Sign up for the Starts With a Bang newsletter
Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
Throughout the Universe, practically all galaxies house a supermassive black hole.
Messier 87, best known as the supermassive galaxy whose black hole was first imaged by the Event Horizon Telescope, has its relativistic jets and the shockwaves created by their material imaged in the infrared by Spitzer, amidst the mass of shining stars (in blue). Messier 87 is the most massive (and second-brightest) galaxy within the entire Virgo cluster of galaxies, and it is the central black hole that generates these relativistic jets.
Whenever actively feeding, they spew out energetic radiation.
This field of view, corresponding to about 1/15th of a square degree, shows the Chandra deep field south, and represents a total of around 2000 hours of total observing time. Approximately 5000 supermassive black holes were detected, with a small amount of hot gas around a few objects appearing as diffuse and extended, rather than point-like, sources.
This activity abounds at the Milky Way’s center, too.
The supermassive black hole at the center of our galaxy, Sagittarius A*, emits X-rays due to various physical processes. The flares we see in the X-ray indicate that matter flows unevenly and non-continuously onto the black hole, leading to the flares we observe over time. In X-rays, no event horizon is visible at these resolutions; the “light” is purely disk-like.
However, at only ~27,000 light-years distant, our black hole is more directly observable.
This 20-year time-lapse of stars near the center of our galaxy comes from the ESO, published in 2018. Note how the resolution and sensitivity of the features sharpen and improve toward the end, all orbiting our galaxy’s (invisible) central supermassive black hole. Practically every large galaxy, even at early times, is thought to house a supermassive black hole, but only the one at the center of the Milky Way is close enough to see the motions of individual stars around it, and to thereby accurately determine the black hole’s mass. Similar techniques could reveal intermediate mass black holes within globular clusters, albeit over longer timescales.
Individual stellar orbits can be traced, revealing its mass.
Size comparison of the two black holes imaged by the Event Horizon Telescope (EHT) Collaboration: M87*, at the heart of the galaxy Messier 87, and Sagittarius A* (Sgr A*), at the center of the Milky Way. Although Messier 87’s black hole is easier to image because of the slow time variation, as well as being intrinsically about 1500 times larger, the one around the center of the Milky Way is the largest as viewed from Earth in terms of angular size. Artificial neural networks were vital to analyzing and processing the data used to recover these images.
More recently, its event horizon has been imaged in radio light.
When multiple masses interact under their own mutual gravitation, the smaller masses tend to get larger kicks, where they get knocked to higher orbits or ejected entirely, often resulting in hypervelocity objects. Meanwhile, the remaining objects wind up even more tightly bound, gravitationally speaking. In the far future, our galaxy’s stellar remnants will almost all be ejected in this fashion, with only a few getting drawn into the central, supermassive black hole instead.
However, supermassive black holes also cause a telltale, indirect gravitational effect: creating runaway, hypervelocity stars.
The star Mira, as shown here as imaged by the GALEX observatory in the ultraviolet, speeds through the interstellar medium at speeds much greater than normal: at about 130 km/s, which is about ten times faster than typical speeds but is still below the escape velocity of the Milky Way. Mira’s trajectory cannot be traced back to the galactic center, indicating that some lesser gravitational interaction gave it its high-velocity kick. Many stars other than Mira, however, have achieved speeds sufficient to gravitationally unbind them from our home galaxy, with a significant fraction of those being traceable back to the galactic center.
Hypervelocity stars were first predicted in 1988: from binary star systems interacting with a supermassive black hole.
In a 2020 study using Gaia and LAMOST data combined, a whopping 591 new hypervelocity stars were discovered, bringing the total to more than 1000. When all the hypervelocity stars are examined, together, scientists find that 15% of them can have the origin of their trajectories traced back from near the galactic center, implying that they were once part of binary systems that interacted with the black hole, receiving ejection-worthy kicks. On the other hand, about 30% of them appear to have an extragalactic origin.
Credit: Xiao Kong/China’s National Astronomical Observatories
Many hypervelocity stars have been discovered, with star clusters, multi-star systems, and globular clusters also producing them.
One of the mechanisms for creating a hypervelocity runaway star is shown above: a many-body interaction in a dense stellar region like a globular cluster. As heavier mass objects sink to the center, lower-mass objects can be kicked out at enormous speeds, possibly even ejecting them from the galaxy entirely.
However, back-tracing those fast-moving stellar trajectories frequently leads to a supermassive black hole.
At the centers of galaxies, orbiting stars experience close interactions with the central supermassive black holes at a galaxy’s core. Stars that pass too close, particularly in multi-star systems, are at risk of receiving hypervelocity kicks and being ejected from the galaxy entirely. In recent years, other mechanisms have also been shown to create hypervelocity stars: perhaps even more abundantly than via receiving gravitational kicks from supermassive black holes at the centers of galaxies.
This technique has been applied within globular cluster Omega Centauri, (likely) revealing an intermediate-mass black hole.
The locations of fast-moving stars identified in the latest study of globular cluster Omega Centauri’s center, along with, on the right, a zoomed-in panel identifying the properties of the seven hypervelocity stars closest to the central region. This is extremely strong evidence for an intermediate mass black hole: a cosmic first.
What about the closest galaxy beyond the Milky Way: the Large Magellanic Cloud (LMC)?
The Large (top right) and Small (lower left) Magellanic Clouds are visible in the southern skies, and helped guide Magellan on his famous voyage some 500 years ago. In reality, the LMC is located some 160-165,000 light-years away, with the SMC slightly farther at 198,000 light-years. Both galaxies, along with Triangulum and Andromeda, make up the only extragalactic objects visible to the naked human eye.
Just 165,000 light-years away, it’s forming stars rapidly, but exhibits no black hole activity.
This is an image of the Large Magellanic Cloud (LMC), one of the nearest galaxies to our Milky Way, as viewed by ESA’s Gaia satellite using information from the mission’s second data release. This view has been compiled by mapping the total amount of radiation detected by Gaia in each pixel, combined with measurements of the radiation taken through different filters on the spacecraft to generate color information. Although spectacular, this galaxy exhibits no evidence for radiative activity in any known wavelength originating from its galactic center; there are no direct signatures of a supermassive black hole.
However, many stars in the Milky Way’s halo are traceable back to the LMC’s core.
This histogram sorts a large number of hypervelocity stars found in one corner of the Milky Way, and assesses (and sorts) them by velocity and likely origin. There are two distinct populations of hypervelocity stars found here: one originating from Sagittarius A* at the center of the Milky Way, and another pointing to an origin from the center of the Large Magellanic Cloud. This strongly suggests an origin from a supermassive black hole for that latter population of stars.
They reveal a central black hole of ~600,000 solar masses.
This illustration shows the origin of a hypervelocity star ejected from the Large Magellanic Cloud. When a binary star system ventures too close to a supermassive black hole, the intense gravitational forces tear the pair apart. One star is captured into a tight orbit around the black hole, while the other is flung outward at extreme velocities—often exceeding millions of miles per hour—becoming a hypervelocity star. This illustration depicts this process, with lines showing the past trajectories of the two relevantly affected stars.
These “stealthy” supermassive black holes could be everywhere.
If the Large Magellanic Cloud (LMC) possesses a large, massive black hole, that object should be responsible for launching many hypervelocity stars due to gravitational interactions with binary stellar systems. A significant population of those stars should be identifiable in the Milky Way’s halo, leading to a predicted overdensity of hypervelocity stars that’s dependent on the mass of the black hole. From this data, scientists have concluded the likely presence of a ~600,000 solar mass black hole at the center of the Large Magellanic Cloud: a black hole that’s 15% the mass of our own in a galaxy that’s just 1% of the Milky Way’s mass.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.
Sign up for the Starts With a Bang newsletter
Travel the universe with Dr. Ethan Siegel as he answers the biggest questions of all
