Binary star systems.

A binary star is truly an awe-inspiring phenomenon — two massive balls of plasma circling each other in a spectacular and reality-bending dance of death.

Just think for a moment about the raw power of our star, the Sun. From the immense thermonuclear fusion at its core and the coronal mass ejections (CMEs) thrown out from the surface to its truly awesome, Solar System-extending gravitational pull, our lone star is truly a magnificent spectacle. Granting life, taking life and bending light years of particulate matter to its will, nothing in the Solar System can compare to our star’s majesty and might. Now think what would happen if you put two of these stellar titans side by side, locked around a common centre of mass…

When two stars are in orbit around a commonly shared centre of mass — called the barycentre — they are known as a binary star system and, as you might expect, the results of their shared central point can be spectacular. Indeed, the endgame for many of these binary star systems are both runaway novas and type la supernovas, with one of the two behemoths accreting material from the other to such an extent that a cataclysmic nuclear explosion is triggered. This explosion either generates a variable star — one in which the brightness of one of the system’s component stars is dramatically increased for a short period — or a Chandrasekhar limit (the maximum stable mass of a white dwarf), at which point a star is close to total destruction.

Not all binary star systems end in this explosive way, however, with many types of system generated and a wide range of processes involved. Indeed, it has been estimated that perhaps up to half of all stars in the Milky Way are binaries, orbiting each other around a common centre of mass at variable distances, velocities and orbit types.

For example, certain binary systems may have orbits that are so vast that they can take decades -or even centuries — to complete, making their detection and observation difficult to near impossible. Others, in contrast, such as eclipsing binaries, are far more easily observed. These are stars in a binary system whose plane passes through, or very near to, Earth’s and so when viewed from our planet they eclipse each other.

The observation of binaries is important to astrophysicists as their common centre of mass and therefore orbital movements allow their component stars’ masses to be directly determined, along with myriad other stellar dimensions such as radius and density. In fact, how binary systems are detected also dictates how they are classified. For instance, optically detected binary stars are called visual binaries, those detected by spectroscopy (an observation that takes advantage of the Doppler effect) are known as spectroscopic binaries, and those discovered by astrometry are referred to as astrometric binaries. For binary star systems whose component stars are close in distance — as described above — these ‘close binaries’ lead to the gravitational distortion of stellar atmospheres and enhanced star evolution, as typified by the type la supernova scenario.

How binary star systems form in the first place is currently not a comprehensively understood process. However, all binaries must be born out of star-forming molecular cloud cores, as the likelihood of one star partnering with another through gravitational capture alone is very low.

Current theory therefore suggests that binary stars are formed in one of two ways. The first method is by molecular cloud fragmentation during the formation of protostars — an early precursor phase to main-sequence stars. Molecular cloud fragmentation is a process in which a potential star-forming cloud of molecular hydrogen is divided into two or more density cores by the interaction of surrounding magnetic fields, as well as under the influence of its own mass and molecular composition. As a result, due to each core typically leading to the birth of a protostar, the splitting of a single core into multiple clouds leads to the creation of more than one star within the same region.

The second theorised possibility for binary star generation is by the ejection of one of three newborn stars (with an equal mass) from a system due to the closer relationship between the other two. This ejection can lead to the phenomenon of ‘runaway stars’, a process that may also be triggered by the collision of two binary systems. Either way, this ejection leaves two stars within the original system — which will likely become a binary — and opens up the possibility of an additional one-star system ‘catching’ the runaway to form another binary. The relationship between a binary star system’s component stars is described in a ‘primary-companion’ relationship, with the larger of the two stars being considered the primary member.

Interestingly, since the first evidence of binary star systems was discovered in the early-19th century, astrophysicists have been attempting to discover whether any component star of a binary was capable of hosting its own satellites. The answer to that has only recently been confirmed, with NASA announcing in September 2012 that not only can a binary system sport component stars with their own planetary satellites, but entire transiting circumbinary systems too — with multiple planets orbiting around the binary’s barycentre.

This discovery was made by analysing the Kepler-47 binary star system, which is located approximately 5,000 light years from Earth in the Cygnus constellation . What is most intriguing, perhaps, is that NASA’s analysis of these planets indicates that one of the two system planets -Kepler-47c — is positioned in a habitable zone, where liquid water could exist. And if water does indeed exist on the planet, then so too could life.

Importantly, binary star systems are not merely two stars that appear close together. If two stars appear visually close then, while they may indeed be part of a binary system, they could just as easily be two independent stars in close proximity to one another. These latter stars are referred to as optical double stars and are in fact usually very far apart, with their chance alignment as viewed from our world creating an optical illusion of influence.

Misleading optical doubles aside, many binary star systems are visible from Earth, with systems such as Alcor in the constellation Ursa Major, and Albireo A in the constellation Cygnus easily viewable. Indeed, Albireo A provides quite spectacular viewing due to its rich amber colouring.

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