The black holes at the center of the Milky Way (Earth’s home galaxy) and Andromeda (one of our closest galactic neighbors) are among the quietest eaters in the universe. What little light they emit varies subtly in brightness, suggesting that they consume a small but steady stream of matter rather than large clumps. The streams approach the black hole gradually and in a spiral, like water swirling down a drain.
There’s a massive new black hole in the Milky Way, and it’s lurking close to Earth. This sleeping giant was discovered using the European space telescope Gaia, which tracks the movements of billions of stars in our galaxy. The stellar-mass black hole, designated Gaia-BH3, is 33 times more massive than our Sun. The previous most massive black hole of this class discovered in the Milky Way was the X-ray binary Cyg X-1, which is estimated to be about 20 times the mass of the Sun. The average stellar-mass black hole in the Milky Way is about 10 times more massive than the Sun.
Gaia-BH3 is just 2,000 light-years from Earth, making it the second-closest black hole ever discovered to our planet. The closest black hole to Earth is Gaia-BH1 (also discovered by Gaia), which is 1,560 light-years away. Gaia-BH1 has a mass about 9.6 times that of the Sun, making it significantly smaller than this newly discovered black hole, the journal Astronomy & Astrophysics reports.
Three stellar-mass black holes in our galaxy: (left) Gaia BH1, (centre) Cygnus X-1 and (right) Gaia BH3, which have masses of 10, 21 and 33 times that of the Sun, respectively. Gaia BH3 is the most massive stellar-mass black hole discovered so far in the Milky Way. ESO/M. Kornmesser
Of course, Gaia-BH3 is small potatoes compared to the supermassive black hole that dominates the heart of the Milky Way, Sagittarius A* (Sgr A*), which has a mass 4.2 million times that of the Sun. Supermassive black holes like Sgr A* are not created by the deaths of massive stars, but rather by the mergers of increasingly larger black holes.
All black holes are marked by an outer boundary called the event horizon, where the escape velocity of the black hole exceeds the speed of light. This means that the event horizon is a one-sided surface that traps light, beyond which no information can escape.
The region around the black hole Gaia-BH3. ESO/Digitized Sky Survey
As a result, black holes do not emit or reflect light, meaning they can only be “seen” when they are surrounded by material that they gradually feed on. Sometimes this means a black hole in a binary system that is sucking material from a companion star, forming a disk of gas and dust around it.
The enormous gravitational influence of black holes generates intense tidal forces in this surrounding matter, causing it to glow brightly as material is destroyed and consumed, also emitting X-rays. In addition, material that the black hole does not consume can be directed toward its poles and ejected in jets at speeds close to the speed of light, accompanied by the emission of light.
All that light could allow astronomers to detect black holes. The question is, how can we detect dormant black holes that aren’t feeding on the gas and dust around them? For example, what if a stellar-mass black hole has a companion star, but they’re too far apart for the black hole to snatch stellar matter from its binary partner?
A diagram showing the locations of the three black holes detected by Gaia. ESA/Gaia Collaboration
In such cases, the black hole and its companion star orbit a point that represents the center of mass of the system. This is also the case when a light companion, such as another star or even a planet, orbits the star.
The rotation of the center of mass causes a wobble in the star’s motion that astronomers see. Because Gaia has a long history of accurately measuring the motion of stars, it is the ideal tool for observing this wobble.
The Gaia black hole team began searching for strange oscillations that could not be explained by the presence of another star or planet and that would indicate the presence of a heavier companion, possibly a black hole.
While aiming at an old giant star in the constellation Aquila, located 1,926 light-years from Earth, the team detected a wobble in the star’s path. This wobble suggests that the star is locked in orbit with a dormant black hole of exceptionally high mass. They are separated by a distance that ranges from the distance between the Sun and Neptune at their widest, to our star and Jupiter at their closest. Thanks to Gaia’s sensitivity, the black hole task force was also able to constrain the mass of Gaia-BH3, determining that it has 33 solar masses.
Artist’s impression of the system containing the most massive stellar black hole in our galaxy. ESO/L. Calçada
“Gaia-BH3 is the very first black hole whose mass we have been able to measure so precisely,” said Zevi Mazeh, a scientist and member of the Gaia collaboration at Tel Aviv University. “At 30 times the mass of our Sun, the object’s mass is typical of the estimates we have for the masses of very distant black holes observed in gravitational wave experiments. Gaia’s measurements provide the first definitive proof that [stellar-mass] black holes of such large mass actually exist.”
However, the Gaia-BH3 system will likely be of great interest to scientists not only because of its proximity to Earth and the mass of its black hole.
The star in this system is a subgiant, about five times larger than the Sun and 15 times more luminous, though it is cooler and less dense than our star. The companion star, Gaia-BH3, is made mostly of hydrogen and helium, the two lightest elements in the universe, and lacks the heavier elements that astronomers (somewhat confusingly) call “metals.”
The fact that this star is “metal-poor” suggests that the star that collapsed and died to create Gaia-BH3 also lacked heavier elements. Metal-poor stars are expected to lose more mass than their metal-rich counterparts over the course of their lives, so scientists wonder if they can maintain enough mass to give birth to black holes. Gaia-BH3 represents the first hint that metal-poor stars can indeed do this.
“The next release of Gaia data is expected to contain much more information, which should help us ‘see’ more of the ‘matrix’ and understand how dormant stellar black holes form,” Seabrok concluded.
The black holes at the center of the Milky Way (Earth’s home galaxy) and Andromeda (one of our closest galactic neighbors) are among the quietest eaters in the universe. What little light they emit varies subtly in brightness, suggesting that they consume a small but steady stream of matter rather than large clumps. The streams approach the black hole gradually and in a spiral, like water swirling down a drain.
Using computer models, the authors simulated how gas and dust near Andromeda’s supermassive black hole might behave over time. The simulations showed that a small disk of hot gas could form near the supermassive black hole and continuously feed it. The disk could be replenished and maintained by multiple streams of gas and dust.
However, the researchers also found that these streams must remain within a certain size and flow speed; otherwise, matter will fall into the black hole in irregular clumps, causing even larger light fluctuations.
When the authors compared their findings with data from Spitzer and NASA’s Hubble Space Telescope, they found spirals of dust previously identified by Spitzer that matched these constraints. From this, the authors concluded that the spirals feed Andromeda’s supermassive black hole.
This close-up view of the center of the Andromeda galaxy taken by NASA’s former Spitzer Space Telescope is marked with blue dotted lines highlighting the path of two streams of dust flowing toward the supermassive black hole at the galaxy’s center (shown as a purple dot). NASA/JPL-Caltech
“This is a great example of how scientists are revisiting archival data to learn more about galaxy dynamics by comparing it with the latest computer simulations,” said Almudena Prieto, an astrophysicist at the Instituto de Astrofísica de Canarias and the University of Munich Observatory and a co-author of a study published this year. “We have data from 20 years ago that tells us things we didn’t recognize when we first collected it.”
Launched in 2003 and operated by NASA’s Jet Propulsion Laboratory, Spitzer studies the universe in infrared light, invisible to the human eye. Different wavelengths reveal different features of Andromeda, including hotter sources of light like stars and cooler sources like dust.
By separating these wavelengths and looking only at the dust, astronomers can see the galaxy’s “skeleton” — places where gas has clumps and cools, sometimes forming dust, setting the stage for stars to form. This look at Andromeda revealed a few surprises. For example, although it’s a spiral galaxy like the Milky Way, Andromeda is dominated by a large ring of dust, rather than distinct arms surrounding its center. The images also showed a secondary hole in one part of the ring through which a dwarf galaxy has passed.
Andromeda’s proximity to the Milky Way means that it appears larger than other galaxies from Earth: With the naked eye, Andromeda would be about six times wider than the Moon (about 3 degrees). Even with a wider field of view than Hubble, Spitzer had to take 11,000 images to create a complete picture of Andromeda.
JPL managed the Spitzer Space Telescope mission for NASA’s Science Mission Directorate in Washington until the mission was shut down in January 2020. Science operations were conducted at the Spitzer Science Center at Caltech. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado. Data are archived in the Infrared Science Archive, which is managed by IPAC at Caltech. Caltech manages JPL for NASA.
A new study of a rare and short-lived type of galaxy has found that some objects harbor dormant supermassive black holes that briefly awaken to rip apart a massive star and devour its remains, turning them into a giant cosmic breakfast.
“Compact symmetric objects,” or CSOs, are active galaxies that shoot out two jets at speeds close to the speed of light. These jets are typical of active galactic nuclei (AGNs), which have supermassive black holes at their centers that feed on surrounding gas and dust — but CSO jets are different, The Astrophysical Journal reports.
This radio image shows two jets ejected from the center of Cygnus A, a galaxy not far from our own. A new paper reports the discovery of a similar object in a much more distant, ancient galaxy. That galaxy has a bright, relativistic jet emanating from its central supermassive black hole pointed toward Earth, making it a blazar. NRAO
While jets from active galactic nuclei can extend 230,000 light-years in both directions, CSO jets are small, extending only 1,500 light-years or so.
Scientists previously assumed that CSO jets are short because they are newly formed or young. Now, a team led by scientists at the California Institute of Technology (Caltech) has determined that these jets simply have short lifespans.
“These CSOs are not young. You wouldn’t call a 12-year-old dog young, even if it has lived less than an adult human,” study leader Anthony Readhead, a professor emeritus of astronomy at the California Institute of Technology, said in a statement. “These objects are a distinct species that live and die over thousands of years, not millions of years, as is typical in galaxies with larger jets.”
A VLBA image of two supermassive black holes, one of which is the compact symmetric object (CSO) J0405+3803a, consuming a star. HL Maness/Grinnell College
To unravel the mystery of CSOs and reveal their true nature, Reedhead and his colleagues spent two years studying 3,000 CSO candidates in previous literature and astronomical data obtained with the Very Long Baseline Array (VLBA) and other high-resolution radio telescopes.
“VLBA observations are the most detailed in astronomy, providing images with detail equivalent to measuring the width of a human hair at a distance of 100 miles [160 kilometers],” Reedhead said.
The team confirmed that 64 of these candidates are CSOs, and also discovered 15 more of these rare galaxies. By analyzing these CSOs, the team concluded that these rare types of galaxies emit jets for only 5,000 years or less and then fade away.
Artist’s impression of a star (foreground) being destroyed as it passes close to a supermassive black hole. ESO/M. Kornmesser
“The CSO jets are very energetic jets, but they seem to be shutting down,” said team member Vikram Ravi, an associate professor at Caltech. “The jets stop flowing out from the source.”
The team has identified a suspect behind the jets: they suggest that CSOs are driven by supermassive black holes tearing apart stars that get too close to them in so-called tidal disruption events (TDEs).
When stars get too close to a black hole, its enormous gravity creates powerful tidal forces inside the star’s body. These tidal forces stretch the star vertically while squeezing it horizontally, a process called “spaghetification.”
A supermassive black hole rips apart and devours a star. Inset: This image from the Very Long Baseline Array shows two supermassive black holes at the centre of galaxies, with the one on the right having just devoured a star. ESA/C. Carreau. Inset: HL MANESS/GRINNELL COLLEGE
These stellar noodles spin around to form a disk of matter that is gradually being devoured by a supermassive black hole. But black holes are messy eaters, and some of this stellar matter is directed toward the poles of these cosmic monsters. From there, some of the material is ejected in jets. This TDE process is accompanied by incredibly bright emissions of light that tell astronomers about the supermassive black holes that feed them.
“We think that one star is torn apart, and then all that energy is funneled into jets along the axis around which the black hole is spinning,” Reedhead explained. “The giant black hole is invisible to us at first, and then when it swallows the star, bam! The black hole has fuel, and we can see it.”
However, not just any star can be the messy cosmic meal that awakens a black hole as a CSO. The team believes that CSOs are only created when a truly massive star is torn apart by a supermassive black hole in a TDE.
“The TDEs we’ve seen so far have only lasted a few years,” Ravi explained. “We think the remarkable TDEs that power CSOs live much longer because the stars that are disrupted are very large, very massive, or both.”
Reedhead and his colleagues were also able to create a “cosmic family album” showing how CSOs and their jets evolve over time. Younger CSOs have shorter jets that are closer to the central supermassive black hole, while older CSOs have longer jets that extend farther from the TDE site.
The team determined that while the vast majority of CSOs will die out, 1% of them will go on to have long-lasting events with extended jets similar to those seen from Cygnus A, a distant supermassive black hole with jets directed toward Earth (a class of objects called blazars).
The researchers suggest that in 1 in 100 long-lived events, the central black hole is fed by additional gas and dust that results from the merger of its parent galaxy with another.
For Reedhead, these results support a theory he first put forward in the 1990s, when only three CSOs were discovered. The idea went largely unrecognized by the broader scientific community when it was first proposed, but should gain traction with this new evidence.
“This hypothesis was almost forgotten because it took years for observational evidence to start accumulating in favor of TDEs,” Reedhead said. “These objects really are a distinct population with their own origins, and now we have to learn more about them and how they came to be.”
“The ability to study these objects on time scales of years to decades, rather than millions of years, has opened the door to a completely new laboratory for studying supermassive black holes and the many unexpected and unpredictable surprises they hold.”