Billions of Stars Travel Through the Intergalactic Medium. Why Were They Expelled From Their Galaxies?

 All stars are born within galaxies. However, there are also lone travelling stars in the emptiness between galaxies. No one knows for sure why they are there, but one thing is certain—only violence can tear stars from their homes. 

The Antennae Galaxies, two merging galaxies. It's possible for stars to be ejected during mergers; Source: Wikipedia

In outer space, stars and galaxies like to stay as close as possible due to their gravity. Stars accumulate to galaxies like the Milky Way, and galaxies group to clusters. The closest larger galaxy cluster is the Virgo Cluster, prominent among amateur astronomers for its relatively bright galaxies such as Messier 87. 

But galaxy clusters aren’t the ultimate; clusters are just small structures on the very large scale, where they grow to superclusters. For example, the Milky Way is part of the Local Group, which in turn is part of the Laniakea Supercluster. 

Superclusters reside in galaxy filaments — the largest known structure in the observable universe is the Hercules-Corona Borealis Great Wall, measuring 10 billion light-years in diameter! These filaments give the universe on the large scale its foam-like structure; streets of bright galaxy superclusters align next to dark patches where only few clusters reside. Think of it the way cities light up at night from the window of an airplane. 

So, as you can see, matter likes to be close to other matter. But the universe isn’t perfect and some objects reside in empty space. On larger scales, examples are void galaxies which reside in galactic voids. On the very small scale, there are rogue planets floating between the stars within a galaxy. 

And then there are rogue stars—stars not belonging to any galaxy, travelling in the vacant space between galaxies. 

In this lonely world, the night sky isn’t illuminated by bright neighboring stars, but instead dark with galaxies shining in the distance. If the Sun were located in interstellar space, the night sky wouldn’t be filled with constellations telling tales of mythology; we’d tell tales of the faint galaxies far away, indulging in a lonely comfort as the Sun continues its journey without destination. 

At first, astronomers thought stars could only exist in galaxies; until 1996. That year, data from the Hubble Space Telescope revealed a population of elder stars in the space between the galaxies of the Virgo Cluster. These stars account for 10% of the cluster’s mass, yet are hundreds of thousands of light years away from any galaxies. 

Even more intergalactic stars were discovered in the Fornax Cluster, another nearby prominent galaxy cluster. Here, it’s estimated that intergalactic stars could make up to 40% of the cluster’s stars. 

Years after these initial discoveries, astronomers discovered intergalactic stars much closer to home—675 stars which reside in the outskirts of the Milky Way. And, just like in the Virgo Cluster, these stars are old: red giants, on the verge of death. 

And recently, the Euclid Space Telescope discovered 1.5 trillion intergalactic stars in the Perseus Cluster! These stars cause a faint background glow to emanate from the cluster, making for a stunning sight.

One of the possible formation mechanisms of intergalactic stars; Source: Wikipedia

But a mystery remains: since all stars form within galaxies, what tears them away? There are various ways for stars to be ejected from their parent galaxies. Let’s take a look at them:

1. Interactions with other galaxies: In the universe, galaxies grow by merging with others. These mergers are very violent events that incinerate new star formation but also send stars on different orbits. While collisions between stars are rare, it’s possible for the interaction between galaxies to eject some stars along the way. The same is true for galactic cannibalism, when larger galaxies swallow smaller ones.
2. Blue stragglers and hypervelocity stars: Estimates point to the existence of just 1000 hypervelocity stars (HVS) in the Milky Way, all on a path to leave the galaxy. But what exactly are they? One thing that defines HVS is that they have a violent path behind them, as well as a precise formation mechanism.
   In order to make one, you need: a multiple-star system and a supermassive black hole (SMBH). Throw one of the stars into the galactic SMBH, and the remaining stars of the system will gain speed to the point that it exceeds galactic escape velocity. And tada, you have intergalactic stars. It’s also possible for the remaining stars in the system to merge, which will turn them into one giant star: a blue straggler. These shine in a bright blue, but their lives end just like other stars: by turning into red giants.
   One example of a star that is both a HVS and a blue straggler is HE 0437-5439 in the constellation Dorado. It lies at a distance of 200’000 light years and was ejected from the Large Magellanic Cloud, one of the satellite galaxies of the Milky Way. It’s currently moving away from us at 723 km/s! In comparison, the Sun travels at 230 km/s around the galactic center. 
3. Star deaths: The final theory for intergalactic stars also requires a multiple-star system. If the system contains one star bound to go supernova, the explosion will expel the other star(s) of the system — these can reach speeds of more than 1000 km/s! 

Given these formation mechanisms, it’s likely that there are many more intergalactic stars than expected. We’re still uncovering them. But just the glow from the Perseus Cluster due to its rogue stars is proof enough that intergalactic space isn’t just empty — perhaps a solace to the stars that reside in this dark space.