Our sun is alone in space; it is a single star 4.26 light years away from its closest stellar neighbour (α Proxima Centauri). For many other stars this is not the case, it has been estimated that between 50%– 66% of all stars are in binary (a two star system) or even larger multiple star systems (with more than two stars).

A binary system can have several configurations – The two can orbit a common centre of gravity (termed a barycentre) which lies between the two, or the lower mass star can orbit the higher mass star just as Earth orbits the sun^[1].

A Detached Binary Star System Credit: Zhatt

These configurations also create three distinct physical types of binary star – Detached Binaries, Semi-detached Binaries and Contact Binaries.

Detached binaries are essentially two stars that orbit far enough apart so that one does not have a significant affect on the other – i.e. each star evolves independently and as normal.

A semi-detached binary is a pair with a more involved interaction. One star fills it Roche Lobe – This is the area of space within which the star’s gravity is stronger than that its partner – the other does not fill its Roche Lobe. This results in a process of matter transfer off the larger star to the smaller one. This matter forms a disk as it spirals round its new star. Eventually friction reduces its speed and it falls on to the surface of the star – it does not fall directly onto the star due to the conservation of angular momentum. A semi-detached binary may evolve form a detached binary after the more massive star swells as it leaves the main sequence by giant formation.

An animation of a semi-detached, eclipsing binary system (with luminosity plotted against time)

Contact binaries occur when both stars fill their Roche Lobes, this results in the outer atmospheres of both stars joining to form a shared ‘common envelope’. This envelope causes friction which slows the orbits of both stars. This decrease in orbital velocity can eventually (but not in every case) cause both stars to merge entirely forming a higher mass, single star.

As well as the three physical kinds of binaries there are also four observational types of binary stars – Visual, Spectroscopic, Eclipsing and Astrometric.

Visual binaries are far enough apart that their identity as a binary can be determined by direct observation using high powered binoculars or telescopes. The brighter star of the pair is generally accepted to be the primary star with the dimmer being the secondary. It is harder to separate two very bright stars, or a very bright and a very dim star in this way as the stars can appear as one object due to glare.

Spectroscopic binaries can’t be separated by standard observation alone due to the stars proximity to one another and/or due to the effect of glare. This may mean that the stars can only be distinguished from one another through analysis of their spectra. The movement of the stars in their orbits causes the spectral absorption lines (the gaps in the light spectrum of a star due to absorption by the various elements that make up the star – see here) to shift towards either the red or blue end of the spectrum due to the Doppler Shift. When a star is moving away from the observer – i.e. us – its spectrum shifts towards the red end – it redshifts. As a star moves towards the Earth its spectrum shifts the opposite way – towards to the blue end – it blueshifts. When these movements have been recorded several times in a regular pattern, astrophysicists can confirm that the star is orbiting a point in space and may even be able to identify its partner. This type gets its name as they are identified through examination of their spectra hence spectroscopic.

Eclipsing binaries

Sometimes the nature of a binary system can be determined by monitoring for any changes in the luminosity of a ’star’. Some stars fluctuate in brightness over a period of days or weeks some of these will be eclipsing binaries. If the stars brightness increases it is likely to be a flare star, a Cepheid variable or a nova. Some ’stars’ luminosity can drop for a short period at then increase back to normal levels, what’s more these ’stars’  may have more than one dip in a luminosity cycle. These are likely to be eclipsing binaries – meaning that whilst they appear as a single star to the naked eye they are in fact pairs of stars that reveal their true nature by passing in front of one another. As can be seen in the animation below when one star passes in front of the other some of the light from the other star is blocked making the apparent brightness drop. This is most noticeable when the dimmer star passes in front of the brighter star. This type gets its name from the way in which the stars eclipse one another hence eclipsing binaries.

An animation of an eclipsing binary(with luminosity plotted against time)

It is important to note that a binary system can fall into several of these groups for example – An eclipsing binary system might also be a spectroscopic system as well.

More Complex Multiple Star Systems

A ternary system – containing three stars – may have all three stars orbiting a shared point however this seems to be uncommon as such set ups would not remain gravitationally stable for long.

More likely is a situation where two stars orbit the larger ‘primary’ centre star, these two stars may also be binary i.e. they orbit each other. An example of such a system is Gliese 570 (which also has a brown dwarf orbiting the primary, talk about crowded!).

Another possibility is a pair of binary stars with a third star on a longer orbit – an example of such a system may be Alpha Centauri AB – The central pair contains the fourth brightest star in the  sky; the slightly larger than the sun the G2V class α Centauri A (for more information on spectral classes of stars see – Spectral Classes Explained) its partner, the smaller and less luminous K1V class α Centauri B (which should not be confused  β Centauri a blue supergiant). They are potentially orbited by our closest star Proxima Centauri however it is currently unclear whether Proxima is actually orbiting the pair or whether it is a matter of a coincidental alignment of trajectories (they may just be moving in the same way through space relative to each other) or perhaps they all shared an origin in a star cluster that has since dispersed.

An artist's impression of the Gliese 570 System Credit: NASADr. Robert Hurt, Infrared Processing and Analysis

Please note – Both the small animations used in this post are free from copyright

^[1] A planet orbiting a star is in actual fact still orbiting the barycentre, though this is a special case where the barycentre lies within the space occupied by the parent star. Multiple planets in a system – such as our own – can actually cause the barycentre to occur outside the parent star if their is enough mass contained within the orbiting planets.

A Binary Star System with a Massive Primary and Low Mass Secondary Credit: Zhatt