From Harvard-Smithsonian Center for Astrophysics: “The Monster in the Middle of the Milky Way Is…Spinning Slowly?”

Harvard Smithsonian Center for Astrophysics

From Harvard-Smithsonian Center for Astrophysics

October 20, 2020

Amy Oliver
Public Affairs
Center for Astrophysics | Harvard & Smithsonian
Fred Lawrence Whipple Observatory

This wide-field view of the center of our Milky Way galaxy shows, in visible light, the vast array of stars contained within this small space. The stars closest to the center of the galaxy, known as S-stars, are orbiting SgrA*, a massive black hole. The orbits of these stars are helping scientists to better understand the black hole and the nature of our galaxy.

This view was created from photographs in red and blue light and forming part of the Digitized Sky Survey 2. The field of view is approximately 3.5 degrees x 3.6 degrees. Credit: ESO and Digitized Sky Survey 2. Acknowledgment: Davide De Martin and S. Guisard.

SgrA* NASA/Chandra supermassive black hole at the center of the Milky Way, X-ray image of the center of our galaxy, where the supermassive black hole Sagittarius A* resides. Image via X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI.

SGR A* , the supermassive black hole at the center of the Milky Way. NASA’s Chandra X-Ray Observatory.

The monstrous black hole at the center of the Milky Way galaxy—now of Nobel Prize fame—is proving yet again to be stranger than fiction. New research from scientists at the Center for Astrophysics | Harvard & Smithsonian (CfA), and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern University has revealed that the supermassive black hole at the center of the Milky Way galaxy is not spinning much, providing more evidence that it is unlikely to have a jet. The paper is published in The Astrophysical Journal Letters.

Supermassive black holes like SgrA*—the monstrous black hole at the center of the Milky Way galaxy—are characterized by just two numbers: mass and spin, but have a critical influence on the formation and evolution of galaxies. According to Dr. Avi Loeb, Frank B. Baird Jr. Professor of Science at Harvard and CfA astronomer, and co-author on the research, “black holes release a huge amount of energy that removes gas from galaxies and therefore shapes their star formation history.”

While scientists know that the mass of central black holes has a critical influence on their host galaxy, measuring the impact of their spin isn’t easy. As Loeb puts it, “the effect of black hole spin on the orbits of nearby stars is subtle and difficult to measure directly.”

To get a better understanding of how Sgr A* has impacted formation and evolution of the Milky Way, Loeb and Dr. Giacomo Fragione, of CIERA, studied instead the stellar orbits and spatial distribution of S-stars—the closest stars orbiting SgrA* and traveling at a speed of up to a few percent of the speed of light—to constrain, or place limits on the spin of the black hole. “We concluded that the supermassive black hole in the center of our galaxy is spinning slowly,” said Fragione. “This can have major implications for the detectability of activity in the center of our galaxy and the future observations of the Event Horizon Telescope.”

Star S0-2 Andrea Ghez Keck/UCLA Galactic Center Group at SGR A*, the supermassive black hole at the center of the Milky Way.

The S-stars appear to be organized into two preferred planes. Loeb and Fragione showed that if Sgr A* had a significant spin, the preferred orbital planes of the stars at birth would become misaligned by the present time. “For our study we used the recently discovered S-stars to show that the spin of the black hole SgrA* must be smaller than than 10-percent of its maximal value, corresponding to a black hole spinning at the speed of light,” said Loeb. “Otherwise, the common orbital planes of these stars would not stay aligned during their lifetime, as seen today.”

The results of the research also point to another important detail about Sgr A*: it is unlikely to have a jet. “Jets are thought to be powered by spinning black holes, which act as giant flywheels,” said Loeb, with Fragione adding that, “Indeed there is no evidence of jet activity in SgrA*. Upcoming analysis of data from the Event Horizon Telescope will shed more light on this issue.”

The find was published just days before the announcement of the 2020 Nobel Prize in Physics, which was awarded in part to scientists Reinhard Genzel and Andrea Ghez for their ground-breaking research which demonstrated that SgrA* is a black hole. “Genzel and Ghez monitored the motion of stars around it,” said Loeb. “They measured its mass but not its spin. We have derived the first tight limit on SgrA*’s spin,” adding that the find wouldn’t be possible without Genzel and Ghez’s original Nobel Prize-winning work.


This work was supported in part by a CIERA Fellowship at Northwestern University, and Harvard’s Black Hole Initiative, which is funded by grants from the John Templeton Foundation and the Gordon and Betty Moore Foundation.

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The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.