Before the invention of the telescope, stars were understood as single points of light, forming patterns known as constellations or asterisms but distinct from one another. The earliest telescopes revealed the surprising fact that a large fraction of stars, when studied at high magnification, split into two distinct points of light: a visual binary star. While in some cases this was pure chance - the random alignment on the sky of two stars at different distances from Earth - in others the stars were true binaries, bound together by gravity and orbiting their common centre of mass. As telescopes improved through the nineteenth and twentieth centuries more and more stars were found to have multiple components - often two but sometimes three stars (a trinary or triple star system) or more (a higher order multiple star). Indeed we now know that most stars more massive than the Sun are born this way, as are as many as 20-30% of Sun-like (or Sol-like, to avoid confusion with local suns) stars.

These systems, so very different from our own Solar System, have captured the imagination of many astronomers. Indeed a large part of my own scientific research focuses on the impact of binary stars on our understanding of galaxies. But these strange environments also captured the imagination of the public and of science fiction writers too.

There are many, many examples to be found in science fiction. Perhaps the most well-known is Tatooine, the sun-scorched homeworld of Luke Skywalker in the Star Wars saga, whose two host stars are shown in key scenes as Luke considers his future. Also well known - although less iconic as a binary - is Gallifrey, homeworld of the Time Lords in Doctor Who with its twin suns. Magrathea, home to the planet builders responsible for Earth in The Hitchhiker’s Guide to the Galaxy was also distinguished by twin suns, Soulianis and Rahm, while the ‘verse of television series Firefly comprised a close multiple with no fewer than five suns, each with their own system of planets but close enough to its neighbours to allow efficient travel between them.

Science fiction regarding such systems has, over the years, both paralleled and at times anticipated ongoing research into the potential habitability of these systems for human-like life, the consequences of stellar multiplicity for their planets, and the very different life that may evolve in such an environment. A recurring theme is that of cyclic history: a companion star with a long period (the technical term for a complete orbit or “year”) may disrupt an ecosystem or society at regular intervals. Again, I’m rather fond of catastrophism as a way of exploring ideas about our place in the Universe, so such stories naturally attract me. For now though, I’d like to focus on just four:

Helliconia

The Helliconia trilogy by Brian Aldiss is an epic history of a planet that is entirely ruled by the orbits of its binary suns. The eponymous planet orbits a Sol-like star, Batalix, in a relatively wide orbit which would normally place it only marginally in the star's habitable zone. However Batalix has a binary companion, Freyr, which is a hot blue giant and has 15 times the mass of our Sun. As a result, Freyr outshines Batalix in Helliconia's skies, even when it's at the most distant point in the system's millennia-long orbital cycle. This is just enough to make Helliconia (barely) habitable for its various native intelligent species, although plunged into an ice-age-like winter which last centuries. As Batalix and Freyr approach one another, Heliconia experiences a thawing and then a blazingly arid summer, before the stars once again separate and the planet slides back towards winter, destroying entire civilisations which have arisen during the brief clement periods. The books follow this cycle through Helliconia Spring (1982), Helliconia Summer (1983) and Helliconia Winter (1985), discussing the astronomy, geography and natural history of a world that endures such extremes, as well as the social and personal histories of the natives of the planet.

The orbital cycles of Batalix and Freyr, and the level of irradiation each provides, have been carefully worked out, and indeed are calculated by a Heliconian scientist character in the first book. As the bulk of Helliconian vegetation and animal life emerges from deep hibernation, and the snows melt within a generation, this woman is part of a cultural renaissance which is enabled by the new abundance. She studies the movement of the heavens and realises that Helliconia and its three sibling planets orbit Batalix in one plane, while Freyr's orbit intersects the system at an angle. The two stars' orbit is also extremely eccentric - stretched out into an ellipse rather than near-circular (like Earth's orbit around Sol). These properties (presented in the first book of the trilogy) are interesting as they imply that this system is a dynamical capture binary - its two stars were not born together (which tends to align stars and planets in a single disk) but instead were born separately and became bound together by gravity when their paths through the Milky Way intersected by chance. Indeed the capture scenario is discussed and confirmed in the second book, where it is revealed that other bodies including Freyr's original stellar companion and Helliconia's moon were ejected (as the law of conservation of momentum requires!). As a result of this dynamic past, Helliconia has two intersecting ecosystems: an older, colder ecosphere with cycles aligned to the original small year and a more Earth-like ecology which follows the great-year cycle of the binary orbit.

The details of this interaction and the resulting orbit are spelled out in an Appendix of the single-volume 1996 edition of the trilogy. How common this kind of capture binary is in the real-world Universe remains unclear, although in some types of binary star (such as the double black holes we see through gravitational waves) their numbers may be significant. Most multiple stars, by contrast, come instead from a single cloud which fragments into two or more pieces, each becoming a star and remaining together thereafter. But in dense regions of the Milky Way, particularly in star clusters or near the centre of the galaxy, stars are much more tightly packed than they are near us, and dynamical capture may be more common.

Interestingly, Aldiss also describes an Earth-origin scientific space station, Avernus, which studies the planet from orbit and beams a Truman Show-style feed back to vast Earth audiences, but whose inhabitants are barred from the surface by microbiological threats. Thus the watchers become essentially cast as voyeurs indulging in their studies while seeing the inhabitants of the planet as little more than characters acting out a role... and the reader is placed right along side them in taking that voyeuristic pleasure in the lives of other sentients. It's an unsettling viewpoint and it makes an interesting comment on the degree of emotional disconnect between scientists and their subjects, and the way science fiction writers and their readers are prepared to experiment on their subjects, playing with ideas such as binary systems of this type, regardless of the suffering that the characters might endure.

Anubelea (Fire Time)

Poul Anderson’s 1974 novel Fire Time tells a story of the triple star system Anubelea. The most massive star, Anu, is a red giant which expanded a billion years ago, scorching its own planetary system into extinction. The second star, Bel, is a star much like our Sun - long lived, yellow-white and host to the habitable planet Ishtar. These two stars are very close in astronomical terms, with a minimum separation just 40 times that of the Sun from the Earth (known as an au). A third sun, Ea, is a low mass, faint and cool red dwarf star, orbiting far further out and appearing as little more than a particularly bright star in the sky.

Every 1041 years, Anu approaches Bel causing storms, droughts and massive temperature rises on the planet Ishtar - the Fire Time. A full half or more of the planet becomes uninhabitable, leading to strife and mass migration as the native life seeks the more temperate southern hemisphere. Given that the lifetime of the sentient native inhabitants is about 8 times that of humans, this is equivalent to a major catastrophe occurring about every 125 years on Earth. Perhaps unsurprisingly, the native culture collapses at these regular intervals, limiting their capacity to develop and preserve learning, technology or even complex skills. Partly because of this, and partly because of geographical isolation, a second less intelligent native species, equivalent to our early hominids, exists on a remote continent, unlike on Earth where the spread of complex culture allowed one species of human to dominate. Another element is added by the intelligent descendants of life from one of Anu’s scorched planets, Tammuz - deposited on Ishtar a billion years ago before its homeworld was wiped out by the expansion of the red giant. Into this mix, human settlers and scientists are deposited, with inevitable complications.

As well as addressing how two native cultures aim to survive or cope with the imminent return of their familiar cyclic catastrophe, Fire Time is notable for the care with which the Anubelea system has been worked out, with a lot of technical information provided in the text in the form of excerpts from textbooks or explanations offered to others. The luminosity, temperature and mass of Bel is provided, together with details of its planets - indeed another of these, Shammuz, is a Mars-analogue and would be considered to be on the very outer edge of Bel’s habitable zone by modern calculations. The luminosity and mass of Anu, its impact on the irradiation of Ishtar during Aphelion and the resultant temperature rise are also given. Ea is described in less detail but is mentioned as hosting a superjovian planet (anticipating the scientific detection of such objects by more than two decades).

Unsurprisingly, given Anderson’s physics degree and acknowledgement of assistance from technical experts in shaping his world, this system proves perfectly plausible from an astronomical perspective. A sun 1.2 times the mass of our Sun, like Anu, would indeed exhaust its core hydrogen fuel at an age between 4 and 5 billion years, swelling to some 200 times the diameter of our Sun and becoming some 280 times brighter, while the lower mass Bel would be longer lived, as the text reports. Given the relatively small difference in mass between the two suns, and the eccentricity of the orbit, both stars will show relatively wide orbits around their common centre of mass - unlike in the Helliconia system (mass ratio > 15) where Freyr always remains within Batalix's orbit and relatively small in the sky. At its most distant, Anu would appear comparable in size to Bel in the night sky of Ishtar. At its closest approach, it would appear some five times larger and much redder - small wonder the locals refer to its coming as the Fire Time.

Ptallaya (A Red Sun also Rises)

Mark Hodder is best known for his steampunk novels. One of his rare stand-alone books is a consciously-deliberate attempt to update the ‘planetary romance’: a story of adventure in which human protagonists are displaced to an alien world and fight for their survival. In this subgenre, the planetary setting can often approach fantasy, with little emphasis on the plausibility of the scenario. Nonetheless, Hodder’s world of Ptallaya, appearing in the novel A Red Sun also Rises (2012), shows evidence for careful world-building and thought put into another example of an interesting triple star system.

This world is unlike Anubelea in which each star hosts its own planetary system and each planet has one primary sun (this is known as an S-type orbit). By contrast, Ptallaya boasts twin suns (the Eyes of the Savior) which form a close pair, rise and set together and provide a Sol-like yellow-white light. Thus at first encounter, Ptallaya appears to be a true circumbinary planet on a p-type orbit, circling both stars in a close binary rather than just one. However, as the novel’s title hints, the situation is more complex and a red giant star (the Heart of Blood) rises when the twin suns set, bringing with it atmospheric turbulence resulting from a drop in temperature, and triggering biological processes in the native life. This star is described as appearing at least ten times bigger than the Sun as seen from the planet’s surface (suggesting a distance of only about 10au), and casts a dim red light. It always appears opposite the Eyes in the sky, and Ptallaya has a months-long day, suggesting a rather more complex geometry for the system. In fact, the only way this system could make sense is if Ptallaya itself is sitting at the inner Lagrange point (known as L1) of what’s known as a hierarchical triple - the Eyes orbit one another as a binary and are orbited in turn by the red giant. Like Earth’s, Ptallaya’s days result from its slow rotation on its own axis. Unlike Earth, however, this is effectively a world orbited by its suns (as medieval Earth was once believed to be) rather than the reverse!

Could such a situation arise? Well, technically the gravitational balancing act could work, but it’s difficult to see how any planet could form in such a situation - one impossible through our current understanding of the way planets condense from disks. It’s also extremely unlikely to be stable - any slight nudge away from that perfect balance point, and Ptallaya will fall towards either the Eyes or Heart. And there are plenty of possible sources for that nudge. Any other planet in the system, or other nearby source of significant size would cause a gravitational imbalance. Any passage of the system through unevenness in the Milky Way’s gravitational potential (rare but inevitable on million- or billion-year timescales) would do the same. Perhaps more significantly, red giant stars drive strong winds into their environment from their surfaces. Impacting on the planet, these could easily generate enough pressure to upset a system which had hitherto been stable. In short, the Ptallaya system is almost certainly a fantasy - but a fun one to consider nonetheless.

Kagash (Nightfall) 

The final system I want to look at is one that has been studied before in detail, but is too good not to mention when we think of multiple stars. The planet Lagash in Isaac Asimov’s short story Nightfall ([1] renamed as Kalgash in the novel of the same name he later co-authored with Robert Silverberg) boasts no fewer than six suns. As a result, its surface never knows true darkness, with one or more of the system’s stars always in the sky. Unfortunately once every 2048 years a rare conjunction occurs: a dim object - either a moon of Kalgash, another planet in the system, or a similar non-luminous body - is able to eclipse the faintest sun, Dovim, at a time when it’s alone in the sky. The inhabitants of Kalgash, psychologically unable to deal with darkness, suffer insanity and destroy their own civilisation in the urge to create fires for the light they give off.

The story is extremely effective, creating a tense and brooding atmosphere as doom approaches with all the predictability of the clockwork heavens. Several of the principle characters are astronomers - and, perhaps because of this, the hypothetical system described has been studied more than once by astronomers in the real world. Another attraction is the complexity of system and level of detail provided in the text. Simply put, Asimov describes a principle Sun called Onos, two pairs of more distant stars which travel together through the sky (Trey and Patru, which are blue, and Tano and Sitha, described as white) and a sixth star, Dovim, which is a dim red dwarf. Such a system could plausibly be stable, but would have to have a hierarchical structure similar to those discussed above. In other words, rather than six stars all independently orbiting a single centre of mass, it is likely that pairs of stars will orbit one another, those pairs will orbit in turn around a common barycentre.

Unfortunately, it is extremely difficult to find configurations which are simultaneously stable, habitable and allow for the eclipse described in the novel. One suggestion, as discussed in a blog by astronomer Sean Raymond, is illustrated above and has the two close pairs orbiting one another. Another, considered by Deshmukh and Murphy, separates the two pairs, placing them either side of Dovim and Onus rather than both on the same side. In each case, the investigators found problems with the configuration as described - either in terms of stability or the eclipse alignments.

Nonetheless, stable six-sun systems do in fact exist. One of the best known is Castor - which together with Pollux is one of the two brightest stars in the constellation Gemini. Appearing to the naked eye as one star, a small telescope will split the light into three - Castor A, B and C. Larger telescopes have revealed that each of these is itself a binary so that Castor is actually a trinary of binaries and comprises stars Aa, Ab, Ba, Bb, Ca and Cb! Another recently discovered example is the sextuple system is TYC 7037-89-1. Despite the unexciting name, this is a truly exciting system - all six stars eclipse, meaning that the system is seen edge on from Earth and is perfectly aligned so that each star is occasionally hidden behind another. This is useful as the way the light changes during the eclipse tells us a lot about the stars themselves, letting us understand the system in ever more detail.

Evolving as we have in the light of a single sun, humanity will likely always find the idea of multiple star systems fascinating. Scientists (including Anderson, who graduated with a physics degree, and Asimov, who had a background as a research chemist) seem to find their complexity and the intellectual challenge of imagining their worlds endlessly compelling. As our understanding of exoplanets in the real world and their hosts grow, more and more are likely to be found which boast binary sunsets… or even the bright skies of Asimov’s Kalgash. Whether such worlds will prove to be habitable, or even inhabited, remains a question only science fiction can answer!