At the heart of our Milky Way galaxy lurks a supermassive black hole, more than four million times the mass of our Sun. Scientists with the international Event Horizon Telescope (EHT) collaboration have now produced the very first image of that supermassive black hole, showing that it has a ring structure. The collaboration made the announcement during a live-streamed press conference this morning from the European Southern Observatory’s headquarters in Munich, Germany, as well as numerous other simultaneous press conferences around the world. Six papers about the research have been published in a special issue of The Astronomical Journal Letters.
In 1933, physicist Karl Jansky noticed a radio signal coming from somewhere in the constellation Sagittarius, near the center of our Milky Way galaxy, which he dubbed Sagittarius A. Later research revealed that the source actually had several overlapping components, one of which (identified in 1974) was particularly bright and compact. It was named Sagittarius A* (pronounced A-star). It’s so named because (per co-discoverer Robert Brown) the radio source was “exciting” and in physics, the excited states of atoms are denoted with an asterisk. Physicists have been convinced since the 1980s that the central component of Sagittarius A*—and the source of all those radio emissions—was likely a supermassive black hole, similar to those thought to be at the center of most spiral and elliptical galaxies.
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’ John Timmer reported back 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 for facilities like ALMA (the Atacama Large Millimeter/submillimeter Array), 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 where supermassive black holes are very bright, ensured sufficient photons. The net result is a telescope that can do the equivalent of reading the year stamped on a coin in Los Angeles from New York City—assuming the coin was glowing at radio wavelengths.
The EHT announced the first direct image ever taken of a black hole at the center of an elliptical galaxy in 2019, located in the constellation of Virgo some 55 million light-years away: Messier 87 (M87). This image would have been impossible a mere generation ago, and it was made possible by technological breakthroughs, innovative new algorithms, and (of course) connecting several of the world’s best radio observatories. The image confirmed that the object at the center of M87 is indeed a black hole.
Last year, the EHT collaboration released a new image of M87* showing what the black hole looks like in polarized light—a signature of the magnetic fields at the object’s edge—which yielded fresh insight into how black holes gobble up matter and emit powerful jets from their cores. A few months later, the EHT was back with images of the “dark heart” of a radio galaxy known as Centaurus Aenabling collaboration to pinpoint the location of the supermassive black hole at the galaxy’s center.
After the successful imaging of M87*, scientists were hopeful that the EHT would soon produce a similar image of Sagittarius A*, which is much smaller, but also much closer, than M87. However, as theoretical physicist Matt Strassler wrote recently:
[T]he measurements of the Milky Way’s black hole proved somewhat more challenging, precisely because it is smaller. EHT takes about a day to gather the information needed for an image. M87’s black hole is so large that it takes days and weeks for it to change substantially — even light takes many days to cross from one side of the accretion disk to the other — so EHT’s image is like a short-exposure photo and the image of M87 is relatively clear. But the Milky Way’s galaxy’s black hole can change on the times scale of minutes and hours, so EHT is making a long-exposure image, somewhat like taking a 1-second exposure of a tree on a windy day. Things get blurred out, and it can be difficult to determine the true shape of what was captured in the image.
Physicists have other means of determining the mass of Sagittarius A*. For instance, UCLA astronomer Andrea Ghez shared the 2020 Nobel Prize in Physics for her work (building on that of co-Nobelist Reinhard Genzel of the University of California, Berkeley) mapping the orbits of stars closest to the center of our galaxy. This provided an indirect means of establishing that the object at its center is indeed a supermassive black hole. (No other known object can be so massive and densely packed.)
But per Strassler, we still don’t know how the Milky Way’s supermassive black hole is oriented or fast it is spinning. Those are the kinds of questions that the EHT collaboration hopes to answer. The EHT observations will enable physicists to directly study the gravitational effects near a black hole and the accretion and outflow dynamics of the matter orbiting the object. It should also yield new tests for general relativity and perhaps answer some nagging fundamental questions about the existence of the black hole’s point of no return: the event horizon.
The new image is based on the same 2017 data as the image of M87*; it just took much longer to process. While M87* was an easier, steadier target, with nearly all images looking the same, that was not the case for Sagittarius A*. The image is an average of the different images from observational data the team collected over the course of multiple days. It took five years, multiple supercomputer simulations, and the development of new computational imaging algorithms capable of making inferences to fill in the blanks in the data.
“The gas in the vicinity of the black holes moves at the same speed—nearly as fast as light—around both Sagittarius A* and M87*,” said Chi-kwan “CK” Chan of the University of Arizona. “But where gas takes days to weeks to orbit the larger M87*, in the much smaller Sagittarius A* it completes an orbit in mere minutes. This means the brightness and pattern of the gas around Sagittarius A* was changing rapidly as the EHT Collaboration was observing it—a bit like trying to take a clear picture of a puppy quickly chasing its tail.”
The final image shows a “glowing, donut-shaped ring of light” just outside the event horizon, encircling a dark center (the black hole’s “shadow”). It also reveals that Sagittarius A* is remarkably similar to M87*, even though our galaxy’s black hole is more than a thousand times smaller and less massive. “This tells us that general relativity governs these objects up close, and any differences we see farther away must be due to differences in the material that surrounds the black holes,” said Sera Markoffco-chair of the EHT Science Council and a physicist at the University of Amsterdam, the Netherlands.
“Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works,” said EHT scientist Keiichi Asada from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. “We have images for two black holes—one at the large end and one at the small end of supermassive black holes in the Universe—so we can go a lot further in testing how gravity behaves in these extreme environments than ever before.”
The next step is to make a movie of the black hole, showing it as it changes over time, which could yield insight into the way gas behaves as it swirls around a black hole, and would also help estimate the spin of the black hole itself .
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