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Intergalactic Travel via Hypervelocity Stars

We’ve been looking at not just interstellar but intergalactic crossings in the past few days, something of an homage to Carl Sagan, whose enthusiasm for continuous acceleration at 1 g and relativistic time dilation was immense in the years shortly after Robert Bussard’s key paper on interstellar ramjets. Without a working ramjet and largely unaided by time dilation, we’re faced with millions of years of flight time to reach M31. What to do?

In a recent paper, discussed here by Adam Crowl on Monday, Robin Spivey ponders ‘autonomous probes that spawn life upon arrival’ as a way of reaching the Virgo cluster, which he wants to do for reasons Adam explained in his post. He’s also counting on continuous acceleration at 1 g for these small ‘seed ships,’ but other than mentioning antimatter, he doesn’t explore how this would be done, and we’ve seen the results Sagan and Iosif S. Shklovskii came up with for antimatter when they worked out the equations.

Let’s assume that the ‘slow boat’ solution is the only practical way to proceed. Here I think Adam’s suggestion that we take our environment with us rather than building a worldship is sensible, flinging a small star and planet out of the galactic core toward the destination. Ray Villard pondered the same question back in 2010 in an online piece called The Great Escape: Intergalactic Travel is Possible. He points to the four million solar mass black hole at the center of the Milky Way as the only conceivable way to impart the needed kinetic energy to a star.

Here’s how Villard describes the mechanism:

The theory is that a star could be slingshot out of a binary star system if the stellar duo swung close to the central black hole. The hole’s gravitational tidal forces would break apart the pair’s gravitational embrace.

The companion star orbiting in the direction of the black hole would pick up momentum and plunge toward the black hole. In accordance with Newton’s third law of motion — action-reaction — the other binary companion would go whizzing off with the same velocity but opposite direction away from the black hole.

In just a few thousand years the star would ascend out of the galactic plane and hurtle deep into intergalactic space. The persistent tug of our Milky Way’s dark matter halo would slow it down but the star would never fall back into the Galaxy.


Image: Using ESO’s Very Large Telescope, astronomers have recorded a massive star moving at more than 2.6 million kilometres per hour (1160 km/sec). Credit: ESO.

We do in fact know about a number of such hypervelocity stars, some of which may be moving fast enough to exceed galactic escape velocity. Consider this: Ordinary stars in the Milky Way have velocities in the range of 100 kilometers per second, while some hypervelocity stars near galactic center show velocities of ten times that, closing on 1000 km/sec. Meanwhile, a team led by Tilmann Piffl (Leibniz Institute for Astrophysics, Potsdam) that has been working with high-velocity stars has calculated escape velocity for objects in the vicinity of our own Solar System. The team uses data from the Radial Velocity Experiment (RAVE) survey.

The result: We would need 537 kilometers per second to get our payload fast enough to escape the galaxy. That’s a high speed, of course, but in terms of small craft, it’s not a lot higher than some studies have shown a solar sail could reach using an extremely tight ‘Sundiver’ maneuver to let itself be whipped out of the Solar System. Piffl’s team has catalogued hypervelocity stars moving at 300 km/sec, and we also know of unbound hypervelocity stars (although it’s a tricky call because of uncertainties about the mass distribution of the galaxy). Even some neutron stars are fast-movers: RX J0822-4300 was measured to move at 1500 km/sec in 2007.

Not all hypervelocity stars come from encounters with the black hole at galactic center. In work described at the American Astronomical Society meeting in January, Kelly Holley-Bockelmann and grad student Lauren Palladino found what may be a new class of hypervelocity stars moving with sufficient speed to escape the galaxy (see Stars at Galactic Escape Velocity). Says Holley-Bockelmann:

“It’s very hard to kick a star out of the galaxy. The most commonly accepted mechanism for doing so involves interacting with the supermassive black hole at the galactic core. That means when you trace the star back to its birthplace, it comes from the center of our galaxy. None of these hypervelocity stars come from the center, which implies that there is an unexpected new class of hypervelocity star, one with a different ejection mechanism.”

As we learn more about what creates hypervelocity stars, can we imagine far future technologies that might help us exploit them? If so, an intergalactic journey opens up. A civilization that somehow harnessed a hypervelocity star for such a journey — or one that arose on a planetary system that had been already flung into intergalactic space — would experience eons in the space between the galaxies, periods that dwarf the lifetime of human civilizations. Villard speculates about the astronomers of such a civilization trying to discover their place in the universe as their ‘worldship’ exited the Milky Way, globular clusters peppering the sky, the galaxy’s spiral arms winding out from a nucleus looking like ‘a fuzzy headlamp.’

Inevitably larger telescopes would yield a view of the universe that revealed myriad other pinwheel structures. Spectroscopy would show they are racing away too. Still the aliens literally wouldn’t know if they’re coming or going. A long-lived civilization’s science archive would note the shrinking and dimming of the Milky Way over geologic time. They might conclude that the eerie pinwheel is speeding away from them. And without a cosmological or stellar framework, they would have no idea of cosmic evolution. They would not even be able to calibrate the vast distance to the Galaxy.

But let’s assume for the sake of argument that a civilization might knowingly set out on a hypervelocity star system, its futuristic powers vast enough to shape the encounter between the star and the galactic black hole so as to direct its journey to the proper destination. Any culture that did this would knowingly be splitting into different evolutionary lines given the immensity of the distances and time involved, leaving behind its own species to grow into another over the course of millions of years. Whether and why any species might choose to make this kind of a journey is an exercise left to the reader, and to the imagination of science fiction writers.

We’ve seen stars manipulated for a variety of purposes in science fiction, as a matter of fact. Tomorrow I’ll wrap up this week of speculations on intergalactic travel with a look at some of the methods that have been employed to move stars around, and the possible SETI implications that arise from all this.

The Piffl paper is “The RAVE survey: the Galactic escape speed and the mass of the Milky Way,” submitted to Astronomy & Astrophysics (preprint). The Palladino paper is “Hypervelocity Star Candidates in the SEGUE G and K Dwarf Sample,” The Astrophysical Journal Vol. 780, No. 1 (2014), with abstract and preprint available.


Comments on this entry are closed.

  • Gregory Benford June 26, 2014, 12:36

    Thanks, Paul, for this series. It’s given me an idea for a story set a mere 100,000 or so years from now.
    The star-hitchhiker method at least gives timescales allowing smart life to seed whole galaxies, if their civilization coheres for the millions of years needed. A society convinced that spreading life, however primitive, through as many stars as possible, could then dispatch life as spores or in stasis packages at large angles to their inter-galactic trajectory, thus delivering life across the whole swath of a galaxy they approach.
    I recall a Poul Anderson (?) story about a starship crew marooned on a planet in the galactic halo or further, with the whole galaxy as a pinwheel in their night sky. Recall what it was? Anderson always had a colossal reach, as Tau Zero proved well.

  • Michael June 26, 2014, 13:35

    An interaction with S2, a star at the centre of the galaxy in orbit around the central black hole would yield velocities of around 5000 km/s!


    Close sling shot manoeuvres of several of these hyper velocity stars would yield even greater velocities still but the highest velocity possible may not put you in a direction towards another galaxy. Interactions between orbiting black holes, neutron stars and white dwarfs would also yield very high velocities.

  • James D. Stilwell June 26, 2014, 14:01

    Intergalactic Travel via Hypervelocity Stars
    by Paul Gilster on June 26, 2014
    An amazing post…
    Arthur C. Clarke wrote briefly about this sort of idea in his “The City and the Stars” about a galactic civilization that had powerful twin reasons for making just such an undertaking…fleeing the disembodied vicious Mad Mind and sheer curiosity about an advanced civilization of beings living on the far side of the universe…Apparently there is no limit to the human imagination…

  • Paul Gilster June 26, 2014, 14:24

    Gregory Benford writes:

    I recall a Poul Anderson (?) story about a starship crew marooned on a planet in the galactic halo or further, with the whole galaxy as a pinwheel in their night sky. Recall what it was? Anderson always had a colossal reach, as Tau Zero proved well.

    That rings a bell with me as well, though I can’t come up with the title. Can any readers help?

  • Eric S. June 26, 2014, 16:03

    Brings to mind the Ringworld series & the Pierson’s Puppeteers moving their world(s)…

  • Steve Kilston June 26, 2014, 16:21

    My personal, curmudgeonly reaction is to put hypervelocity (anything faster than 1000 km/s) concepts in a class with Icarus, his hubris, and his wax-supported wings. He failed to heed the warning about getting too close to the sun, so the wax melted, and the comparable warning I give for hypervelocities has to do with the problems resulting from collisional destruction, even by pebbles. (Deceleration remains another challenge as well.) Solve those problems and I’ll be enthusiastic.

  • Eric June 26, 2014, 16:33

    World Without Stars. Read it back in college, from an old copy of my dad’s. Not a bad story, I recall. Don’t remember many particulars.

  • Alex Tolley June 26, 2014, 17:07

    If M31 is traveling at 110 kn/s and will reach our galaxy in 4 bn years, then a hypervelocity star traveling at 1100 km/s would reach M31 in 400 my, about as long as the Earth has had terrestrial plants and vertebrates. Any life is going to hugely evolve in that time period, including the intelligent ones. Is there going to be much point in doing anything over that huge timescale unless you know that your world might be destroyed in less time? I could imagine that a planet near the galactic center with higher risk of destruction by a nearby supernova might be a candidate for such a maneuver to preserve it.

  • Edg Duveyoung June 26, 2014, 17:08

    Each of us is an emergent phenomenon of a culture.

    We know the minds of the past would be nonplussed by what today’s child takes as common sense.

    Who are we to speculate, now, as we learn to crawl and dream of flight?

    Strange that we gather here and plan for what we’d do if . . . . when the minds of the past show us so clearly that we simply cannot know what our minds will LONG FOR years hence.

    It’s not shop-talk we do here. The masthead contains the word “dreams.”

  • Paul Gilster June 26, 2014, 19:49

    Eric writes:

    World Without Stars. Read it back in college, from an old copy of my dad’s. Not a bad story, I recall. Don’t remember many particulars.

    Hey, that’s it! Can’t believe I forgot the title because it’s come up here before (a few years back, I think). Thanks, Eric!

  • NS June 26, 2014, 23:46

    When it comes to seeding other worlds with Earth life, the SF we should keep in mind is “Invasion of the Body Snatchers” and “The Andromeda Strain”. And IIRC there’s an Arthur C. Clarke story about how the indigenous life on another planet (Venus?) is accidentally destroyed by garbage left behind by visiting Earth astronauts. Until we know a lot about any particular world we intend to seed or visit we should be doing our best to make sure Earth life (or garbage) DOESN’T get there. Both for the mainly moral reasons illustrated in the above SF stories, and the scientific one that there are a lot of questions about biology etc. that can only be answered by investigating the indigenous life of many worlds, an opportunity that could be irretrievably lost if we are not very careful about where Earth life goes.

  • Eniac June 27, 2014, 1:10

    With plenty of time on their hands, future generations might quite easily accelerate the Earth and sun together onto an intergalactic trajectory by arranging successive close encounters with other stars, each precisely chosen and aligned such that it results in an increase in velocity. The amount of thrust needed to arrange for the first of such encounters decreases exponentially with the distance/time allowed for it, so it could be quite modest. Subsequent encounters can be done faster, because each encounter provides an opportunity to take precise aim for the next.

    Say we might gain 10 km/s per encounter, only 100 would be needed to get up to the desired 1000 km/s. The process will still take millions of years, but compared to the time spent on intergalactic coasting it is not that bad…

  • Brady June 27, 2014, 5:21

    There is another science fiction book about sling-shotting a star system around our central black hole, to get back to mother Earth. Yes, back to.
    I read it back in the 1980s, I think. The book started off with a group of humans growing up on an alien planet and raised by aliens. The humans were a product of an “information package” sent out from old Earth to the stars (a globular cluster?) This package included our DNA code as well as “cultural information”, like a Mozart symphony but nothing about how to build a violin (not enough space in the “information package” for everything). The group of humans eventually decided to say a friendly goodbye to their alien “parents” & return to their home planet. They used planets on a star system in a sling shot effect with the Galaxy’s central supermassive black hole. Time dilation was involved too, if I remember rightly. When they arrived back in Earth space they are attacked by cannibalistic aliens. Don’t remember how it ended. It would be nice to review how the time dilation effects worked in that book. Does this ring a bell with anyone?

  • swage June 27, 2014, 8:53

    Such an undertaking would require forces of staggering imagination.

    ‘We would need 537 kilometers per second to get our payload fast enough to escape the galaxy. That’s a high speed, of course, but in terms of small craft, it’s not a lot higher than some studies have shown a solar sail could reach’

    How small? What studies? Any links?

  • Paul Gilster June 27, 2014, 9:34

    swage, I watched Geoffrey Landis work this out on the proverbial back of an envelope in his office at Glenn Research Center in Cleveland — he came up with roughly 500 km/sec. But sure, there are papers working these numbers. I don’t have the references in front of me right now, but I’m about to get into an extended series on sail technologies where this is going to come up, so I’ll dig some of these references up and publish them in coming days. Remember, these high numbers are for a solar sail making a ‘Sundiver’ maneuver for a propulsive slingshot effect. They demand advances in materials and a very close solar pass.

  • ljk June 27, 2014, 9:56

    Let us not forget Fritz Zwicky’s plan to use the entire Sol system as a starship with our sun as the propulsion system:



    Zwicky was introduced in the revamped Cosmos series for his other ideas such as dark matter, so he is officially famous now. :^)

  • Paul Gilster June 27, 2014, 12:24

    Not to worry — I’ve got a piece on Zwicky scheduled for next week. Had hoped to get it in Friday’s post but I ran out of room.

  • Al Jackson June 27, 2014, 14:52

    Hypervelocity stars were originated by (unjustly unknown astrophysicist) Jack Hills.


    This paper speaks to stars with planets during ejection from a system with a black hole:

    Things don’t really look all good from the planets , in the long run, running into your mother sun or being tidally disrupted would not make your day.

    The may be 1000 HV stars in the Galaxy , look here for the section on HVs:


  • swage June 27, 2014, 16:22

    Thanks to Mr. Gilster and others. That came off a bit more harsh than it was actually intended. I’d love to see the actual work. It helps me to put certain things into the right perspective, because i do care especially for actual capabilities. I bid my excuses.

  • stephen June 28, 2014, 5:29

    You mention a binary star passing close to a black hole. What if the star system has more than two components? The surviving components could support multiple life-bearing planets.

    You mention the enormous antimatter mass ratio necessary. They could use a supermassive star as a fuel source; its mass would be converted to antimatter a little at a time for propulsion. They might be able to extend its lifetime artificially

    It would be part of a multiple star system, with the inhabited planets orbiting one of the other stars.

    Impacts with intergalactic debris would generate enormous energy; but maybe they’d know how to store that energy, or convert it to matter and/or antimatter.

  • Paul D. June 28, 2014, 12:26

    There’s another ramjet-like concept that might be suitable for extreme relativistic speeds. It doesn’t exploit nuclear fusion or matter annihilation, but rather carefully transfers momentum and energy between the spacecraft, an inert propellant, and passing stationary matter.

    To see how this might work, consider a toy example: a car in which the wheels drive a pump that expels an inert reaction mass backwards at variable speed. The speed is adjusted so that, in the reference frame where the ground is stationary, the expelled propellant is also stationary. Ignoring friction, this system accelerates the car (thrust from the expelled propellant > drag from the pump on the wheels), because all the kinetic energy remains in the vehicle, and the mass of the vehicle is decreasing.

    A similar scheme could work at relativistic speeds, without incurring the exponential growth in mass ratio that bedeviled Sagan and Shklovskii.

  • Eniac June 30, 2014, 22:36

    I did not see the, for me, most likely source of hypervelocity stars mentioned. If one component of a tightly bound binary goes supernova and loses much of its mass, the other will be thrown out at a good fraction of its orbital velocity, which, I think, can well be in the thousands of km/s if the pair is really tight.

  • Michael July 1, 2014, 13:27

    @Eniac June 30, 2014 at 22:36

    ‘I did not see the, for me, most likely source of hypervelocity stars mentioned. If one component of a tightly bound binary goes supernova and loses much of its mass, the other will be thrown out at a good fraction of its orbital velocity, which, I think, can well be in the thousands of km/s if the pair is really tight.’

    I get around 1700 km/s for a 1 solar mass star ejection


  • Paul July 1, 2014, 14:30

    Eniac: the likely source of hypervelocity stars is from binary (or higher) systems ripped apart during a close encounter with the massive black hole at the center of the galaxy. One star is left orbiting the hole (or drops into it), and the other receives a boost that, by the Oberth effect, leads to a large velocity at infinity.

  • Eniac July 3, 2014, 15:05

    Paul: This is indeed one likely source, and it has been mentioned here. But, it works only for stars that actually emanate from the galactic center. I understand there are many that don’t, and for those the supernova explanation needs to be evoked. Michael above provides the relevant link.