What Are Binary Stars?

Binary stars are two stars that share a common center of mass. The brighter star of the two is classified as the primary star, while the dimmer of the two is known as the secondary star. Binary stars may also be classified by the mode in which they are observed, such as visual binaries, spectroscopic binaries, eclipsing binaries, or astrometric binaries.

The term binary star, as the name suggests, is a star system that consists of two paired stars, in the most rudimentary sense. Many of the single points of light we see in the night sky are actually two or more stars orbiting together: roughly half of all Sun-like stars have at least one companion, and among the very brightest stars, multiples are even more common. (The lowest-mass red dwarfs, which vastly outnumber Sun-like stars, are mostly solitary, so most individual stars in the Galaxy are in fact single.) The most common form of a multiple star system is a binary star. These pairs come in an array of configurations that help scientists to classify stars, and help study the potential for nearby life. Let’s now take a look at the classification of these binary stars.

Classification

Binary stars are composed of two stars that share a common center of mass. The brighter star of the two is classified as the primary star, while the dimmer of the two is known as the secondary star. If both stars are equally bright, the designation given by the discoverer is taken as the standard. The first feature on which binary pairs can be classified is their orbit. Wide binaries are stars with orbits that keep them spread apart from one another. These stars evolve separately, with very little influence from their companion. Sometimes these wide binary systems might have once contained a third star that booted the distant companion outward before being ejected itself. Close Binaries, on the other hand, evolve in proximity and can transfer their mass between one another. The primary stars in some of close binary systems end up consuming material from their companion, sometimes exerting such a strong gravitational force that it pulls the secondary  star into itself.

Binary stars may also be classified by the mode in which they are observed. Visual Binaries are two stars with a wide enough separation that both can be viewed through a telescope, or even with a pair of binoculars. 5-10% of stars come under the category of visual binaries. Another classification of binary systems is a spectroscopic binary. These appear close to one another, even when viewed through a telescope. Scientists must measure the wavelengths of light that the stars emit and determine their binary nature based on the features of these measurements. Eclipsing Binaries are at an angle to one another. They are at such an angle in relation to Earth that one passes in front of the other, forming an eclipse. This feature is based more on line-of-sight than any significant feature of the pair. The last type of classification are known as Astrometric Binaries. Believe it or not, these are stars that scientists believe dance around space; more specifically, their companion cannot be identified, but only inferred. Such a companion might be too dim to be seen or might be blocked out by glare from the primary star.

Discovery And Evolution

The first discovery of a binary star was a visual binary (we will go into more detail on this in the upcoming section). In 1617, at the request of a fellow scientist, Galileo Galilei had the opportunity to observe them. With the help of his telescope, he directed his attention to Mizar, the middle star in the handle of the Big Dipper; at first, he thought he had found an optical aberration, but was later astonished by what he saw. While observing Mizar, which appeared to be a single star, he found that there were actually two stars side by side; later, it was confirmed that the star system was actually a combination of six different stars! In 1802, Sir William Herschel, who cataloged around 700 pairs of stars, was the first to use the term binary for these star pairs.

Now, when it comes to the evolution of binary stars, there usually isn’t just one way in which they can be formed. The rarest way is by stars traveling around the galaxy. This occurs when a massive moving star catches a smaller one as it passes by, creating a binary pair. However, the most common way in which a binary star is formed is when an envelope of gas forms, because it can collapse within itself, thereby causing a split. Although two stars in a binary system evolve together, they do not necessarily develop at the same speed.

How a pair of stars evolves depends on their distance from one another. Wide binaries have a minimal effect on each other. Close binaries, however, impact each other’s evolution, which involves the transferring of mass from one star to another, which changes the composition of the stars. If one star in a close binary system ends up exploding as a supernova, leaving behind a dense remnant (a neutron star, sometimes seen as a rapidly spinning pulsar, or a black hole), the companion is often disrupted as well. If it survives, it continues to orbit the newly formed body, perhaps passing on even more of its material.

Another crucial aspect of a binary star system is that it can help scientists calculate the mass and size of both stars as the pair pulls on one another; scientists can make predictions about the size and other essential parameters, such as the temperature and radius of the star. Stars in multiple systems also have a significant impact on the potential for life. A host of planets have been found orbiting around many different star systems. The orbits of these stars can drastically affect life, which needs a relatively stable system to develop in, although binary and multiple star systems might not seem conducive for the sustenance of life. However, we must take into consideration the movement of planets around the stars; the varying amount of light, heat and radiation they receive could produce conditions that could create life.

The closest star system to our solar system, Alpha Centauri, is a binary star system. The two stars in the binary star system of Alpha Centauri are called Alpha Centauri A and Alpha Centauri B, respectively. The third star, Proxima Centauri, sits roughly one-fifth of a light-year away (about 13,000 sun-Earth distances). Astrometric measurements published in 2017 confirmed that Proxima is gravitationally bound to the pair, looping around them once every roughly 550,000 years, which makes Alpha Centauri a true triple system. While no stars in the habitable zone have been found in the binary star part of Alpha Centauri, the planet Proxima Centauri b was announced in 2016 as being in the livable region of its star. However, scientists are divided as to whether a red dwarf star such as Proxima Centauri has stable enough “space weather” to prevent radiation or heat surges, which would diminish the chance for life on a nearby planet. In conclusion, we can safely state that binary stars serve as a vibrant and plentiful frontier for scientists to work, as further research may answer existing and brand-new questions in the field of stars and their behavior.