Runaway star

Stellar cannonballs may be invaders from another galaxy

Contributed by

Our Milky Way galaxy is a collection of gas, dust, dark matter, and a couple of hundred billion stars. Most of those stars orbit the galactic center in a pinwheel-shaped disk about 100,000 light years across and a few thousand light years thick, but there’s also a vast roughly spherical halo of stars around the galaxy stretching out about 100,000 light years, itself.

Most of the stars in the halo are moving around the Milky Way in nice, normal orbits. However, over time a handful have been discovered that are weird: They’re moving too fast.

Milky Way map

A map of the Milky Way based on real observations. Not shown is the huge spheroidal halo of stars and hot gas surrounding the disk of our galaxy. Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)


These stellar bullets are screaming around space much faster than the stars around them. Sometimes their velocity is so high that the galaxy’s gravity can’t hold on to them: Their destiny is to escape the galaxy forever.

We call these high-velocity stars. A really big question is actually a pretty a simple one: Where did they come from? There are lots of possible origins for these stars (which I’ll get to in a sec), but a new one has just been found, and I’ll be honest, it surprised me: They are coming from the Large Magellanic Cloud (or LMC), a satellite galaxy of the Milky Way.

That startled me for a lot of reasons, but the biggest is that the LMC is over 150,000 light-years from us, and that’s a long way to travel for a star even at high speed. But a paper just published outlines how it works, and it’s pretty convincing.

The main piece of evidence is that a lot of these high-velocity stars are seen in the constellations of Leo and Sextans. That’s significant, because if you map out the location and orbit of the LMC around the Milky Way, the LMC is headed in that direction (think of it as watching a car zoom past you on a road, and you can see it’s headed toward the east; it might be in front of you at this exact second, but you can extrapolate where it will be in a few minutes). It orbits our galaxy at about 380 kilometers per second, which is really fast, and if you could eject stars from it they would preferentially be found moving in the direction of the LMC itself.

That’s pretty good circumstantial evidence but, to be honest, it’s not enough. Can stars like this, in fact, be ejected from the LMC?

This animation from the journal paper shows nearly two billion years of time as the LMC moves across our sky and the stars ejected from it. The curved path is due to the way the map is shown (like how maps of the Earth can distort the poles); the diustance is listed in kiloparsecs (1 kpc = 3260 light years) and time in millions of years ago. Credit: Boubert et al


There are many ways to get stars blowing through space at high speeds. One is if the star starts out in life as part of a binary star, two stars orbiting one another. If they pass really near a black hole, one star can get swallowed by it while the other gets ejected at a pretty substantial clip. We think this happens in our Milky Way when a binary encounters the gigantic black hole at the galaxy’s exact center. Some high-velocity stars seen are consistent with this, but that doesn’t explain the excess seen toward Sextans and Leo. Plus, there’s no evidence the LMC has a big black hole like ours, so that doesn’t really cover the observations.

There are other ways (for example, encounters with other stars in a dense stellar cluster can kick stars pretty hard), but it’s hard to account for both the number and distribution of these stars seen.

contact binary

Artwork depicting a contact binary: two stars orbiting so close together they share a single atmosphere. Credit: ESO/L. Calçada


One way seems to fit the bill, though. You start with a binary system, where at least one of the stars is high-mass, more than 8 times the mass of the Sun. Eventually, that star will turn into a red giant, swelling hugely in size. The other star can then draw material off the giant, increasing its own mass. If they are close enough together, they can actually become what’s called a contact binary, a peanut-shaped object which is essentially two stars sharing the same atmosphere! When this happens, the two stars actually can spiral in, getting very close together. As that happens, their orbital speed around each other increases.

Then, catastrophe: The more massive star explodes in a spectacular supernova! If it loses enough mass in the explosion, it no longer has enough gravity to hold the binary together, and the companion star gets flung away at high speed. A-ha! A high-velocity star.

This seems a little unlikely, though. How often does this happen?

Turns out, a lot! The scientists doing the study decided to find out just how common an occurrence this is in the LMC, so they did two things: They used a physical model of how stars form and evolve in the LMC to see how many high-velocity stars you can get this way, and then used a second physical model of the LMC and Milky Way system to see if the gravity of the two galaxies changes the way the stars behave (for example, the gravity of the LMC may slow down the stars ... but the ones shot out ahead of the LMC in its orbit get the galaxy’s velocity added to them, as a ball thrown out a car window gets the car’s speed added to its own).

Runaway star

Artist’s illustration of a runaway star ejected from the LMC (background, using an actual image of the satellite galaxy). Credit: ESO


Their model simulated nearly 2 billion years of time, and what they found was pretty cool: Over that time, more than 860,000 stars will have escaped the LMC, making up about 80% of the high-velocity stars seen in the Milky Way’s halo! That shows that it’s extremely plausible that the stars actually seen come from our companion galaxy.

There were other interesting tidbits to come out of this as well. Because these stars were once part of a contact binary, they may have started off lower mass, but gained mass before getting flung out into the Universe. If they wound up with more than about 8 times the Sun’s mass, they, too, would explode over time. The model predicts that about half the stars ejected from the LMC exploded on their way here. These supernovae leave behind either a dense neutron star or a black hole, which means thousands of these objects — tiny, but possessed of super-strong gravity — are blazing past our galaxy even now.

Now, don’t fret: They’re too far away to hurt us in any realistic way, but I do hope some science fiction author hears about this and devises a fun story based on them.

Interestingly, a lot of high-velocity stars are high-mass blue stars (called B stars in the astronomical stellar classification system). That, too, is naturally explained by them once being in a contact binary, where they gained enough mass to fall into this category.

So, how do we prove this? High-velocity stars can be found in a number of ways. In general, it’s through their spectrum; when you break the light up from a star into thousands of narrowly sliced colors, you can learn a lot about them, including their speed. But that’s a hard measurement to make on a large scale.

You can also take images of lots of stars in the sky, wait a few years, then do it again. Stars moving rapidly enough in space will move noticeably in such a survey (if the observations are accurate enough). And there is such a survey: Gaia, which is mapping a billion stars in the Milky Way. Over the course of its multi-year mission it may find quite a few of these runaway stars.

So, is this idea of cannonball stars from the Large Magellanic Cloud correct? Maybe. I do like it, and it explains a lot. The good news is it’s testable, making predictions about the numbers, locations and types of stars we should expect to see in the Gaia survey. Time will tell, and we won’t have to wait too long, since the survey results needed for this will be released over the next few years.

Every time I think I’ve heard everything about astronomy, something new comes along. Alien invader stars from another galaxy! Science is just so much fun.