Kareem El-Badry – MacArthur Foundation

About Kareem’s work

Kareem El-Badry is an astrophysicist who studies how stars form, evolve and interact. El-Badry leverages astronomical data sets and theoretical modeling to study binary star systems, black holes, neutron stars, and other stellar bodies. Its ability to extract information from the enormous amounts of data collected during space observation missions has led to numerous discoveries, from our galaxy’s neglected dormant black holes to new classes of stars and coupled systems.

In his early work, El-Badry developed a method for identifying binary stars in spectrographic data sets. More than half of stars exist in binary systems, but they are often too close together to be differentiated with available technology. El-Badry overcame this challenge through targeted statistical analysis of existing spectral data. His analysis of data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) project identified more than 3,000 binary systems in the Milky Way, and in subsequent work with data from the Gaia mission he discovered more than a million binary systems, the largest sample known to date. As a postdoctoral researcher, El-Badry used spectral disentangling (that is, the separation of light spectra emitted by different stellar bodies) to demonstrate that several purported dormant black holes were, in fact, stars that had been stripped of their outer layer of hydrogen and were contracting, making them appear abnormally bright. The inflated luminosities of bare stars have led researchers to overestimate their masses and, therefore, identify their companions as black holes. El-Badry then showed that this process is how most Be-class stars form, that is, by removing material from a companion star in a nearby binary, thus solving a decades-old mystery. In more recent work, he developed a method to identify dormant or non-interacting black holes in large binaries. A black hole too far from another stellar body to steal its matter cannot be seen via X-ray emissions or gravitational waves. El-Badry determined that in such cases, dynamic interactions between the black hole and its orbiting star cause characteristic radial velocity variations and “wobble” of the stars across the plane of the sky. These measurements can then be used to reconstruct the orbit and mass of the star’s dark companion. Using this method, El-Badry and his colleagues have so far discovered two dormant black holes, including the closest known black hole to Earth, as well as dozens of neutron stars.

El-Badry is very prolific and has contributed to many other areas of stellar astronomy, including his discoveries about high-velocity stars, triple star systems, the fate of massive stars, and galaxy formation. As more astronomical data becomes available and new observation missions are launched, El-Badry is poised to further expand our knowledge of the wonders of our universe.