Classification is never easy. Whether it’s monkey species, astronomical objects or elementary particles, there are seemingly endless ways to organize and group things. For centuries biologists have used “family tree” diagrams as their approach of choice for tracing living organisms’ lineages. And now astronomers are borrowing from biology to classify stars this way, too.
DNA can reveal how organisms are related, and the chemical makeup of a star can similarly be used to determine its ancestry. Stars are thermonuclear forges, fusing light elements such as hydrogen and helium into heavier ones including carbon and oxygen. When stars die they eject this material into space, where it can in turn form new suns. Each subsequent generation of stars will thus become more enriched with heavy elements—and thus their chemical composition offers information about their stellar genealogy. By surveying the chemistry of several stars in our galactic neighborhood, a team of astronomers has now grouped them into a distinct family tree, and uncovered clues about their origins. The results were published in February in the Monthly Notices of the Royal Astronomical Society.
“The chemical composition of these stars can be used as the ‘DNA’ of the gas from which they formed,” says Paula Jofré, lead author of the study and an astronomer at the University of Cambridge. “This can then be used to build up an evolutionary tree, like in biology.”
Using data gathered by a European Southern Observatory telescope in La Silla, Chile, Jofré and her colleagues studied 22 nearby stars similar to the Sun, focusing on seventeen different chemical elements as a stand-in for DNA. Their analysis successfully grouped most of the stars into two well-known families—but it also found that some of the stars belong to a new, previously unknown third group.
Biologists have devised many methods to craft family trees. The most influential emerged in the 1850s from Darwin’s theory of evolution, and relied upon the principle of genetic inheritance to group organisms by hereditary similarities in shape and form. The advent of DNA sequencing in the 20th century helped establish more rigorous and quantitative “phylogenetic” studies of how organisms are related, with molecular markers and morphology both informing the construction of the trees.
Of course, stars do not pass on DNA or obey laws of natural selection—so they cannot evolve like biological organisms on Earth, says Robert Foley, co-author and professor of human evolution at the University of Cambridge. “In this case it’s very, very different from true evolution, where you have reproduction and true heritability,” he says. “But it’s enough to suggest that information is passed down in time in ways that might reflect shared history.” Specifically, the recycling of elements through generational stellar enrichment creates a common ancestry, allowing phylogenetic techniques to trace family histories and chemical evolution.
Jofré, Foley and their co-authors are not the first to practice this sort of celestial phylogenetics. Astronomer Didier Fraix-Burnet of the Institut de Planétologie et d’Astrophysique in Grenoble, France, first began working on phylogenetic studies with astronomical objects 16 years ago.
“I used to work on galaxies, and was puzzled by the difficulty to classify these complex objects,” Fraix-Burnet says. “I imagined the possibility to use the phylogenetic approach on galaxies.”
Fraix-Burnet calls his work “astrocladistics,” after cladistics: a term biologists often use interchangeably with phylogenetics to describe the technique of using a tree-shaped diagram as a visual aid to show shared, inherited characteristics. Mapping inherited characteristics is a natural way of displaying how organisms are related, but it can also be used to illustrate how groups evolve over time.
Jofré’s team decided to use an approach that would go beyond simple classification to understand the stellar families’ formation and evolution. The researchers used a phylogenetic computational approach called the “distance method,” which scales the tree’s branches such that longer ones indicate greater chemical (and thus evolutionary) change between the stars. “We are going one step further,” Jofré says. “Once we have the classification, we use the branch lengths to study also what happened to the system.”
The results verified the two commonly identified families of stars in the disk of the Milky Way—the younger stars in our galaxy’s central thin disk and the older stars in the thick disk, which envelopes the thin disk like a diffuse cloud. However, they also found a third group that did not match either of the two known stellar families, raising questions as to this newfound group’s origins.
To uncover additional clues about each family’s history, the team turned to the stars’ ages and motions. The stellar motions helped confirm that the families were indeed separate. The ages narrowed down where and how they were born, and revealed the speed of each family’s chemical evolution. Based on these assessments, the team speculates that the mysterious third family of stars may have formed from a galactic merger or a similar major, single event early in the Milky Way’s history. Alternatively—and perhaps less likely—the stars in this family could simply be late-bloomers from the thick disk or early-birds from the thin disk, scattered over time into our solar system’s vicinity from their distant birthplaces.
Now that some of the difficulties in translating biological methodologies into astronomy have been smoothed out, Jofré hopes that this approach can be scaled up and applied to more stars. With a larger sample, she hopes to pin down the origins of the new third family as well as several other outlier stars that do not fit into any of the three identified groups. This may someday help astronomers find a common ancestry binding all the stars in the Milky Way, giving us a better understanding of the origin story of our galaxy.