Using NASA's James Webb Space Telescope, Northwestern astrophysicists gained the longest, most detailed glimpse yet of the supermassive black hole at the center of the Milky Way. They found the black hole's accretion disk emits a constant stream of flares with no periods of rest. This is a screenshot from a video showing the 2.1 micron data taken on April 7, 2024. (Credit: Farhad Yusef-Zadeh/Northwestern University)
Supermassive black hole at Milky Way’s center never sleeps
EVANSTON, Ill. — Four million times more massive than our Sun, the black hole at the Milky Way’s center has revealed itself to be a restless giant. New research from Northwestern University shows that Sagittarius A* maintains a constant state of activity, producing multiple flares daily and never truly falling quiet, a discovery that’s forcing astronomers to rethink how supermassive black holes operate. Using NASA’s James Webb Space Telescope, researchers watched as Sagittarius A* produced an endless stream of infrared flares, ranging from subtle flickers to dramatic bursts of energy.
In findings published in The Astrophysical Journal Letters, researchers detail their observations of Sagittarius A*, a supermassive black hole located about 26,000 light-years from Earth. Weighing about 4 million times the mass of our Sun, Sgr A* is relatively quiet compared to black holes in other galaxies, making it an ideal laboratory for studying how these cosmic giants behave in their more subdued states.
“Flares are expected to happen in essentially all supermassive black holes, but our black hole is unique,” says lead study author Farhad Yusef-Zadeh from Northwestern University. “It is always bubbling with activity and never seems to reach a steady state. We observed the black hole multiple times throughout 2023 and 2024, and we noticed changes in every observation. We saw something different each time, which is really remarkable. Nothing ever stayed the same.”
The research team observed Sgr A* for a total of 48 hours, spread across seven different viewing sessions over the course of a year. Using JWST’s near-infrared camera (NIRCam), they tracked the black hole’s activity at two different infrared wavelengths simultaneously, something no previous telescope could do for such extended periods.
“In our data, we saw constantly bubbling brightness,” explains Yusef-Zadeh. “And then boom! A big burst of brightness suddenly popped up. Then, it calmed down again. We couldn’t find a pattern in this activity. It appears to be random. The activity profile of the black hole was new and exciting every time that we looked at it.”
The research revealed that Sgr A*’s accretion disk, the collection of gas and dust swirling around the black hole, produces five to six major flares each day, along with numerous smaller fluctuations in brightness. These variations span timescales from mere seconds to several months, creating a dynamic display of cosmic activity.
What makes these observations groundbreaking is JWST’s ability to stare at Sgr A* continuously from its position in space, free from the limitations that hamper ground-based telescopes. Earth’s atmosphere and day-night cycle typically force ground observatories to take intermittent snapshots rather than continuous observations.
The ability to observe in two wavelengths simultaneously provided researchers with a more complete picture of the black hole’s behavior. This dual-wavelength approach revealed an unexpected finding: changes in brightness at the shorter wavelength (2.1 microns) occurred slightly before those at the longer wavelength (4.8 microns), with delays ranging from a few seconds to 40 seconds.
These time delays offer crucial clues about the physical processes occurring near the black hole’s event horizon, the boundary beyond which nothing can escape its gravitational pull. The team believes two distinct mechanisms drive this cosmic light show.
The frequent faint flickers likely arise from turbulent fluctuations within the accretion disk, where hot plasma becomes temporarily compressed. Meanwhile, the bigger, brighter flares probably result from magnetic reconnection events, dramatic collisions between magnetic fields that release energy in the form of particles traveling near the speed of light.
“A magnetic reconnection event is like a spark of static electricity, which, in a sense, also is an ‘electric reconnection,'” says Yusef-Zadeh.
The study also revealed that Sgr A* maintains an average brightness about 100 times that of our Sun which is relatively dim for a supermassive black hole. This modest output occurs because only a small fraction of nearby material actually falls into the black hole. Most of it gets deflected by powerful forces surrounding the void.
The research team measured magnetic field strengths between 40 and 90 Gauss in the regions producing infrared light, a range that helps explain both the constant flickering and the occasional bright flares. These magnetic fields extend across a region larger than our entire solar system, influencing how material moves near the black hole.
These findings may help us better understand galaxy evolution. Since nearly every large galaxy harbors a supermassive black hole at its center, studying Sgr A*’s behavior helps astronomers understand how these cosmic giants influence their host galaxies over billions of years.
JWST’s observations also demonstrate that black holes are far more dynamic than previously thought. Rather than existing in stable states punctuated by occasional outbursts, they appear to be constantly active at various intensities. Looking ahead, the research team has submitted a proposal to observe Sgr A* for an uninterrupted 24 hours.
“When you are looking at such weak flaring events, you have to compete with noise,” says Yusef-Zadeh. “If we can observe for 24 hours, then we can reduce the noise to see features that we were unable to see before. That would be amazing.”
The research team’s proposed 24-hour continuous observation marks the next step in understanding these mysterious cosmic objects. By capturing an uninterrupted day of Sagittarius A*’s activity, astronomers hope to determine whether its seemingly random variations actually follow subtle patterns. This knowledge could reshape our understanding of how galaxies evolve.
Paper Summary
Methodology
The research team used JWST’s NIRCam instrument to observe Sgr A* across seven different epochs between 2023 and 2024, totaling approximately 48 hours of observation time. They collected data simultaneously at 2.1 and 4.8 micron wavelengths. The team had to carefully account for nearby stellar sources that could contaminate their measurements. The observations were spaced across different epochs to capture both short-term variations and longer-term changes in the black hole’s behavior.
Results
The study revealed several major findings: (1) Sgr A* exhibits constant variability with no true quiescent state; (2) the black hole produces 5-6 major flares daily plus numerous smaller fluctuations; (3) changes in brightness at 4.8 microns lag behind those at 2.1 microns by 3-40 seconds; (4) two distinct types of brightness variations suggest different physical processes at work; and (5) magnetic field strengths in the emission region range from 40-90 Gauss. The team also found evidence for loop structures in the way the brightness changes relate to spectral characteristics.
Limitations
The research faced several technical challenges, including difficulties with telescope guidance in the crowded galactic center region and occasional data gaps due to observing constraints. Some observations were affected by nearby bright stars, requiring careful data processing to isolate Sgr A*’s emission. The team also notes that while 48 hours of total observation time represents a significant improvement over previous studies, even longer continuous observations would be beneficial for detecting potential patterns in the flaring activity.
Discussion and Takeaways
The findings suggest that black holes are more dynamically active than previously thought, with constant variability rather than distinct active and quiet periods. The time delays between different wavelengths provide new insights into the physical processes occurring near the event horizon. These results have implications for understanding how supermassive black holes influence galaxy evolution and interact with their surroundings.
Funding and Disclosures
This research was supported by NASA through the James Webb Space Telescope (JWST) Guest Observer programs 2235 and 3559. Additional support was provided by the National Science Foundation (NSF) and the Space Telescope Science Institute (STScI). The study also benefited from resources at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).
Publication Information
The study, “Non-stop variability of Sgr A* using JWST at 2.1 and 4.8 micron wavelengths: Evidence for distinct populations of faint and bright variable emission,” was published in The Astrophysical Journal Letters on February 18, 2025. The research represents collaborative work between researchers from Northwestern University, Space Telescope Science Institute, NASA/GSFC, Macquarie University, Harvard & Smithsonian, and the National Radio Astronomy Observatory.
