How Do Stars Go Rogue?

Rogue stars are moving so quickly they’re leaving the Milky Way, and never coming back. How in the Universe could this happen?

Stars are built with the lightest elements in the Universe, hydrogen and helium, but they contain an incomprehensible amount of mass. Our Sun is made of 2 x 10^30 kgs of stuff. That’s a 2 followed by 30 zeros. That’s 330,000 times more stuff than the Earth.

You would think it’d be a bit of challenge to throw around something that massive, but there are events in the Universe which are so catastrophic, they can kick a star so hard in the pills that it hits galactic escape velocity.

Rogue, or hypervelocity stars are moving so quickly they’re leaving the Milky Way, and never coming back. They’ve got a one-way ticket to galactic voidsville. The velocity needed depends on the location, you’d need to be traveling close to 500 kilometers per second. That’s more than twice the speed the Solar System is going as it orbits the centre of the Milky Way.

There are a few ways you can generate enough kick to fire a star right out of the park. They tend to be some of the most extreme events and locations in the Universe. Like Supernovae, and their big brothers, gamma ray bursts.

Supernovae occur when a massive star runs out of hydrogen, keeps fusing up the periodic table of elements until it reaches iron. Because iron doesn’t allow it to generate any energy, the star’s gravity collapses it. In a fraction of a second, the star detonates, and anything nearby is incinerated. But what if you happen to be in a binary orbit with a star that suddenly vaporizes in a supernova explosion?

That companion star is flung outward with tremendous velocity, like it was fired from a sling, clocking up to 1,200 km/s. That’s enough velocity to escape the pull of the Milky Way. Huzzah! Onward, to adventure! Ahh, crap… please do not be pointed at the Earth?

This artist’s impression shows the dust and gas around the double star system GG Tauri-A.
This artist’s impression shows the dust and gas around the double star system GG Tauri-A.

Another way to blast a star out of the Milky Way is by flying it too close to Kevin, the supermassive black hole at the heart of the galaxy.

And for the bonus round, astronomers recently discovered stars rocketing away from the galactic core as fast as 900 km/s. It’s believed that these travelers were actually part of a binary system. Their partner was consumed by the Milky Way’s supermassive black hole, and the other is whipped out of the galaxy in a gravitational jai halai scoop.

Interestingly, the most common way to get flung out of your galaxy occurs in a galactic collision. Check out this animation of two galaxies banging together. See the spray of stars flung out in long tidal tails? Billions of stars will get ejected when the Milky Way hammers noodle first into Andromeda.

A recent study suggests half the stars in the Universe are rogue stars, with no galaxies of their own. Either kicked out of their host galaxy, or possibly formed from a cloud of hydrogen gas, flying out in the void. They are also particularly dangerous to Carol Danvers.

Considering the enormous mass of a star, it’s pretty amazing that there are events so catastrophic they can kick entire stars right out of our own galaxy.

What do you think life would be like orbiting a hypervelocity star? Tell us your thoughts in the comments below.

10 Replies to “How Do Stars Go Rogue?”

  1. Stars only live so long. So chances are if the star is not heading towards any galaxy in our local group, it would burn out before reaching anything else. But the experience would be awesome. Watching our galaxy fade away and finally seeing what the heck the Great Attractor is.

    1. An interesting aspect of these super speedy halo stars, in some interstellar travel future, is that a spacecraft might get a good speed boost out of it by a close gravitational assist flyby. There are some really very fast stars out there, as in this paper:
      “occasionally with velocities 10^5 km/s (one third the speed of light)”

      If the Sun suddenly became such a beast and we saw it happen, heading towards us, it would already be 1/3 of the way upon us. Because of the light travel time lag. That’s fast.

  2. I cannot imagine the loneliness of an alien race that discovers that its star is a rogue and that they are so far out in the intergalactic medium that even at light speed they are a million years from anywhere. Just how crushing would that be do you think?

    The only bonus would be the view.

    1. It doesn’t matter. Our nearest star is so far far away that it isn’t even visible to the naked eye at night and was discovered only in 1915, by Robert Innes. We *are* alone in an empty hole of nothingness, imagine that. Of course there’s other stuff around, but it is all the way out there beyond our reach.

      1. Of course our “nearest” star is only ~8.5 light minutes away, so I assume you’re referring to Proxima Centauri, which is not very much closer than Alpha Centauri (4.24 LY vs. 4.37 LY according to the Wikis). Its lack of visibility in our sky has much more to do with its small, dim self than its distance: A & B Centauri are both very much visible to the naked eye, despite A being not much larger or brighter than our own Sun (Sol & Rigil Kentaurus are both G-class stars), and if B were to suddenly lose its brighter primary it would still be the 21st brightest star in our sky, despite being only 90% Sol’s mass, and considerably dimmer in the visible wavelengths. Bottom line: Proxima is hard to see because it’s a runt, not because it’s far away.

        The issue of reach is a separate one entirely. One can rationally argue that all it would take to get humans to the Centauri system within their lifetimes — I say nothing of their families and friends left behind, relativistic (yet still sub-light) speeds being what they are — is the profligate expenditure of money, effort and natural resources, as the technology already exists, at least in prototype & principle. A combination of solar sails, nuclear propulsion, and ion drive systems all (in theory) could get a crew of explorers to that system within a few decades by the crew’s reckoning, despite the ~4 light year gap.

        Granted, this is all speculation, and it would probably be a bad idea to even try at this exact point in time, the rough equivalent technologically speaking of circumnavigating the Earth’s oceans in a bathtub. And yet even at this level, we really aren’t THAT far off from the eventual outriggers, caravels, brigantines and steamships (by comparison w/said bathtub) that will begin to make the effort worthwhile to the few brave, crazy, egotistical and utterly necessary First Starfarers.

        But as J. pointed out, we are very lucky in terms of our placement in space, for were we millions of LY away — instead of the Sol-centric dozen LY bubble which currently nets us quite a selection of star systems — even the metaphorical equivalent of an Apollo program (as related to my bathtub example) would not be enough to make the rest of the universe even remotely reachable. Barring the development of inter-dimensional travel or the like, such a civilization would in fact be truly, utterly alone — a lone house on a tree-less island in the midst of an endless sea.

        By comparison, we’re more like two-year olds in a fenced yard who have just figured out that we actually maybe COULD go next door by opening the latch on the gate… if only we could reeeeach the latch….

      2. Proxima is as dim as most stars are. But if one were near it, it would look like a giant object melted to plasma by the continuous nuclear war inside of it (because that’s what it is). Blindening to watch. And that we cannot see because of the huge distance. With microlensing astronomers have even found planets around such stars which they cannot even see. Btw, many astronomers would’ve loved living outside of a galaxy. Alot of work is today put into adjusting observations for gas, dust, radiation and gravity around us inside of this busy galactic arm.

        Interstellar travel is impossible for humans. I’ve watched the talks online of a couple of conferences about interstellar travel (google Icarus or 100 year starship) and although these knowledgeable people have the best will and hopes, they say that we don’t know how to do it, even with unlimited budget. Even fusion propulsion might not cut it. As someone put it: “-We need more than one visit by the tooth fairy.”

        I think that life from Earth will go interstellar pretty soon. But it is too impractical for humans as we know them to do it.

      3. I guess we would have to settle on a definition for the term “near;” from 1AU the star would appear a bit brighter than our full Moon, or about as bright as Sol appears now to the Voyager probes. Even compared to other red dwarf stars it’s considered dim, being big and bright only in relative comparison to objects like brown dwarfs or gas giants.

        As for the rest, I’m a bit more optimistic, preferring to think of such travel as “implausible” rather than “impossible.” Still, either way, interstellar travel remains unreal for the time being, and “impractical” is certainly a true assessment at this point no matter how hopefully one looks at it.

  3. “What do you think life would be like orbiting a hypervelocity star?”

    In the words of Carl Sagan (who was referring to the view from a planet orbiting a halo star, rather than a “rogue star” per se): “Not a sunrise, but a galaxy-rise; the dawning of a hundred billion Suns.”

  4. Of course the sun is mostly made of plasma, not atoms such as hydrogen and helium. Living in the solar system of a rogue star would be like living anywhere else. Who would notice the movement?

  5. Wow, with warp drive we traveled to Proxima Centauri in only a few weeks. Unfortunately our drive is down; however, we know there is a viable planet orbiting Alpha Centauri A only about a tenth of a light year away. Practically next door. We got rockets and enough fuel to get up to 30,000 mph and coast there. Whew. It will only take a little over 2100 years to get there. What are we waiting for?

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