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Scattered across the universe are stars that shine subtle hints of a shared ancestry. They might be thousands of light-years apart, almost indistinguishable from other, nearer stars that surround them, but similarities—their ages, their compositions, and their orbits—reveal a common origin story. And astronomers can now use these stellar kin-groups to reconstruct the chaotic, often violent, history of galaxies.
Galaxies are the bustling cities of the universe, home to almost all its stars, and through their histories, we can learn about the grand forces that shape the organization of matter.
To trace these stellar genealogies, scientists look to the stars’ conception. Stars are rarely born alone. Instead, a single massive cloud of gas and dust—called a giant molecular cloud—filled with a particular proportion of elements will spontaneously collapse and fragment, popping out tens, hundreds, and sometimes thousands of stars at once. The famous Orion Nebula, a delight to backyard astronomers, is one such active star-forming region.
But shortly after their birth, these stars begin to leave the nest. While initially tightly knit, they do not possess enough gravity to bind to each other. Any little disturbance—a passing neighbor nebula, gravitational interactions with each other—sends the stars farther away from each other on a path chosen through sheer random chance, like spilled salt scattering on the counter. In only a few million years, the stars, once birthed from the same gas cloud, can be tens of thousands of light-years apart, each one set to follow its own orbital trajectory around the galaxy.
It’s in the details—a similar orbital origin point, a mutual age, a common abundance of heavy elements—that stars retain a blueprint of their birthplace. And scientists are now able to start charting these pasts.
Shortly after their birth, stars begin to leave the nest.
By matching these properties to that of the sun, for example, in 2014 astronomers were able to find our own first-known stellar sibling. Located in the constellation Hercules and sitting 110 light-years away, the star, named HD 162826, can be seen with a small telescope. The discovery came as a surprise. Even though the sun likely has thousands of siblings, they were thought to be so far away by now as to elude confirmation of a match. But HD 162826 just happened to be close enough that the team could confirm its abundance of elements and reconstruct its orbital motion, indicating that four and a half billion years ago, that star and our sun likely shared a point of origin.
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Essentially all stars belong to one such family group or another. If the group is young enough, such as the well known Pleiades cluster, then the identification is easy. There simply hasn’t been enough time for the members to scatter away, and the stars still live in the same neighborhood. But most stars cannot be matched to their siblings; it takes extensive surveys covering millions, if not billions, of stars to tease out the signatures of a few kin groups. The vast majority of those identified groups are easily identified as members of the Milky Way.
But among the 300-odd billion stellar denizens of the Milky Way, there are some groups that simply … don’t belong. They have an odd abundance of elements, or strange orbital properties. They don’t share the genetic properties of the bulk of our galaxy’s stars.
Take, for example, the oddly named Gaia Sausage, discovered in 2018. The “Gaia” comes from the European Space Agency’s Gaia spacecraft, which has amassed, so far, a catalog of more than 2 billion stars with detailed information on their properties. The “sausage” part is less obvious. When astronomers plot the circular motion versus the radial motion of stars in our galaxy, a large elongated, sausage-like lump appears in the plot, made up of a collection of stars unlike the others in the Milky Way.
But these stars in the Gaia Sausage haven’t remained clustered together. They are currently scattered all around the Milky Way, their thin threads of commonality all that betrays their shared history. They have a common proportion of heavy elements, or “metallicity.” And their orbits are extremely elliptical, bringing the stars brushing up against the core of the galaxy then flinging them back out more than 60,000 light-years away. In their commonality, they tell the story of a once-mighty galaxy eventually consumed and obliterated.
Cosmologists give this process an innocent-sounding name: hierarchical galaxy formation. But what that means is that our contemporary Milky Way, like all massive and proud galaxies in the universe, has lived a violent life, and only acquired its present-day bulk through devouring its neighbors.
Billions of years ago, there were no galaxies. In this period, some 100 million years after the Big Bang, there weren’t even any stars. There was just a relatively smooth continuum of hydrogen, helium, and dark matter spread across the universe. But within that continuum, there were small density differences, places of slightly higher or lower density. Over time—hundreds of millions of years—those regions of higher density accreted more and more material, forming the gravitational seeds that would eventually grow up to become galaxies and form stars of like-histories. As smaller galaxies venture too close to large ones, their constituent stars get consumed and scattered within them.
We can only understand the details of galactic mergers through computer simulations that recreate these titanic events, tracing them through the shared lineages of these ancient, subsumed stars. Through these simulations, cosmologists see just how cataclysmic the process is.
In the case of a minor merger, when a large galaxy such as the Milky Way consumes a small one, the process might look something like this: The smaller galaxy feels a different gravitational pull at different locations, causing it to stretch out. It elongates, turning from a dense spherical clump to a long, thin stream. This stream then plunges headlong into the consuming galaxy, where its individual stars are then strewn about into random orbits.
In the details, stars retain a blueprint of their birthplace.
After a few billion years, you would never know that anything had ever happened.
The only clues we have today are the common origin of those far-flung stars. But the Gaia Sausage is not the only dispersed ancestral group. There are other such strands of these hidden histories. The Sagittarius Stream, discovered in 2002 is a “ghost” remnant of the Sagittarius Dwarf Elliptical Galaxy. It consists of a thin strand of stars and gas ripped from its parent galaxy looping around the core of the Milky Way; we seem to have caught this merger event towards the end, but not yet at its final moments.
And there are more such streams: the Arcturus, the Helmi, the Palomar 5. There is even the Monoceros Ring, a community of stars that make three complete circles around the Milky Way. Each stream represents a distinct kinship, a shared heritage from a once-intact galaxy otherwise lost to history.
Some former galaxies were defeated and swallowed by the Milky Way so long ago that we can barely discern them. Hypothesized in 2020, the Kraken galaxy is thought to have contributed roughly 10 percent of the Milky Way’s present-day collection of globular clusters—tight dense clumps of stars that retain their own shape. In this case, the remnant population of the Kraken resisted further obliteration.
These smaller galaxies consumed by the Milky Way also alter their host; they change the composition, structure, and chemical makeup of their newfound home. Another name for the Gaia Sausage is Gaia Enceladus, named after the mythological giant buried under Mount Etna and responsible for its earthquakes. For the Milky Way, the collision with Gaia Enceladus disrupted its prominent thin disk of stars, puffing it out and causing the formation of an ancillary, thicker disk.
Some ancient star cousins might even be in our neighborhood of the galaxy. Recently, a team of astronomers discovered 20 stars within a few thousand light-years of the sun that all might share a common ancestry. Given their particular makeup of heavy elements and the scattered nature of their orbits, the astronomers think that they are the leftovers of a truly ancient galaxy, named Loki, which merged with ours when the Milky Way was first coalescing in the darkness of the early universe, some 11-12 billion years ago.
But those who live by the sword, die by the sword—even galaxies. The Milky Way has merged, acquired, destroyed, and cannibalized countless neighbors, engulfing their surviving stars and matter within its vast bulk. But those conflicts were all unequal, against foes far smaller than our own galaxy.
Sitting 2.5 million light-years away is a true peer to the Milky Way, the Andromeda Galaxy, resplendent with more than 1 trillion stars, and roughly equal to our own galaxy in total mass. And it’s headed right for us. In about 5 billion years, our two galaxies will begin to merge together, a process that will take more than half a billion years to complete.
The new mega-galaxy will be unrecognizable. No spiral arms, no flat disk. Just an elliptical lump of stars mixed together in randomized orbits. Perhaps some future astronomer, living in an age well after our own sun has died, will scan their heavens and find a population of stars that share a common age and metallicity; a family born in the Milky Way but flung distantly through that reorganized cosmos, the faintest of patterns tying them back to a vanished homeland. 
Lead image: McCarthy’s PhotoWorks / Shutterstock
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Paul M. Sutter
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Paul M. Sutter is a research professor in astrophysics at the Institute for Advanced Computational Science at Stony Brook University and a guest researcher at the Flatiron Institute in New York City. He is the author of Your Place in the Universe: Understanding our Big, Messy Existence.
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