Paula Jofré makes stellar connections

Paula Jofré, 36
Galactic and stellar astrophysics
Universidad Diego Portales, Santiago, Chile

Jofré, 36, is an astrophysicist at Universidad Diego Portales in Santiago, Chile, where she studies the inner lives and histories of stars. She measures the wavelengths of the light that stars emit to figure out which chemical elements the stars contain, and in what proportions.

Then she does something unusual: She borrows a technique from biology to trace the stars’ evolution. Much like an archaeologist examining the DNA in ancient human remains to trace a population’s history, Jofré uses modern stars’ contents to track how their stellar ancestors moved around the Milky Way. Her best-known work, and the research of which she’s most proud, uses those elements as a proxy for DNA to chart the first family tree of the Milky Way’s stars.

She got the idea as a postdoc at a University of Cambridge event organized by an art history student. The focus was how scientists visualize their results. There, she met Cambridge anthropologist Robert Foley, who showed her how evolutionary trees can trace relationships of members of a species over time. Suddenly she realized that stars, too, pass down bits of themselves to successive generations. Perhaps, she thought, these generations could also be traced back in time.

Soon, she and Foley hashed out the stellar family tree project at dinner in a Cambridge dining hall — “very much like a Harry Potter room,” she says, where all the fellows wear academic gowns. Stars obviously don’t procreate like animals, the pair agreed, but dying stars do pass on their chemistry.

That happens because stars forge heavy elements, such as carbon and iron. When the stars die, they often explode and spread those elements throughout the cosmos. The next generation of stars, born from collapsing clouds of gas containing those elements, picks up elements from the earlier generation.

And thus a family is born. Stars from the same gas cloud should have almost identical chemistry, something like how siblings have similar DNA. The analogy is close enough that Jofré, Foley and colleagues built a three-branched tree showing the relationships of 21 of the sun’s sibling stars in 2017 in the Monthly Notices of the Royal Astronomical Society. The team also reported that two of the branches were known groupings — one was the Milky Way’s thin disk of stars, and the other was the older thick disk that surrounds it. The third branch revealed new connections, showing that Jofré’s technique does more than map known star relationships. The approach can reveal new information about past stellar nurseries.

All in the family

Borrowing techniques from biology, Paula Jofré and colleagues mapped out chemical relationships between the sun and 21 of its sibling stars. Three main branches emerged: younger stars in the Milky Way’s thin disk (red, including the sun), much older stars that could be in the Milky Way’s more dispersed thick disk (dark blue) and a third branch that lies in between (light blue). Six stars (black) had no clear relationship to the others, but more observations could help tie them in.

Milky Way star family tree

Source: P. Jofré et al/Monthly Notices of the Royal Astronomical Society 2017

By expanding to more groups of stars, “we could use these trees to learn something about the evolution of our whole galaxy,” Jofré says. “That has been so exciting.”

Other astronomers call the technique original and inventive, if a little unorthodox.

“Paula Jofré impressed me as being very innovative,” says astronomer Kenneth Freeman of the Australian National University in Canberra. “She sees things that other researchers do not see.”

Payel Das of the University of Oxford, a collaborator on the Milky Way project and a close friend, calls Jofré “really brave” as a researcher. “She’s very confident, which is really nice. I think especially now — we’re going through this crisis of women in physics and science and all this — we need this confidence.”

Jofré has never shied away from unpopular paths. Before she graduated from an all-girls high school in Santiago, a guidance counselor spoke to her class about the importance of choosing a career that would leave time for family. One shouldn’t choose a career in, say, astronomy.

“The whole class looked at me,” says Jofré, who had been interested in astronomy since childhood. The moment only strengthened her resolve. “This woman trying to say, please don’t do that, was for me argument to say, please do it.”

The question of whether astronomy was compatible with a family came up sooner than Jofré expected: Her first child was born before she and her partner, Thomas Maedler, finished their PhDs. Their second was born during her first postdoctoral fellowship. Being the only parents in their graduate cohort was difficult. “You feel quite lonely when you’re the only one,” Jofré says.

But contrary to the guidance counselor’s warnings, parenthood has been grounding for the two and helped keep them focused on what’s important — “which is not like, papers, papers, papers, papers, papers,” as Maedler puts it. “We’re always this little nucleus, the four of us, this little atom that is walking around.”

For Jofré, science has been inextricably entwined with family — not just the sun’s, but her own.