A Spider Stellar Engine Could Move Binary Stars Halfway Across a Galaxy

Eventually, every stellar civilization will have to migrate to a different star. The habitable zone around all stars changes as they age. If long-lived technological civilizations are even plausible in our Universe, migration will be necessary, eventually.

Could Extraterrestrial Intelligences (ETIs) use stars themselves as stellar engines in their migrations? In broad terms, a stellar engine uses a star to generate work. A simple example is solar panels, which use the Sun’s radiation to generate electricity that we use to perform work. But the scaled-up idea is to use the star to produce thrust. That thrust could be used to move the star itself. An ETI capable of doing that would be a Type II civilization on the Kardashev Scale.

To most of us, it seems like a wildly improbable idea. But who knows what’s out there? If an ETI can survive long enough, it may become a Type II civ. The stellar engine idea dates back to science fiction author Olaf Stapledon. A couple of decades after Stapledon, astronomer Fritz Zwicky also discussed manipulating stars with advanced technology, even turning them into spacecraft. In the decades since, the idea has persisted, and other researchers have delved into it. In 1988, Leonid Shakdov developed the first detailed stellar engine model called the Shakdov Thruster.

The Spider Stellar Engine

In new research, Clement Vidal, from Vrije Universiteit in Brussels, Belgium, examines how an advanced civilization could use a binary star as a stellar engine. The paper is titled The Spider Stellar Engine: a Fully Steerable Extraterrestrial Design?

“Since about half the stars in our galaxy are in binary systems where life might develop too, we introduce a model of a binary stellar engine,” Vidal writes. “We apply the model to candidate systems, spider pulsars, which are binary stars composed of one millisecond pulsar and a very low-mass companion star that is heavily irradiated by the pulsar wind.”

"Pulsars are what remains of some massive stars. At the end of their lives, some massive stars collapse to form neutron stars. When these neutron stars spin rapidly, they produce beams of radiation from their poles."

Vidal is concerned with stellar engine technosignatures. Research has focused on hypervelocity stars as potential stellar engine technosignatures because they’re easily observable. Other researchers have also proposed other stellar engine concepts, but according to Vidal, they’re “poorly linked to observable technosignatures.”

Technosignatures

Vidal’s main goal in this work is to determine what types of technosignatures a binary stellar engine would emit. He discusses what potential signatures might be emitted by acceleration, deceleration, steering, and maneuvers such as gravitational assists or captures. However, unlike some other researchers, he focuses on a specific type of binary system: spider pulsars, which are a subclass of binary millisecond pulsars.

Understanding Pulsars

Pulsars are what remains of some massive stars. At the end of their lives, some massive stars collapse to form neutron stars. When these neutron stars spin rapidly, they produce beams of radiation from their poles. If the radiation is aimed at Earth, then we can observe the pulses of energy. These pulses have exquisitely precise timing, and astronomers use them to determine cosmic distances.

Spider Pulsars

A spider pulsar is a pulsar with a companion, usually a red dwarf, a brown dwarf, or even a planetary-mass object. They’re called spider pulsars because it’s as if the pulsar spins a web of powerful beams of radiation that strips away the companion’s mass, eventually destroying it.

Mechanics of the Spider Stellar Engine

Vidal’s paper describes the payload as a pulsar with about 1.8 solar masses and the propellant as its low-mass companion star with between 0.01 and 0.7 solar masses.

How the Mechanism Works

In essence, the gravitationally bound binary system is the vehicle, and the smaller companion star is the propellant. The spider pulsar generates thrust by expelling propellant out of the gravitational system, and the propellant is the matter stripped from the companion.

The binary pair orbits a common center of gravity. The idea behind this binary stellar engine (BSE) is that as they orbit, the pulsar’s radiation strikes the companion or propellant star. A close binary is more effective because the closer the pulsar is to the propellant, the more thrust is generated. The assumption is that a Type II civilization would have the technology to moderate this thrust to serve their purposes by timing the radiation and heating the outer layers of the propellant star with X-ray or gamma radiation.

Acceleration and Deceleration

To decelerate, the BSE would produce active thrust in the opposite direction of travel. It could also use a passive magnetic sail deployed from the pulsar to transfer momentum to the interstellar medium.

Steering the Engine

The BSE steers by selectively evaporating the star during different orbital phases. “To choose a direction, it suffices to evaporate the companion star once per orbit, at a specific orbital phase, in order to create consistent thrust in one direction,” Vidal explains.

These various maneuvers and manipulations with the BSE would emit technosignatures. Have astronomers observed any candidate BSEs in the Milky Way? Possibly. “Could our galaxy host a kind of fully steerable binary stellar engine that we proposed? This is a plausible hypothesis in the context of the stellivore hypothesis, which reinterprets some observed accreting binary stars as advanced civilizations feeding on stars,” Vidal writes.

The Stellivore Concept

A stellivore is a hypothesized type of civilization first proposed by Vidal that has the technology to consume its home star via accretion. They use the star's energy to sustain their existence. Vidal writes that rather than consume the energy, they could use it to migrate to a more favorable location in the galaxy.

Types of Spider Pulsars

There are two types of spider pulsars: Black Widows and Redblacks. The distinction is in the mass of the companion. In a black widow (BW), the companion is less than 0.1 stellar masses. In a redblack, the companion is between 0.1 and 0.7 stellar masses. Spider pulsars are different from other pulsar binaries because they evaporate their companions rather than accrete them. When pulsars accrete too much material, they can form black holes. Spider pulsars don’t tempt the same fate. Vidal calls these spider stellar engines (SSEs) rather than binary stellar engines (BSEs).

Previous Research

Previous researchers have studied the original BW, and Vidal writes, “The 3D motion of the system appears to be nearly aligned with the spin axis of the MSP.” This fits in with the SSE interpretation because perfect alignment is necessary to produce maximum thrust. A stellivore civilization would have a destination in mind, and Vidal says that he’s found a potential destination for the original Black Widow pulsar. He says that the pulsar will reach this target star in about 420 years while also acknowledging the uncertainty in this determination.

Steering and Technosignatures

PSR J1959+2048, the original BW, also modulates itself, which could be interpreted as steering. However, it also displays other characteristics and moderation that call into question the ‘steering’ interpretation.

Suitability and Advantages

Ultimately, Vidal’s SSE may have a shorter duty cycle than other proposed stellar engines, limiting its usefulness. However, it has advantages in steering over others. “Transposing it on a smaller scale, it might also be an inspirational design for advanced propulsion solutions, or for planetary defense purposes such as deflecting asteroids,” Vidal writes.

A Broader Context

The idea may seem preposterous to some, but that’s incidental. Many ideas in history seemed preposterous until they weren’t.

Vidal isn’t claiming that we’re seeing the technosignatures of stellar engines. He’s arguing that it’s worth pursuing the idea of observing them. He sees these candidates and predictions of what their signals might look like as clues and as starting points for further investigation.

“Spider pulsars thus offer observable stellar engine technosignature candidates, with decades of data, active studies that discover, model, and monitor these dazzling systems,” he concludes.

Conclusion

In summary, the research by Clement Vidal presents a fascinating perspective on how advanced civilizations might manipulate their stellar environments through binary stellar engines. Such constructs challenge our understanding of astrophysics and pose significant implications for the search for extraterrestrial intelligence. The theoretical framework established around spider pulsars not only broadens our conception of stellar dynamics but may also provide a new direction to pursue potential signals from extraterrestrial cultures capable of profound engineering feats across galactic scales.

References

For more information, refer to the works cited:

• Clement Vidal, "The Spider Stellar Engine: a Fully Steerable Extraterrestrial Design?" arXiv
• Universe Today, "A Spider Stellar Engine Could Move Binary Stars Halfway Across a Galaxy".