How an Astrophysicist Uses Binary Stars and Gaia to Reveal Hidden Black Holes

How an Astrophysicist Uses Binary Stars and Gaia to Reveal Hidden Black Holes

When Kareem El-Badry picked up an unexpected call at 7 a.m., he assumed it might be a fundraising solicitation. Instead, the caller told him he had been named a MacArthur Fellow — one of this year’s recipients of the prestigious so-called “genius grant.” The recognition, he says, validates his work and gives him the freedom to take bolder intellectual risks.

El-Badry, an astrophysicist at the California Institute of Technology, studies binary star systems — pairs of stars born together that orbit one another — to detect otherwise invisible mass in our galaxy. Rather than relying on bright accretion disks, he searches for black holes by observing the gravitational tug they exert on a companion star.

“One of the cool things about binary stars is that if one of the stars becomes a black hole, we can still study it by noticing its gravitational effects on the companion star that's still alive,” El-Badry explained.

His work uses high-precision astrometric data from the European Space Agency’s Gaia mission to identify stars whose motions betray an unseen companion. By reexamining existing catalogs and looking for subtle anomalies, El-Badry finds candidates that other searches — focused on luminous signals — might miss.

He describes the sky as an interconnected web rather than a collection of isolated objects. “Everything in space is interacting with everything else in some way,” he says. Adding a companion to a system, he notes, multiplies the range of behaviors and yields richer, more interesting physics.

Part of his approach is simple patience and careful pattern recognition. “Over the last five or six years I’ve built intuition,” he says. Often a tiny unexpected feature in the data prompts the thought: Hmm, I’m surprised to see that — and that surprise can point the way to new discoveries.

El-Badry likens modern sky surveys to Google Maps: you can zoom out for a global view of the sky or zoom in on a small region where larger telescopes reveal finer details. Applying that perspective to stellar motion catalogs enables the identification of candidate systems where a visible star’s orbit demands an invisible partner.

Why does this matter? Astronomers estimate the Milky Way should contain between 100 million and 1 billion black holes, yet only about 20–30 are confidently identified so far. El-Badry’s methods aim to close that huge gap and reveal the hidden population of compact objects shaping our galaxy.

For casual stargazers, he hopes this research encourages people to see more than static points of light. “They’re not just pretty lights,” he says. “Many of them are doing something interesting — pulsating, transferring mass, or orbiting unseen companions such as black holes.”

Asked what keeps him up at night, El-Badry offers a modest reflection: there are far more fundamental mysteries in astronomy than solved ones, and making progress on any single question is a rare, exhilarating achievement.