Binary Star Systems: The Ultimate Guide

When we gaze at the night sky, it’s easy to assume that each twinkling point of light stands alone. Yet astrophysical observations paint a different portrait. Double stars, often romantically termed binary systems, are where two celestial bodies dance around a mutual center of mass. Expand this framework, and you encounter triple, quadruple, and even more complex starry assemblies. Recent astronomical research suggests that the majority of stars we observe are part of binary or multiple star systems, making our own solitary Sun an outlier in the universe.

Double and multiple stars come in a variety of orbital configurations and classifications, from visually distinguishable binaries through telescopes or binoculars to those discerned only via their spectral signatures. The fascinating dance of these stars, revolving around a shared center of mass, is not just a breathtaking sight for amateur and professional astronomers alike, but it also provides crucial data that contributes to our broader understanding of the universe.

While binary star systems are common, as we add more stars to the mix, the frequency decreases. Binary and solitary stars are believed to be comparably common, with some estimates suggesting that over 50% of the star systems in our galaxy are binary systems! Triple stars, while not as common as binaries, are still frequently observed and probably make up around 5–10% of star systems. However, as we move to quadruple systems and beyond, their prevalence diminishes rapidly. Quintuple systems and those with even more stars are rare in our galaxy as well as in the universe as a whole. This is simply due to the improbability of such systems forming, let alone remaining intact over millions, if not billions, of years.

Differences between optical and true double star systems

Some of the close-appearing pairs or close groupings of stars in the night sky are genuine binary or multiple-star systems that share a real, physical proximity, gravitationally bound and orbiting a shared center of mass. Others, however, are mere optical illusions—coincidental alignments of stars that appear close from our perspective on Earth but are, in reality, separated by vast distances in space. Understanding the distinction between these types is crucial for both amateur and professional astronomers.

True binary and multiple-star systems are born from the same molecular cloud, these stars remain gravitationally intertwined throughout their lifetimes. Their celestial dance, driven by gravitational forces, sees them orbiting a common center of mass, influencing each other’s stellar evolution, especially if they’re in close quarters.

One of the most celebrated examples that highlights the complexities of differentiating between these two is the star Albireo in the constellation Cygnus. To the naked eye, Albireo appears as a single star, but even a modest telescope or astronomy binoculars reveal it as a double. For a long time, Albireo was considered a prime example of a binary system, with its contrasting blue and orange components offering a stunning visual for amateur astronomers. However, further studies using distance measurement techniques like parallax measurements and proper motion analysis have cast doubt on this. The two stars are several light-years apart and may not share a gravitational bond, making Albireo a prominent example of an optical double star.

Differentiating between optical and true binaries requires a combination of observational techniques. By studying the parallax shift of stars at different times or analyzing their proper motion against more distant celestial backdrops, we can gain insights into their true nature. Additionally, spectroscopy provides a window into the composition and motion of stars, revealing the signatures of genuine binaries even when they’re too close to be individually resolved.

How do Binary Stars orbit each other?

Every object with mass exerts a gravitational force, and in a binary star system, both stars pull on each other. Contrary to a common misconception, the two stars in a binary system don’t orbit around the more massive star. Instead, they both orbit a point called the barycenter. The position of this barycenter depends on the masses of the two stars. If one star is significantly more massive than the other, the barycenter will be closer to the heavier star. In cases where the stars have similar masses, the barycenter might be almost equidistant between them.

Kepler’s First Law of Planetary Motion, which also applies to binary star systems, states that the orbit of a celestial body about another is an ellipse, with the barycenter at one of the two foci. This means most binary stars don’t move in perfect circles around the barycenter. The elongation of their elliptical orbits is termed ‘eccentricity’. A higher eccentricity indicates a more elongated orbit, while an eccentricity of 0 would mean a perfectly circular orbit. This can cause the apparent angle and separation of a double star to change over years or centuries as they move about in their orbits.

The time it takes for a star in a binary system to complete one full orbit around the barycenter is called its orbital period. Binary stars closer together will have a shorter orbital period than those further apart.

As stars age and evolve, their masses can change, especially if one star becomes a type of supernova or sheds its outer layers to become a white dwarf. Changes in mass directly impact the orbital dynamics, potentially leading to changes in the position of the barycenter, the orbital period, and even the stability of the binary system.

How many stars can orbit each other at a time?

The below table provides some approximate statistics for the frequency of multiple-star systems observed in our own Milky Way galaxy.

Number of Stars in System	1	2	3	4	5+
Frequency (Low Est)	35%	31%	4%	1%	0.5%
Frequency (High Est)	56%	58%	9%	2%	0.75%

The same gravitational interplay of Kepler’s laws scales up when we talk about star systems with more than two members. The most common and stable configuration for triple- or higher multiple-star systems is hierarchical. This means that two stars might orbit closely around their mutual barycenter (forming an “inner binary”), while a third star orbits this pair at a much greater distance. The same concept extends to systems with more than three stars. For instance, in a quadruple system, two pairs of binaries might orbit each other, as is the case with the stars of the Epsilon Lyrae system. A quintuple (which also happens to be the case with Epsilon Lyrae) would involve a fifth star orbiting in a triple system with one of the pairs hierarchically or orbiting both pairs at a much greater distance. A sextuple (six-star) system would involve two close pairs with a third further out, while a septuple would add a third member to one of the three pairs, and an octuple would consist of four pairs.

A few other possible septuple (seven-star) systems may exist but are not confirmed to be gravitationally bound at this time. While systems beyond septuple are postulated, they become extremely rare and are challenging to confirm due to the complexities involved in their observational and modeling requirements.

While our focus often shifts to bright, prominent stars, the universe is teeming with fainter stars like dull red dwarfs, compact white dwarfs, and sub-stellar brown dwarfs. These bodies, although significant in number, are challenging to detect beyond a few hundred light-years from our Solar System owing to their dimness. Their elusive nature means that they could very well be components in higher-order septuple, octuple, etc. systems or even dominate some of them, yet remain undetected. This potential “hidden population” presents both a mystery and an opportunity for future astronomical investigations, including research into dark matter, where they may account for the “unexplained” mass in many galaxies throughout the Universe.

Invisible Ones Through a Telescope

Not all double or multiple stars are readily apparent through a telescope. Some orbit so distantly that their association is not immediately obvious. Others are too close to their larger companions and/or so dim that they are not readily apparent. This calls for astrometric measurement and spectroscopy. These methods are used to discover double/multiple stars, look for exoplanets, and pin down the orbits of known multiple-star systems.

Astrometric Measurement of Binary Star Systems

Astrometry is the science of measuring the cosmos, and before the development of modern cosmology and affordable large telescopes, it was the primary discipline of amateurs and professionals alike, who spent hours recording hundreds or thousands of stars’ positions with a small refracting telescope each night. With such a setup, the eyepiece would have reticled crosshairs, and the telescope would only point along the meridian or north-south line. Often, the sole job of the observer was to record the precise time at which an object traversed the telescope’s field of view and the markings on the reticle. This is how many binary stars were measured and discovered early on. Before telescopes, simpler mechanical transit tools operating on similar principles were used to pin down the positions of all of the stars visible to the naked eye with astonishing accuracy by ancient astronomers in many civilizations.

The trouble is that many double stars move at speeds too slow to discern their movements on human time scales. The bulk of double and multiple systems that we can observe directly with telescopes are further apart than the gas giant planets such as Saturn orbit from the Sun, taking decades, centuries, or even thousands of years to make one orbit. This means that the sluggish movements of doubles have to be discovered by systematically re-examining the relative positions of the stars each year, which also allows astronomers to examine their very slow motions against the background sky, or proper motion. We can also measure the parallax, or apparent wobble of stars as Earth goes around the Sun, to determine that a given pair or group of stars are similar distances away from us and thus likely to be associated.

As our Sun and nearby stars move throughout the Milky Way Galaxy, nearby stars can drift across the background each year. If two stars have similar or identical proper motion and parallax, this can be an indication that they are at least close together and on a similar path through space. If the apparent angle or distance between the two changes over time, it’s likely a double. Systematic measurements of the stars’ positions with simple crosshair reticles were done by eye well through the early 1900s until astrophotography became sufficiently reliable for such measurements. Today, photography and software tools replace the crosshairs, but the process is the same.

Astrometry is particularly helpful in the present day for determining whether or not wider systems are truly in orbit or mere optical doubles. For instance, Proxima Centauri, the nearest star to our Sun, is actually part of a triple star system with the nearby binary star Alpha Centauri, which is about 0.1 light-years away from it. However, Proxima lies over two degrees away in the sky from Alpha Centauri, and many astronomers thought that it was simply moving through space alongside our Sun and Alpha Centauri by coincidence. However, Proxima Centauri’s proper motion and parallax have been measured to a high degree of accuracy with tools like the Hubble Space Telescope, revealing a slight curve in its trajectory. This indicates it is in a 550,000-year orbit around Alpha Centauri A and B, themselves much closer to each other. Interestingly, Proxima is actually in a very elliptical orbit around its larger companions, coincidentally at its furthest point (apastron). It will get about twice as close to Alpha Centauri as it is presently in 200,000 years.

In a reverse situation, we can use the primary star’s slight wobble to infer the existence of companions. When a star has a companion, be it another star, a brown dwarf, or a planet, it doesn’t remain stationary. Instead, both bodies orbit around their common center of mass. For the primary star, this results in a small, periodic wobble in its position, which can be noticed if the wobble is not in the same plane of movement as the star’s proper motion or Earth’s orbit. 

By tracking tiny positional shifts over time and ruling out normal movements and observational errors, astronomers can infer the presence, mass, and orbital details of unseen companions in orbit around a star. The amplitude and period of this wobble can give clues about the mass and distance of this companion from the star. However, the amplitude has to be pretty large for astrometric measurement; it doesn’t work as well for less massive objects like brown dwarfs or exoplanets and drops off in usefulness the further away the primary star is from us. For more sensitive measurements, spectroscopy is needed to take advantage of orbital velocity’s effects on starlight itself.

Doppler Shift & Spectroscopic Binaries

For measuring stars and other objects too close to another star to detect, we rely on the radial velocity method. While astrometry focuses on the motion across our line of sight, the radial velocity method concentrates on motion along the line of sight, towards or away from us. This motion causes a Doppler shift in the star’s spectrum. When a star moves towards Earth, its spectral lines shift to the blue end (blue-shifted) of the spectrum, and when it moves away, they shift to the red end (red-shifted). This is the same process that was used on a larger scale to determine that most galaxies in the universe are moving away from us and thus that the universe is expanding.

A star with a sufficiently massive companion will exhibit periodic red and blue shifts in its spectral lines due to its orbital motion. By studying these shifts, astronomers can determine the star’s velocity changes and deduce the presence of an orbiting companion. The size and frequency of these velocity changes can indicate the mass and orbit of the unseen body. 

The radial velocity method is also used for detecting exoplanets where the planet is relatively massive and close by, such as Earth-mass planets orbiting closely around a very small red dwarf star, or a “hot Jupiter” around a Sun-like star. This causes a more pronounced wobble at a relatively high frequency. However, this technique does not work as well with the more subtle gravitational effects of, say, an Earth-like planet around a Sun-like star, where the wobble is less pronounced and extremely infrequent.

Building on the foundation of the radial velocity method is the concept of spectroscopic binaries. These are binary star systems that often cannot be resolved as two separate stars through a telescope due to their close proximity to one another or the angle from which we observe them. However, their dual nature becomes apparent when we analyze their light spectrum. For many spectroscopic binaries, the spectrum will show two sets of spectral emission lines: one for each star. These emission lines correspond to the precise chemical makeup and current internal processes of the star. In a close binary system, the stars’ emission lines will shift position over time, representing the to-and-fro motion of the stars, and often the two stars’ lines shift at different intervals. 

By analyzing the shifts of each star’s emission lines, astronomers can determine various details of the binary system, such as their relative velocities, the shape and orientation of their orbits, and in some cases, even their masses. For example, if both sets of lines are equally shifted (equal amplitude in Doppler shifts), it suggests that the two stars are of similar mass. On the other hand, if one set of lines shows a larger shift than the other, it indicates a disparity in the masses of the two stars. Further examination can allow astronomers to extract data on the composition and relative temperature of each star.

One well-known example of a spectroscopic binary system with stars of roughly equal mass is Mizar. Located in the handle of the Big Dipper (part of the Ursa Major constellation), Mizar was historically known to telescopic observers as a binary star with a second, naked-eye visible companion, Alcor. However, with the advent of spectroscopy, each star comprising the Mizar system itself was revealed to be a spectroscopic binary, with two almost equally bright stars orbiting each other closely. Mizar A comprises two similar blue-white stars, while Mizar B consists of a white main-sequence star with a small red dwarf companion, barely detectable in Mizar B’s spectrum at all.

Besides Mizar B, another example of a spectroscopic binary where the stars have significantly different masses is Spica, the brightest star in the constellation Virgo. Spectroscopic observations of Spica have shown that it consists of a massive and hot blue giant primary star and a smaller companion. The spectral lines from the primary star (Spica A) show larger Doppler shifts than those from its smaller companion (Spica B), indicating that the primary is being influenced more by the gravitational force of its companion than vice versa. The differences in their spectral line shifts clearly reveal the disparity in their masses.

Eclipsing Binaries

Finally, in some fortunate close stellar alignments, the orbital plane of a binary system is oriented edge-on from our perspective. This means that the stars periodically eclipse one another. Observations of the system’s brightness over time yield a ‘light curve’, displaying brightness dips when one star obscures the other. The shape, depth, and duration of these dips in brightness provide insights into the stars’ respective sizes, luminosity, orbital distance, and orbital period. A classic example of an eclipsing binary system is the bright star Algol in the constellation Perseus. Algol’s brightness dims noticeably for several hours every 2.87 days, indicating the time it takes for the dimmer star to pass in front of its brighter companion.

Is our Sun a double star?

In the 1980s, some researchers proposed the existence of an object, which they named Nemesis, to explain perceived periodicities in the Earth’s mass extinctions. The hypothesis was that a dim red dwarf star or a smaller, star-like brown dwarf, orbiting the Sun at a great distance, periodically perturbed the Oort Cloud – a vast, spherical shell of icy objects encircling the Solar System. These perturbations, according to the theory, sent a cascade of comets toward the inner Solar System, leading to comet impacts and consequent mass extinctions on Earth. The authors of the paper argued that mass extinctions on Earth follow a particular time span of around 80 million years, which would correlate to a very distant orbit; however, such a cyclical nature of mass extinctions on Earth – let alone caused solely by giant asteroid and comet impacts – does not exist in the fossil record.

For many years, serious attempts were made to find Nemesis, and compromises in the theory were made – the extinction part was dropped, and perhaps it was a smaller body, such as a gas giant planet several times bigger than Jupiter. The existence of a massive planet, let alone a small star, orbiting our Sun or closer than 4 light-years away in any context has been disproven entirely. This is mainly because even a relatively cold and thus virtually invisible brown dwarf or “super-Jupiter” planet would still glow at infrared wavelengths, enough to be picked up by the extensive all-sky surveys conducted with NASA’s WISE telescope.

Nonetheless, the search continues with WISE and other data. While the fringe idea of a massive body causing periodic mass extinctions was shelved by astronomers, the core idea of the massive body disturbing icy objects in the outer Solar System hasn’t. “Planet Nine”, hypothesized based on observed clustering in the orbits of some distant objects past the orbit of Neptune, is thought to be a massive planet (roughly 5-10 times the mass of Earth) influencing the orbits of these icy bodies from afar – it would have to be some 50-100 times further from the Sun than Neptune and Pluto to remain undetected. However, astrometric measurements and extensive sky surveys have cast doubt on this theory, too, leading to newer proposals for a smaller, Mars-sized body closer in by other researchers. Whether such a ninth planet actually exists is anyone’s guess.

Fun Double Stars for Binoculars & Telescopes

Besides the ones we’ve covered in their corresponding sections, there are thousands and thousands of double stars that are exciting through binoculars or a telescope.

Many of the stars we are familiar with in prominent constellations are actually doubles, which is unsurprising given how common we now know binary star systems to be. Take, for instance, Sirius in the constellation Canis Major. It’s renowned as the brightest star in our night sky, with an apparent magnitude of -1.46, and owes this status to being larger and brighter than our Sun as well as being situated just 8 light-years away from us. Its white dwarf companion, Sirius B, shines at magnitude 8.4, with a maximum separation of about 11.5 arc seconds. Sirius B’s elliptical orbit takes it on a 50-year trip ranging from 3 to 11 arc seconds away. As of the time of writing (2023), they are nearing their maximum separation, and as such, there is arguably no better time than to give splitting Sirius a shot. Even a 4” telescope can reveal Sirius B, but the glare from Sirius A itself is an immense challenge for spotting the stellar remnant.

Another bright and more easily split gem is Rigel, usually Orion’s brightest star at magnitude 0. Its bluish companion, Rigel B, registers at 6.7. Interestingly, Rigel B itself is a spectroscopic binary, converting the system into a triple spectacle. With a separation of around 9.5 arcseconds, resolving this pair is not challenging in terms of distance, but the glare from brilliant Rigel A can often wash out the companion at lower magnifications.

The North Star, Polaris, located in Ursa Minor, isn’t just significant for navigation; it’s a fascinating triple star system. The 2nd-magnitude yellow-white giant Polaris A is accompanied by two companions: Polaris Ab with a magnitude of 9.2 and Polaris B with a magnitude of 8.7. The closer pair, Polaris A and its smaller yellow-white companion Polaris Ab, are unable to be detected without spectroscopy due to their proximity. Fortunately, the distant and noticeably yellower Polaris B, set apart by about 18 arc seconds, is readily visible with a small telescope. Polaris’ lack of apparent motion means it is a great star for testing your telescope’s optics, and Polaris B’s subtle glow provides an ideal test to dial in focus and make sure your collimation is dead-on; any problems will wash it away in Polaris A’s glare.

Situated on the borders of the constellation Andromeda, 2nd-magnitude Almach beckons for attention and is often overlooked on account of all of the deep-sky objects that lie nearby. It’s a shame since this system is a visual treat, providing observers with vibrant color contrast and little challenge to resolve. The golden-yellow primary contrasts beautifully with its bluish 5th-magnitude companion, making it one of the most colorful double star systems visible in a small telescope. At 10 arc seconds apart, separating them will require modest magnifications – 50x or more – but almost any telescope with decent optics will do the job.

Another enchanting system is the 3rd-magnitude Cor Caroli in Canes Venatici. Comprising two stars, it serves as a colorful wintertime analog to Albireo, hence its moniker, “Winter Albireo”. The bright blue giant star, A, is orbited by a fainter yellowish 5th-magnitude companion about 20 arcseconds away – similar in separation to Polaris but with less contrast in brightness.

Other popular double stars are less prominent but still visible to the naked eye. For instance, 5th-magnitude 61 Cygni in the constellation Cygnus is particularly noteworthy. This binary star system isn’t exceptionally bright, but its historical significance as the first star (outside of our solar system) to have its distance measured via parallax makes it a favorite among stargazers. 61 Cygni’s pair of orange dwarfs are just 11.4 light-years from us and are moving rapidly across the sky, about 1 arc second every two years. This has led to its moniker as the “Flying Star” or “Piazzi’s Flying Star” as it rapidly moves against the backdrop of more distant stars. At 29 arc seconds of separation, 61 Cygni can be separated with 10×50 binoculars or pretty much any telescope.

Situated in the constellation Lyra, not far from the brilliant star Vega, Epsilon Lyrae is often referred to as the “Double Double.” This moniker is a nod to its unique nature. To the sharp unaided eye, Epsilon Lyrae appears as a close-together pair of two bluish 4th-magnitude stars, and even the weakest opera glasses or binoculars clear up any uncertainty owing to their extremely wide separation of just under 3.5 arc minutes (about 1/8 of the angular diameter of the Moon or 208 arc seconds).

The true marvel of Epsilon Lyrae, however, only unveils itself when observed through a telescope at high power. If you have a look at Epsilon Lyrae through a good telescope, each of the two stars further divides into two, hence presenting as a quartet of stars, or two binary systems. This gives Epsilon Lyrae its distinctive “Double Double” appearance, making it a favorite observation target for amateur astronomers. If you’re at a northerly latitude, waiting for the constellation Lyra to be directly overhead is ideal; that way, you’re peering through the smallest possible amount of Earth’s turbulent atmosphere to resolve the four stars of the system. If not, at least wait for Lyra to get as high in the sky as possible to split the Double Double. 

At 2.3 and 2.6 arc seconds, neither the Epsilon^1 or Epsilon^2 pairs are much of a challenge for even a small instrument, but a fairly steady sky and 80x or greater magnification are required. All of the stars shine at around magnitude 5, except for Epsilon B (the second star of Epsilon^1) which is visibly a bit dimmer at magnitude 6.1. Interestingly, Epsilon Lyrae is not a quadruple star system but, in fact, a quintuple – the second pair of stars, Epsilon^2 or C/D, are joined by a much fainter fifth star (E), which shines around 12th magnitude. However, at just 0.1 arc seconds away from the stars C/D, it cannot be resolved as a separate object with amateur telescopes.

Further Reading

There are far too many binary stars in the night sky for you to observe that we could possibly cover in this article. Even our constellation guides only scratch the surface of the brightest dozen or so double and multiple systems of each constellation. Online resources such as Cloudy Nights and astronomers’ blogs provide plenty of scattered information on observable double stars, but it’s really best to get a book. Notably, Burnham’s Celestial Handbook is a treasure trove, replete with detailed and accurate descriptions of myriad double stars; there really isn’t anything like it that has been made since. Other dedicated books on doubles are certainly worth picking up. Argyle’s Observing & Measuring Visual Double Stars and Agnes Clark’s Discovering Double Stars are among our favorites.