Neutron stars are the highly compact and cool remnants left at the end of the life of many stars. They have been of recurrent interest to science fiction writers and audiences since they were first hypothesised.

Compact Remnants

When a massive star exhausts the fuel available for nuclear fusion it collapses under gravity. This collapse creates a shock wave which expands outwards as a supernova, leaving behind the core of the star as a remnant. This stellar collapse model was developed by Fritz Zwicky (amongst others) in the 1930s and was being reported in the science-fact articles of science fiction pulp magazines as early as 1939. An article by Howard A Lower, for example, appeared in Astounding Science Fiction in October of that year and described “Hunting Big Game” - searching for supernovae, and trying to understand the remnants they left behind, including black holes and neutron stars:

“Dr. Zwicky suggested that the occurrence of a supernova might be due to the transformation of an ordinary star, composed mainly of electrically charged particles, into a collapsed neutron star of enormous density and exceedingly small stellar radius.”

For the most massive stars, the remnant left by this process collapses to the point where light cannot escape, creating a black hole [1]. For stars a little less massive - whose remnants are similar to the mass of our sun - the collapse is stopped when atoms themselves break down, crushing protons and electrons together to create a structure entirely made of tightly-packing subatomic particles known as neutrons. These neutrons resist further collapse due to a quantum mechanical effect known as degeneracy pressure, and the surviving body - perhaps just 10 km across - is a neutron star.

Neutron stars are born rapidly spinning, and with magnetic fields strong enough to convert that spin to electromagnetic light (as a pulsar). However once spun down, a neutron star is dark and shines only when material from a companion star or other source falls onto it.

A Matter of Gravity

Perhaps the first first aspect of neutron stars to attract the attention of writers was their extraordinarily high surface gravity. A typical neutron star would have a gravity on their surface of perhaps a hundred billion times that on Earth. This is associated with equally strong tides - these are a stretching effect caused by the difference in pull strength between two points (for example between ocean and rock, or between head and feet) and so depends not just on the strength of the gravitational attraction but also how rapidly it is changing. For compact objects, such as black holes or neutron stars, these tides can be strong enough to tear apart any normal material approaching their surface.

This effect - often known as spaghettification - is now quite well-known from decades of popular science and science fiction, but it was effectively unknown in the mid 1960s when Larry Niven wrote his short story “Neutron Star” (Worlds of If, Oct 1966). This is one of Niven’s Tales of Known Space series, and introduces his freelance hero Beowulf Shaeffer, who is commissioned to investigate why a spaceship failed to return from a previous scientific investigation of the neutron star BVS-1. Entering a hyperbolic orbit that will bring him within one mile of the surface, he begins to experience the strong gravity and eventually to realise that he is in danger of destruction (like his predecessors) by the tidal force.

This was amongst the earliest science fiction stories to consider these exotic objects scientifically, complete with a broadly correct description of the properties of a neutron star and its formation. Niven also included a description of the apparent blue-shift of starlight as seen from the infalling ship:

“The ship turned with peculiar sluggishness. And the stars behind were blue, not red. All around me were blue-white stars. Imagine light falling into a savagely steep gravitational well. It won't accelerate. Light can't move faster than light. But it can gain in energy, in frequency. The light was falling on me, harder and harder as I dropped.”

Shaeffer eventually realises his danger. However, as Niven himself has noted in interviews and introductions to the story, we can confidently calculate that Shaeffer’s actions at that juncture wouldn’t have been sufficient to allow him to survive the orbit, and it’s rather inconceivable that any space-going civilisation would not have understood the danger of tidal forces. The story was also wildly inaccurate in estimating that the first neutron star wouldn’t be located until the twenty-seventh century.

The damaging gravity of neutron stars was also the theme of Arthur C Clarke’s very short story "Neutron Tide" (appearing in Galaxy SF, May 1970). As Clarke himself has admitted in introductions to the story, this tale of the United States Space Navy spaceship Flatbush’s catastrophic encounter with a neutron star exists only for the sake of the pun in the final line (spoiler in footnote [2]).

Dragon’s Egg

An earlier discovery date was proposed by physicist and author Robert L Forward in his well-regarded science fiction novel Dragon’s Egg (1980). This novel begins with the discovery of the titular neutron star as an anomaly in the constellation Draco in 2020. Its path through the Galaxy brings it close enough to Earth to make a brief crewed investigation plausible in 2050.

However the bulk of the story focuses on the perspective of native life on the surface of this young, rapidly spinning and highly magnetic pulsar. These natives, the Cheela, have a physical structure based on the strong-force interactions that dominate in nuclear cores (rather than the electromagnetic interactions that form our atoms) and are tiny in physical extent. As a result physical processes happen at a much accelerated rate, such that milliseconds of duration are perceived as the equivalent of human years. This means that the entire evolution of Cheela science and civilisation - from hunter-gatherer to overtaking human science and leaving the planet - takes place within the month-long human visit (to orbit, not the 67 billion gravity surface), and is in part inspired by it.

Forward took advantage of the fictional setting to include a lot of gravitational and particle astrophysics in the text, almost to excess at times. However many of the aspects of the physics are key to the world-building, for example in the way that the Cheela are sensitive to the pulsar’s strong magnetic field meaning that the Cheela perceive directions as hard (crossing field line takes effort) and soft (travelling along the field direction is relatively easy), or in the fact that their tiny eye-analogues can only detect short-wavelength ultraviolet light.

Interestingly, the youth of the neutron star - only half a million years - proves to be relevant to the history of both the Cheela and the human visitors. It goes some way to explaining why the conditions are only now right for sentient life to evolve on the crust of the young star. At the same time, the novel implies that the radiation flux of the supernova that formed the Dragon’s Egg was also responsible for the mutations that led to the evolution of anatomically modern humans. 

As a physics professor, Forward admitted that his novels tended to act somewhat as disguised textbooks, intended to teach as well as entertain. Indeed, he argued that scientific veracity should take priority over following a planned plot where possible - including in an article “When Science Writes the Fiction” that discussed the creation of Dragon’s Egg in the academic volume Hard Science Fiction edited by George E Slusser and Eric C Rabkin in 1986. He pointed out that some key plot elements (for instance, worship of the approaching star Sol by the Cheela, and the configuration of asteroids used by the humans as protection from tidal forces [3]) resulted from working out the necessary physics rather than as planned plot points. As he noted:

“If you are going to write hard-core science fiction, good hard-core science fiction, you had better prepare yourself for a lot of work, for you are going to have to make your intellect (and personal computer) do what “common sense” does for the writer of ordinary, earth-bound, human-bound stories.” - "When Science Writes the Fiction", Robert L Forward, in Hard Science Fiction, eds. Slusser and Rabkin (1986).

Kicking Off

An aspect of neutron star physics that has attracted some interest is the fact that they are in motion relative to Earth. When writing in the 1980s, Forward argued (in line with then-theory) that the strong magnetic fields of young neutron stars would interact with interstellar fields and provide a propulsive force along the spin axis. Hence his young pulsar was fast moving in its approach to the Solar System, while Sol was a polar star on Egg. However soon after Forward wrote Dragon’s Egg, observational evidence suggested that pulsar motion might instead result primarily from asymmetries during the supernova that occurs during the birth of every neutron star. If the emission from the supernova is not spherically symmetrical, then there is a resulting thrust in one direction, and these so-called natal kicks are now believed to be ubiquitous amongst neutron stars and (in some cases) to be powerful enough to eject neutron stars entirely from their host galaxies.

While the evidence for this interpretation is strong, alternatives have been also been explored.

Eric Kotani (a pen name of Yoji Kondo) was another astrophysicist author, with a specialism in supernovae. His novel Death of a Neutron Star (1999) appeared as a tie in to the Star Trek: Voyager television series. In this story, the neutron star in question is part of a binary star, and is gradually being stripped of its surface layers by its companion. USS Voyager investigates when it appears that the loss of material may result in the star falling below the Chandrasekhar mass threshold. When this happens, electron degeneracy theoretically takes over from neutron star degeneracy as the dominant supporting pressure in a star and Kotani suggests that this shift in forces would result in the star becoming unstable and exploding in a small supernova-like transient. As in the case of natal kicks, this detonation would be asymmetric, and so provide an impulse to change the direction of motion of the surviving neutron star in the binary. In the novel, the Voyager crew is forced to intervene when nearby species try to weaponise this detonation and its effects. In particular, as the characters note, delaying or accelerating the explosion by as little as a millisecond can change the direction of the runaway star - targetting it towards or away from inhabited planets.

Rather like Forward, Kotani shows something of a tendency to use his writing as a means of science communication, and the novel includes some rather extensive info-dump briefings containing scientific information on neutron stars. Unfortunately this implies some rather unlikely ignorance of basic physics and astrophysics on the part of the experienced Voyager crew, whose characterisation is perhaps less well developed.

One interesting aspect of the story from an astrophysical point of view is the significance placed on gravitational waves. As Kotani explains (and indeed had calculated in his own academic work), in a close neutron star or black hole binary, a large fraction of its orbital energy is converted to gravitational wave radiation. This results in the binary components spiralling together. In the story, some of this gravitational wave energy is captured by Voyager for its own use. When this novel was written, the existence of gravitational wave radiation was a strong but unproven hypothesis. The first gravitational waves from merging black holes were detected in 2015, and from merging neutron stars in 2017. These latter events are indeed accompanied by a small supernova-like flare of visible light known as a kilonova caused by nuclear reactions in neutron-rich material.

However while Kotani imagines that this would result from one neutron star being disrupted while the other survives to act as a runaway, it is now clear that the kilonova results from surface material being dragged from both stars by tidal forces, while the remains of both stars merge into a new compact object. We now know that a short gamma ray burst is also released by the merger.

Life, but not as we know it

While Dragon’s Egg set the bar for imagining a truly alien species and perspective on the surface of a neutron star, other authors have also explored the idea of life in this bizarre environment.

Stephen Baxter’s Flux (novel, 1993) is told from the point of view of a group of "Human Beings". However it soon becomes clear that these are very different to the humans we know. The protagonists live inside the mantle of a neutron star, between its crust and the quantum-foam of its dense core. Just ten microns high, their anatomy is built on the bonding of tin nuclei, rather than the hydro-carbon and water that make up our tissues. Their distant ancestors were manufactured by original "Ur-Humans" to breathe superfluid neutrons and interact with their environment (they can sense magnetic flux lines, for example) and its pre-existing complex ecosystem. This includes tree and grass analogues that draw heavy elements down from the crust, where they burn in the neutron-rich “air” and are consumed by animal analogues. Unfortunately, the continued existence of life in the Star is being threatened by a series of strengthening star quakes which caused glitches, disrupting the magnetic flux lines. And these glitches, in turn, appear to be caused by the powerful alien Xeechee for reasons of their own. The effects on the Mantle-living, super-fluid humans are devastating:

“When the Star suffered a Glitch the Mantle could not sustain its even, gently slowing pattern of rotation. The super-fluid Air tried to expel the excess rotation from its bulk by pushing the arrays of vortex lines - lines of quantised vorticity - out towards the Crust. And the lines themselves suffered instabilities, and could break down…” (paperback edition, 1994, pg 182)

The construction of these microscopic “Human Beings” is sufficiently far beyond our current technology to count as magic in the Clarke’s Third Law sense. However Baxter’s writing captures their essential humanity in their interactions and reactions to the surprisingly-plausible (if highly speculative) world he constructs for them.

Glitches are a very real feature of neutron stars. They are observed as abrupt discontinuities in the millisecond-spin rates of pulsars, and are believed to be associated with cracks, or quakes, in the rigid crust of the star which relieve pressure caused by their rapid rotation. Robert L Forward also considered this problem for his neutron-star-resident Cheela. The sequel to his novel Dragon’s Egg was Starquake (novel, 1985), in which a devastating crust quake affects both surface-dwelling Cheela and human visitors in orbit alike. As with Forward’s other work, this novel is accompanied by a technical appendix which includes references to genuine academic papers on neutron star glitches.

A distinctive neighbourhood

As well as on their surfaces and in their interiors, neutron stars also provide environments for unfamiliar forms of life in their immediate environs. Formation of a single neutron star requires a massive star to have reached the end of its nuclear burning lifetime, throwing off a vast mass of material, even before this is irradiated by a catastrophic supernova explosion. Since neutron stars often occur in stellar binaries, they are also likely to have a nearby companion - these can range from normal stars, to highly distorted objects feeding material onto the compact object, to still more massive black holes. Either way, relatively little life is likely to survive the process, but it is not outside the realms of possibility for life to have escaped the system before the supernova, or to colonise the system after the chaos has had time to settle.

Stephen Baxter returned to the environment of a neutron star with his novella Ghost Wars (also a part of the Xeelee universe, appearing in Asimov’s SF in Jan 2006). A large part of this story is set in a binary system in which the supernova forming a pulsar partially disrupted a main sequence companion. The young neutron star continues to consume material from this disrupted star - which nonetheless itself manages to possess a habitable world. As we’re told:

“No biosphere could have survived the supernova detonation, but once the system became stable again, any surviving worlds could have been reborn. Comets or outgassing could create a new atmosphere, a new ocean. And life could begin again, perhaps crawling out of the deepest rocks, or brought here by comets - or even delivered by conscious intent; this was a galaxy crowded with life. How strange, Hex thought, a planet that might have hosted not one but two iterations of life.”

Of course, with the neutron star still feeding, the second iteration will be short lived. Indeed, the peculiar environment and history of this dying world proves to be a clue to the original homeworld of an alien race - and they key to understanding their psychology and motivation.

A different perspective - of both alien psychology and human - can be found in the Star Trek: The Next Generation episode “Imaginary Friend” (1992). Here the USS Enterprise is undertaking a scientific analysis of the nebula in the environs of a neutron star, and an entity native to the region enters the ship. This energy-based life form learns to see the Enterprise through the eyes of a lonely human child, and so judges the unfairness of the adult crew’s actions from that perspective. It is only later that contact with Captain Picard helps to explain the need to protect children from harm. Curiously the formation of a nebula around a neutron star is described as a "unique environment", which it certainly would not be - although the source of the gas (whether from a binary companion or wisps of gas accreted by chance) would certainly be of interest. Here the neutron star itself isn’t key to the story, but it, and the associated nebula, does provide the high energy environment in which an energy being can thrive.

Larry Niven returned to the environs of a neutron star with his novel The Integral Trees (1984). This is set in a distant solar system comprising a wide main-sequence, neutron star binary. Unlike in most of these stories, the neutron star is old - estimated at a billion years old rather than the few hundred or thousand expected for fast-rotating pulsars. The neutron star (Levoy’s Star or Voy) is in the process of tearing apart a gas giant planet (Goldblatt’s world or just Gold) that has entered its gravitational Roche radius. The gas stripped from the outer layers of the planet has formed a doughnut-shaped gas torus. Denser material torn from the planetary interior has formed a smaller ring, embedded in this torus, which is rich in water, oxygen and even rocks and dirt, and which is at a habitable pressures and temperatures:

“The Smoke Ring was the thickest pasty of the gas torus around Levoy’s Star. At its median it was as dense as Earth’s atmosphere a mile above sea level: too dense for stability. It must be constantly leaking into the gas torus. But the gas torus was stable: dense, but held within a steep gravitational gradient. Molecules continually returned from the gas torus to the Smoke Ring, and from the Smoke Ring to the storm of atmosphere surrounding Goldblatt’s world.” (pg 56, Orbit edition, 1994)

Given this stability, the system has existed long enough to develop life of its own. This region has also been colonised by the descendants of a survey ship crew, whose fragmented culture echoes distorted versions of the authoritarian state their ancestors left centuries before.

The Integral Trees and its sequel, The Smoke Ring (novel, 1987), describe the life in this environment, in particular the great trees that extend a hundred kilometres or more in extent, with tufts of branches and vegetation at either end which turn into the orbital direction to stabilise the trunk. This results in an overall shape which resembles a mathematical integral symbol, hence the name. Most of the humans in the ring live in colonies in these tree tufts, with little or no surviving technology.

While we are not aware of any neutron stars with habitable rings, we do know about debris disks what have formed around compact objects from the remnants of planetary systems. These are relatively common around white dwarfs, which go through a less dramatic red dwarf and planetary nebula phase at the end of their burning lifetimes, rather than a supernova. However Niven’s premise that one or more outer gas giants might have survived a supernova, and then been perturbed inwards (perhaps by the second star) is certainly plausible. Interestingly, Niven dedicates The Integral Trees to Robert L Forward (author of Dragon’s Egg and Starquake, and professional astrophysicist) who helped ensure the physics of the system was consistent and possible.

Physics Playgrounds 

Since they were first hypothesised, neutron stars have formed part of the milieu of science fiction. Indeed the Science Fiction Encyclopedia describes the terminology as “much used in SF”. Indeed neutron stars and pulsars often seem to appear as part of the background setting of science fiction stories. The Star Wars universe features a Neutron Star Class of bulk cruiser, for instance. A neutron star - white dwarf binary system provides the setting for episodes of the television series Stargate Universe, where radiation from the pulsar affects crucial equipment. Another is used as a solar system destroying threat in the 1979 Doctor Who story "The Creature from the Pit". Neutron star systems also provide background and setting for several episodes of the Star Trek series Enterprise, The Next Generation and Voyager, acting as a source of simultaneous astronomical authority and exciting exoticism in the context of the adventures. However the plot seldom revolves around their properties in these cases, although a few exceptions do exist.

An interesting reference to the use of pulsars as the most stable clocks in the universe (a tool being used in modern astrophysics) can also be found in the novella Hot Pursuit by John M Ford (Asimov’s SF, Sept 1980). Here the passage of time is key to the plot, and there is frequent reference to chronometers and the pulsars used to calibrate them.

Substance extracted or ejected from the core of neutron stars - neutronium - is also a common feature of science fiction, as the densest solid material currently known, although as I've discussed before this wouldn’t be stable when removed from the high gravity environment. In Robert Silverberg’s short story “The Iron Star” (Amazing Stories, Jan 1980), for example, an alien first contact encounter takes place in a young neutron star - black hole binary system where a human commercial research vessel is currently in pursuit of a minute sample of the precious material.

Since the recognition that binary neutron star mergers can result in both gravitational wave flares and electromagnetic short gamma ray bursts, this catastrophic ending has also attracted interest. The television series Threshold (2005-2006), for example, used an incoming neutron star merger shockwave to motivate the actions of alien invasions (although the short-lived series was cancelled before the plan and its consequences were resolved).

More often though, as in the examples of Dragon’s Egg, Flux, Starquake, Death of a Neutron Star and others above, authors use the exotic environments of neutron stars as playgrounds in which to explore the peculiarities of gravitational physics, and their potential implications. As Harold Lower noted in his 1939 article in Astounding Science Fiction,

“Physicists have been doing such surprising things to atoms lately that one has great difficulty in trying to keep up with developments. But it seems probable that things can happen in the interior of stars that as yet cannot be duplicated in the laboratory.”

This remains true almost a century later. At times, the extremely hard science fiction that can result from this curiosity can be hard going on readers. Reviews of these books are often divided, with many readers noting that explaining the science occasionally seems to take priority over pacing or character. Something of an exception here is the work of Stephen Baxter, who makes such effective use of the environment in his world building that explanations are closely integrated with the plot and character development. The fact that infodump-heavy, hard science fiction stories such as Niven’s Neutron Star and Forward’s Dragon’s Egg have nonetheless won some of the highest awards in science fiction demonstrates the fascination that such unfamiliar physics can have for audiences.

Neutron stars have risen to a new prominence in popular science and astronomy press releases in recent years as new telescopes have revealed new insights into the gamma ray bursts, gravitational wave transients and pulsar phenomena associated with them. They continue to form a popular part of science fictional world-building, and as the examples discussed here they also provide a remarkable playground for exploring exotic physics and its consequences for humans and for other life. No doubt neutron stars will continue to play this role far into the future.