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Researchers at Harvard and Northwestern University derived the first tight limit for the spin of the supermassive black hole at the center of the Milky Way.

The recent location of 40 new “S-stars” near the Sagittarius A* black hole prompted the research, according to Astronomy professor Avi Loeb, co-author of the study and researcher at Harvard’s Center for Astrophysics. Loeb and his co-author developed a measuring technique based on the spatial distribution of those stars around the black hole.

The pair — who published their work in the Astrophysical Journal Letters earlier this month — used the stars’ unique geometry to measure the black hole’s spin. Loeb said that his research builds on the work of Reinhard Genzel and Andrea Ghez, who won the 2020 Nobel Prize in Physics for discovering the mass of the supermassive black hole.

“It’s just like if you know the first name of a person and you don’t know their last name, then you can’t really identify them or how they grew up,” Loeb said. “In the same way astrophysical black holes are characterized by two numbers: mass and spin. So if you know just one of them it only gives you partial information.”

The spin of the black hole is extremely slow, said co-author Giacomo Fragione, a researcher at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics.

“We realized that we actually can use these geometrical configuration[s] to constrain the spin,” Fragione said.

Black holes are generally characterized by their mass and spin, provided that their charge is neutral and therefore negligible. Measuring the mass of a black hole is relatively straightforward, but determining the spin is more complex.

Loeb and Fragione, however, used the recently discovered S-stars and their close proximity to Sagittarius A* to estimate its spin.

The S-stars that orbit the supermassive black hole are dispersed throughout two disks; stars in each disk rotate in opposite directions around the supermassive black hole. Examining the stars’ positions relative to the black hole allowed Loeb and Fragione to determine an upper limit for the spin.

They reasoned that if the orbitals of S-stars stabilize over time, there must be a constraint on the magnitude of the spin. The spin of the supermassive black hole must be less than 0.1, per their calculations.

“We now have a tight constraint, and we know that it has a low spin. Why is that important? It says something about how this black hole grew to become what it is right now,” Loeb said.

Loeb said he hopes scientists will discover more stars near black holes and use their orbitals and the unique geometry they form to estimate the black holes’ spin.

“If there are stars closer in, one can put even tighter constraints on the spin, as long as they reside in the same orbital plane as the stars farther up. So hopefully more stars will be found,” Loeb said. “These stars not only enable us to test Einstein’s theory of general relativity, Einstein’s theory of gravity, but they also allow us to set tighter constraints on the spin.”