stars maybe deduced. By obtaining the repeated intrinsic light curves, obtained by photoelectric photometery, the nature of the objects can be revealed.

Photometric Binaries can be considered like spectroscopic binaries except that the inclination of the orbit is aligned at 90°. 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, 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 jagged.

Stars with the closest orbits, are often called contact systems, can transfer material from one star across to another. This causes small and obvious deviations in their evolution. Most obvious examples are the W UMa class of eclipsing variables, which are so close that an eventual merging of stars are possible. These objects are categorised as FK Coma Berenices type stars which show fast rotational velocities with extreme magnetic properties. The eventually outcome is thought to be R Corona type variables — theoretically merging of two star with white dwarfs, causing cores below the 1.4M⊙ solar masses.

Examples include; W Crucis, β Lyrae, Zeta ζ 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 violent reactions by producing an outburst of energies in very short periods such as in the dwarf novae like U Geminorum. Some are not as violent outbursts but may also show slow periodic changes, indicating either atmospheric instabilities or changes in both stars. Most of these types 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 thick, high pressure atmosphere. Here the stellar internal core temperature is confined, akin to some 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 the classic example. Processes of novae production is also not necessarily one off events. 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 some 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 1.4 solar masses. At this mass the white dwarf is dangerously balancing to a point of instability. Surface matter is suffering enormous gravitational forces, and where atomic forces that are normally much greater compared to gravity, but in these kind of stellar objects it can be overcome. Most astronomical objects can easily prevent electrons physically merging with the protons in the atom — the balance point being properly called the degenerate pressure. If any new 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 may collapses. Atomic nuclei are then squashed into the protons, with the end product being neutrons and huge amounts of liberated energy. An implosion then occurs, shrinking the Earth-sized object to one about five kilometres across all in just 0.4 seconds! Mosts stars cannot cope with these incredible changes, and therefore the energy produced throughout the stellar core, then dramatically explodes the entire mass back into space. Normally this should produces small neutron stars, but the terrible violence produced from the collapse is far greater than the gravity holding it together.


Larger Optical or Dynamical Stellar Systems

Multiple Stars

These are groups of stars that contain between three and twenty components, all under the same 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 close pairs, orbiting smaller companions in much longer orbital periods. Alpha Centauri, is an example. The close pair A×B 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. Such 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 close pairs, with one solitary star in orbit. The other star is rejected from the system altogether at 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 (ασ) Orionis and α Crucis.

Open Star Clusters

Open Star Clusters(OSCs) or Open Clusters are relatively young objects compared to the Sun. They each can contain anywhere between 50 and 1,000 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. Relative strengths of this binding force will depend 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 IC 2602 and NGC 3572 in the η Carinae Nebula.

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

Globular Star Clusters

Globular Star Clusters (GSC) or Globular Clusters when compared to most 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 of which contain several million stars. Two of the 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 globular star clusters.

All the stars in cluster seem to be also controlled by one or more hard binaries, existing near 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 the 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; Omega Centauri, 47 Tucane, M22 in Sagittarius, M13 in Hercules, M30 in Capricornus, M4 and M80 in Scorpius, ω Centauri and 47 Tucanae.


Last Update : 29th December 2017

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