The stars move ever on. What seems like a fixed distance due to the limitations of our own longevity morphs over time into an evolving maze of galactic orbits as stars draw closer to and then farther away from each other. If we were truly long-lived, we might ask why anyone would be in such a hurry to mount an expedition to Alpha Centauri. Right now we’d have to travel 4.2 light years to get to Proxima Centauri and its interesting habitable zone planet. But 28,000 years from now, Alpha Centauri — all three stars — will have drawn to within 3.2 light years of us.
But we can do a lot better than that. Gliese 710 is an M-dwarf about 64 light years away in the constellation Serpens Cauda. For the patient among us, it will move in about 1.3 million years to within 14,000 AU, placing it well within the Oort Cloud and making it an obvious candidate for worst cometary orbit disruptor of all time. But read on. Stars have come much closer than this.
In any case, imagine another star being 14,000 AU away, 20 times closer than Proxima Centauri is right now. Suddenly interstellar flight looks a bit more plausible, just as it would if we could, by some miracle, find ourselves in a globular cluster like M80, where stellar distances, at the densest point, can be something on the order of the size of the Solar System.
Image: This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Located about 28,000 light-years from Earth, M80 contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. Globular clusters are particularly useful for studying stellar evolution, since all of the stars in the cluster have the same age (about 12 billion years), but cover a range of stellar masses. Every star visible in this image is either more highly evolved than, or in a few rare cases more massive than, our own Sun. Especially obvious are the bright red giants, which are stars similar to the Sun in mass that are nearing the ends of their lives. Credit: NASA, The Hubble Heritage Team, STScI, AURA.
These thoughts are triggered by a paper from Bradley Hansen and Ben Zuckerman, both at UCLA, with the interesting title “Minimal Conditions for Survival of Technological Civilizations in the Face of Stellar Evolution.” The authors note the long-haul perspective: The physical barriers we associate with interstellar travel are eased dramatically if species attempt such journeys only in times of close stellar passage. Put another star within 1500 AU, dramatically closer than even Gliese 710 will one day be, and the travel time is reduced perhaps two orders of magnitude compared with the times needed to travel under average stellar separations near the Sun today.
I find this an interesting thought experiment, because it helps me visualize the galaxy in motion and our place within it in the time of our civilization (whether or not our civilization will last is Frank Drake’s L factor in his famous equation, and for today I posit no answer). All depends upon the density of stars in our corner of the Orion Arm and their kinematics, so location in the galaxy is the key. Just how far apart are stars in Sol’s neighborhood right now?
Drawing on research from Gaia data as well as the stellar census of the local 10-parsec volume compiled by the REsearch Consortium On Nearby Stars (RECONS), we find that 81 percent of the main-sequence stars in this volume have masses below half that of the Sun, meaning most of the close passages we would experience will be with M-dwarfs. The average distance between stars in our neck of the woods is 3.85 light years, pretty close to what separates us from Alpha Centauri. RECONS counts 232 single-star systems and 85 multiple in this space.
Hansen and Zuckerman are intrigued. They ask what a truly patient civilization might do to make interstellar travel happen only at times when a star is close by. We can’t know whether a given civilization would necessarily expand to other stars, but the authors think there is one reason that would compel even the most recalcitrant into attempting the journey. That would be the swelling of the parent star to red giant status. Here’s the question:
As mentioned above, this stellar number density yields an average nearest neighbor distance between stars of 3.85 light years. However, such estimates rely on the standard snapshot picture of interstellar migration − that a civilization decides to embark instantaneously (at least, in cosmological terms) and must simply accept the local interstellar geography as is. If one were prepared to wait for the opportune moment, then how much could one reduce the travel distance, and thus the travel time?
Maybe advanced civilizations don’t tend to make interstellar journeys until they have to, meaning when problems arise with their central star. If this is the case, we might expect stars in close proximity at any given era — ruling out close binaries but talking only about stars that are passing and not gravitationally bound — to be those between which we could see signs of activity, perhaps as artifacts in our data implying migration away from a star whose gradual expansion toward future red giant phase is rendering life on its planets more and more unlivable.
Here we might keep in mind that in our part of the galaxy, about 8.5 kiloparsecs out from galactic center, the density of stars is what the authors describe as only ‘modest.’ Higher encounter rates occur depending on how close we want to approach galactic center.
Reading this paper reminds me why I wish I had the talent to be a science fiction writer. Stepping back to take the ‘deep time’ view of galactic evolution fires the imagination as little else can. But I leave fiction to others. What Hansen and Zuckerman point out is that we can look at our own Solar System in these same terms. Their research shows that if we take the encounter rate they derive for our Sun and multiply it by the 4.6 billion year age of our system, we can assume that at some point within that time a star passed within a breathtaking 780 AU.
Image: A passing star could dislodge comets from otherwise stable orbits so that they enter the inner system, with huge implications for habitable worlds. Is this a driver for travel between stars? Credit: NASA/JPL-Caltech).
Now let’s look forward. A gradually brightening Sun eventually pushes us — our descendants, perhaps, or whatever species might be on Earth then — to consider leaving the Solar System. Recent work sees this occurring when the Sun reaches an age of about 5.7 billion years. Thus the estimate for remaining habitability on Earth is about a billion years. The paper’s calculations show that within this timeframe, the median distance of closest stellar approach to the Sun is 1500 AU, with an 81 percent chance that a star will close to within 5000 AU. From the paper:
Thus, an attempt to migrate enough of a terrestrial civilization to ensure longevity can be met within the minimum requirement of travel between 1500 and 5000 AU. This is two orders of magnitude smaller than the current distance to Proxima Cen. The duration of an encounter, with the closest approach at 1500 AU, assuming stellar relative velocities of 50km/s, is 143 years. In the spirit of minimum requirements, we note that our current interstellar travel capabilities are represented by the Voyager missions (Stone et al. 2005); these, which rely on gravity assists off the giant planets, have achieved effective terminal velocities of ∼ 20 km/s. The escape velocity from the surface of Jupiter is ∼ 61 km/s, so it is likely one can increase these speeds by a factor of 2 and achieve rendezvous on timescales of order a century.
My takeaway on this parallels what the authors say: We can conceive of an interstellar journey in this distant era that relies on technologies not terribly advanced beyond where we are today, with travel times on the order of a century. The odds on such a journey being feasible for other civilizations rise as we move closer to galactic center. At 2.2 kiloparsecs from the center, where peak density seems to occur, the characteristic encounter distance is 250 AU over the course of 10 billion years, or an average 800 AU during a single one billion year period.
You might ask, as the authors do, how binary star systems would affect these outcomes, and it’s an interesting point. Perhaps 80 percent of all G-class star binaries will have separations of 1000 AU or less, which the authors consider disruptive to planet formation. Where technological civilizations do arise in binary systems, having a companion star is an obvious driver for interstellar travel. But single stars like ours would demand migration to another system.
We can plug Hansen and Zuckerman’s work into the ongoing discussion of interstellar migration. From the paper:
Our hypothesis bears resemblance to the slow limit in models of interstellar expansion (Wright et al. 2014; Carroll-Nellenback et al. 2019). In a model in which civilizations diffuse away from their original locations with a range of possible speeds, the behavior at low speeds is no longer a diffusion wave but rather a random seeding dominated by the interstellar dispersion. Even in this limit, the large age of the Galaxy allows for widespread colonization unless the migration speeds are sufficiently small. In this sense our treatment converges with prior work, but our focus is very different. We are primarily interested in how a long-lived technological civilization may respond to stellar evolution and not how such civilizations may pursue expansion as a goal in and of itself. Thus our discussion demonstrates the requirements for technological civilizations to survive the evolution of their host star, even in the event that widespread colonization is physically infeasible.
It’s interesting that the close passage of a second star is a way to reduce the search space for SETI purposes if we go looking for the technological signature of a civilization in motion. Separating out stars undergoing close passage from truly bound binaries is another matter, and one that would, the authors suggest, demand a solid program for eliminating false positives.
Ingenious. An imaginative exercise like this, or Greg Laughlin and Fred Adams’ recent work on ‘black cloud’ computing, offers us perspectives on the galactic scale, a good way to stretch mental muscles that can sometimes atrophy when limited to the near-term. Which is one reason I read science fiction and pursue papers from people working the far edge of astrophysics.
The paper is Hansen and Zuckerman, “Minimal conditions for survival of technological civilizations in the face of stellar evolution,” in process at the Astronomical Journal (preprint). Thanks to Antonio Tavani for the pointer on a paper I hadn’t yet discovered.