Enlarge / The star's orbit, shown here in light, is influenced by the far more massive black hole, indicated by the red orbit. (credit: ESO/L. Calçada )

As far as black holes go, there are two categories: supermassive ones that live at the center of the galaxies (and we're unsure about how they got there) and stellar mass ones that formed through the supernovae that end the lives of massive stars.

Prior to the advent of gravitational wave detectors, the heaviest stellar-mass black hole we knew about was only a bit more than a dozen times the mass of the Sun. And this makes sense, given that the violence of the supernova explosions that form these black holes ensures that only a fraction of the dying star's mass gets transferred into its dark offspring. But then the gravitational wave data started flowing in, and we discovered there were lots of heavier black holes, with masses dozens of times that of the Sun. But we could only find them when they smacked into another black hole.

Now, thanks to the Gaia mission , we have observational evidence of the largest black hole in the Milky Way outside of the supermassive one, with a mass 33 times that of the Sun. And, in galactic terms, it's right next door at about 2,000 light-years distant, meaning it will be relatively easy to learn more.

Enlarge / Scientists have found a large black hole that “hiccups,” giving off plumes of gas. (credit: Jose-Luis Olivares, MIT)

In December 2020, astronomers spotted an unusual burst of light in a galaxy roughly 848 million light-years away—a region with a supermassive black hole at the center that had been largely quiet until then. The energy of the burst mysteriously dipped about every 8.5 days before the black hole settled back down, akin to having a case of celestial hiccups.

Now scientists think they've figured out the reason for this unusual behavior. The supermassive black hole is orbited by a smaller black hole that periodically punches through the larger object's accretion disk during its travels, releasing a plume of gas. This suggests that black hole accretion disks might not be as uniform as astronomers thought, according to a new paper published in the journal Science Advances.

Co-author Dheeraj "DJ" Pasham of MIT's Kavli Institute for Astrophysics and Space research noticed the community alert that went out after the All Sky Automated Survey for SuperNovae (ASAS-SN) detected the flare, dubbed ASASSN-20qc. He was intrigued and still had some allotted time on the X-ray telescope, called NICER (the Neutron star Interior Composition Explorer) on board the International Space Station. He directed the telescope to the galaxy of interest and gathered about four months of data, after which the flare faded.

Enlarge / A new image from the Event Horizon Telescope has revealed powerful magnetic fields spiraling from the edge of a supermassive black hole at the center of the Milky Way, Sagittarius A*. (credit: EHT Collaboration)

Physicists have been confident since the1980s that there is a supermassive black hole at the center of the Milky Way galaxy, similar to those thought to be at the center of most spiral and elliptical galaxies. It's since been dubbed Sagittarius A* (pronounced A-star), or SgrA* for short. The Event Horizon Telescope (EHT) captured the first image of SgrA* two years ago. Now the collaboration has revealed a new polarized image (above) showcasing the black hole's swirling magnetic fields. The technical details appear in two new papers published in The Astrophysical Journal Letters. The new image is strikingly similar to another EHT image of a larger supermassive black hole, M87*, so this might be something that all such black holes share.

The only way to "see" a black hole is to image the shadow created by light as it bends in response to the object's powerful gravitational field. As Ars Science Editor John Timmer reported in 2019, the EHT isn't a telescope in the traditional sense. Instead, it's a collection of telescopes scattered around the globe. The EHT is created by interferometry, which uses light in the microwave regime of the electromagnetic spectrum captured at different locations. These recorded images are combined and processed to build an image with a resolution similar to that of a telescope the size of the most distant locations. Interferometry has been used at facilities like ALMA (the Atacama Large Millimeter/submillimeter Array) in northern Chile, where telescopes can be spread across 16 km of desert.

In theory, there's no upper limit on the size of the array, but to determine which photons originated simultaneously at the source, you need very precise location and timing information on each of the sites. And you still have to gather sufficient photons to see anything at all. So atomic clocks were installed at many of the locations, and exact GPS measurements were built up over time. For the EHT, the large collecting area of ALMA—combined with choosing a wavelength in which supermassive black holes are very bright—ensured sufficient photons.

Enlarge (credit: NRAO/AUI/NSF/NASA )

Virtually anything in space could be a potential meal for a supermassive black hole , and that includes entire stars. Even stars much bigger than our Sun can fall victim to the black hole’s extreme gravity and be pulled in toward its gaping maw. It is a terrifying phenomenon, but how often does it really happen?

Tidal disruption events (TDEs)—when the tidal forces of a black hole overwhelm a star’s gravity and tear it apart—are thought to occur once every 10,000 to 100,000 years in any given galaxy. TDEs can be detected by the immense amounts of energy they give off. While observations of them are still pretty rare, an international team of researchers has now discovered a whopping 18 of them that previous searches had missed. Why?

Many TDEs can be found in dusty galaxies. Dust obscures many wavelengths of radiation, from optical to X-rays, but long infrared wavelengths are much less susceptible to scattering and absorption. When the team checked galaxies in the infrared, they found 18 TDEs that had eluded astronomers before.

Enlarge / Inset shows the JWST image of the galaxy in infrared, along with the X-rays from the black hole seen by the Chandra. While the X-ray source is far smaller than the galaxy, X-rays are much harder to remove. (credit: X-ray: NASA/CXC/SAO/Ákos Bogdán; Infrared: NASA/ESA/CSA/STScI; Image Processing: NASA/CXC/SAO/L. Frattare & K. Arcand )

Researchers combing through some of the earliest galaxies in the Universe have found one that appears to have an actively feeding central black hole. Based on the amount of radiation it's emitting, the researchers estimate that it accounts for roughly half of the mass of the entire galaxy it's in—an astonishingly high fraction compared to modern galaxies.

The fact that such a large object can exist only half a billion years after the Big Bang places severe limits on how it could possibly have formed, strongly suggesting that supermassive black holes formed without ever having gone through an intermediate step involving a star.

Old X-rays

The earliest galaxies in the Universe that we know about have been identified using the James Webb Space Telescope, which took advantage of a galaxy cluster in the foreground that magnified more distant ones through gravitational lensing. Using the lens provided by a specific cluster, the Webb identified 11 galaxies that were imaged as they existed less than a billion years after the Big Bang.

Enlarge / This is the first image of Sgr A*, the supermassive black hole at the center of our galaxy. It’s the first direct visual evidence of the presence of this black hole. It was captured by the Event Horizon Telescope (EHT). (credit: EHT Collaboration)

It's probably not realistic to call a supermassive black hole "quiet." But, as far as these things go, the one at the center of our galaxy is pretty quiet. Yes, it emits enough energy that we can image it , and it occasionally gets a bit more active as it rips something nearby to shreds. But supermassive black holes in other galaxies power some of the brightest phenomena in the Universe. The object at the center of the Milky Way, Sgr A ^* , is nothing like those; instead, people get excited about the mere prospect that it might wake from its apparent slumber.

There's a chance that it was more active in the past, but any light from earlier events swept past Earth before we had observatories to see it. Now, however, scientists are suggesting they've seen echoes of light that might be associated with an Sgr A ^* outburst that took place about 200 years ago.

Looking for echoes

Audible echoes are simply the product of sound waves reflected off some surface. Light travels as a wave, as well, and it can reflect off objects. So, the basic idea of light echoes is a pretty straightforward extrapolation of these ideas. They may seem counterintuitive because, unlike sonic echoes, we never experience light echoes in normal life—light travels so fast that any echoes from the world around us arrive at the same time as the light itself. It all gets indistinguishable.

Astronomers today unveiled new images of the black hole at the center of the M87 galaxy, showing both a fluffier version of the black hole's glowing ring and its powerful jet together in the same image for the first time. The Event Horizon Telescope (EHT) first imaged the black hole in 2017; this new image is based on data collected by the Global Millimeter VLBI Array (GMVA), which captured radio emissions in a slightly different but scientifically significant wavelength. The details of the new observational data, image processing methods, and associated computer simulations are described in a new paper published in the journal Nature.

“This is the first image where we are able to pin down where the ring is relative to the powerful jet escaping out of the central black hole,” said co-author Kazunori Akiyama of MIT’s Haystack Observatory, who developed the imaging software used to visualize the black hole. “Now we can start to address questions such as how particles are accelerated and heated and many other mysteries around the black hole more deeply.”

As we've reported previously , the EHT is actually a collection of telescopes scattered around the globe, including hardware from Hawaii to Europe and from the South Pole to Greenland. The "telescope" is created by a process called interferometry, which uses light captured at different locations to build an image with a resolution that is the equivalent of a giant telescope (a telescope so big, it’s as if it were as large as the distance between the most distant locations of the individual telescopes).