How Sagittarius A* was found lurking in the center of the Milky Way

The supermassive black hole in our Milky Way weighs about 4.3 million solar masses, equal to 1.4 trillion Earths.
By | Published: February 13, 2024

Finding the Milky Way in the night sky is fairly easy. A short stroll to a dark, isolated location affords a ready snapshot of its splendor, as a small fraction of its 100 billion to 400 billion stars blaze overhead like a sprawling celestial stream.

But finding its mysterious heart, the galactic center, is tougher. A broad band of interstellar dust (the Great Rift) blocks our view, forcing observers to utilise radio astronomy to explore it.

Fifty years ago, two young astronomers seeking evidence of starbirth in this radiation-blasted hinterland stumbled upon the singular colossal object that lurks at this barycenter of our everything: a supermassive black hole.

The exoticism of black holes — collapsed patches of space-time so tightly compressed that not even light can escape their gravitational grip — had by 1974 been mathematically predicted and theoretically discussed for decades (not to mention famously proposed then dismissed by Albert Einstein). Supermassive black holes, among the biggest known, are now thought to occupy most galactic nuclei, ranging from hundreds of thousands to billions of times more massive than the Sun.

The one at our Milky Way’s core clocks in at about 4.3 million solar masses, equal to 1.4 trillion Earths, its unseen gaping maw as wide as the orbit of Mercury at perhaps 27 million miles (44 million kilometers). Viewed from our vantage point in one of the Milky Way’s ragged spiral arms and 26,000 light-years from the galactic center, it lies near the borders of the constellations Sagittarius and Scorpius, deep in our southern sky and visually close to the Butterfly Cluster.

For centuries, astronomers pondered what was at the galactic center. Early ideas posited the presence of a large star — maybe Sirius, mused 18th-century philosopher Immanuel Kant. But in 1933, physicist Karl Jansky detected strong radio emissions from the region.

The advantages of multiple telescopes

Twenty years later, scientists identified a discrete, bright radio source known as Sagittarius-Scorpius, broadly coincident with the center. By 1954, it acquired the name Sagittarius A, following a convention of identifying celestial objects by constellation then adding capital letters to denote increasing levels of magnitude. In 1958, the International Astronomical Union accepted Sagittarius A as the true-zero coordinate point of galactic latitude and longitude.

By the 1960s, astronomers were increasingly eying long-baseline interferometry — using multiple telescopes spread across wide geographical areas to yield the observing power of a single detector — as a tool to scrutinize the galactic center.

A key player in this emerging arena of inquiry was the National Radio Astronomy Observatory (NRAO) site at Green Bank in West Virginia. Its three telescopes, each spanning 85 feet (26 meters), were teamed with a 45-foot (14 m) mountaintop telescope 22 miles (35 km) away in Huntersville, creating a tenfold increase in resolution.

In June 1972, astronomers Dennis Downes and Miller Goss, then working at West Germany’s Max Planck Institute, submitted a proposal to the NRAO to observe the galactic center. They cited a growing interest in quasars and active galactic nuclei, stressing the “paramount importance to pursue investigations of compact structure in Sagittarius A” and adding that interferometry and the center’s relative closeness meant “we can observe details on a much finer linear scale than is possible in external galaxies.”

But trouble obtaining travel funds prevented them from visiting the U.S. They subsequently took up new academic posts, and the urgency at NRAO to fulfill their proposal diminished. In October 1973, MIT graduate student Fred Lo observed “just a hint” of the galactic center, but stopped short of publication. “As a student, I was very cautious,” said Lo in a 2016 interview. “I never published the result.”

An image of the area surrounding Sagittarius A*, the supermassive black hole at the center of the Milky Way galaxy, in X-ray and infrared light. Credit: X-ray: NASA/UMass/D.Wang et al.; IR: NASA/STScI.
An image of the area surrounding Sagittarius A*, the supermassive black hole at the center of the Milky Way galaxy, in X-ray and infrared light. Credit: X-ray: NASA/UMass/D.Wang et al.; IR: NASA/STScI.

An intense signal

Elsewhere, astronomers Bruce Balick, later of the University of Washington, and Robert Brown of NRAO tendered their own bid to investigate star-forming regions near the Milky Way’s heart. Early in 1974, they observed Sagittarius B2, a giant molecular cloud 390 light-years from the galactic center, but discerned no bright stars.

Then – “for the hell of it,” Balick later recounted in 2022 interview with Mashable – on Feb. 13-15 they turned the Green Bank instruments onto the galactic center itself. They detected a sub-arcsecond source that exhibited an intense radio signal, much more powerful than expected.

It was a thrilling moment, said Balick: peering across 26,000 light-years to spy the glow of the Milky Way’s pulsating heart as it appeared when Neanderthals walked on Earth. The men notified David Hogg, assistant director for Green Bank operations, who had responsibility for timelining the NRAO’s observing schedule. Hogg realised that Balick and Brown had used a configuration and radio wavelength that Downes and Goss also desired to pursue.

“Two groups unknowingly competed to make the detection observations, both based on very different scientific rationales,” Balick wrote in a chapter on the discovery for 2005’s The New Astronomy: Opening the Electromagnetic Window and Expanding Our View of Planet Earth. “The other group … correctly anticipated the astronomical significance of the discovery but could not make their way from Europe to Green Bank when their observations were scheduled in the fall of 1973.”

Aware that conflicts in the groups’ research interests might ignite controversy and competition, Hogg asked Balick and Brown to wait for him to communicate with Downes and Goss. Perhaps, he suggested, they might work together on a future paper.

Over the next three months, Balick moved to California while Brown remained in West Virginia, but the pair corresponded by mail. In one exchange, evidently anxious that other astronomers (including Lo) were snapping at their heels, Balick told Brown: “We’d better publish fast if we want to beat him into print.”

Their six-page paper, “Intense Sub-Arcsecond Structure in the Galactic Center,” graced the Astrophysical Journal in December 1974. “The unusual nature of the sub-arcsecond structure and its positioned coincidence with the inner one-percent core of the galactic nucleus,” they wrote, “strongly suggests that this structure is associated with the galactic center.”

Left unsaid was its potential black-hole candidacy. Instead, Balick and Brown described it more benignly as a “structure” that “defines the galactic center.” Physicists Donald Lynden-Bell and Martin Rees had earlier argued that the Milky Way and other galaxies might host black holes, but such ideas remained radical, and the fear of professional ridicule rarely saw them committed to publication.

The object’s nature, Balick and Brown concluded with well-placed caution, “cannot be established until observations are substantially improved.” Both were 30-something, newly minted Ph.D.s and neither would risk their academic futures by embarking on outlandish claims. “If this turned out not to be a black hole, my career would tank,” Balick said in the 2022 interview. “There was a political decision being made here, not a scientific one, on what we should say.”

For his part, Goss showed no animosity at an opportunity lost. “We might have done it,” he reflected. “But they were ahead of us slightly, which was fair enough,” he said in the same Mashable article. In Goss’ mind, it was exciting that the discovery had been made.

But that discovery remained nameless for almost a decade. One early suggestion was the Galactic Center Compact Radio Source (GCCRS), a clumsy tongue-twister that never caught on. Then, one morning in 1982, Brown thought back to his doctoral dissertation in atomic physics. Highly excited energy states tended to be marked with an asterisk. And his and Balick’s find was nothing if not exciting.

“Scratching on a yellow pad one morning, I tried a lot of possible names,” said Brown, who died in 2014. “When I began thinking of the radio source as the ‘exciting’ source…the name Sagittarius A* occurred to me by analogy brought to mind by my Ph.D. dissertation, which is in atomic physics.” Pronounced “Sagittarius-A-star” and abbreviated as Sgr A*, the new nomenclature stuck.

But its nature as a black hole took another quarter-century to prove. Infrared and submillimeter observations in the 1990s confirmed its highly concentrated mass. Measurements of stars orbiting near Sgr A* ruled out other options, strengthening the argument beyond all reasonable doubt in favor of its black-hole candidacy.

By 2008, one of these so-called “S-stars” (known as S2) had been observed through an entire 16-year orbit, following a highly elliptical path that carries it as close as 120 astronomical units from Sgr A*, four times as far as Neptune lies from the Sun. (One astronomical unit, or AU, is equal to the average Earth-Sun distance of 93 million miles or 150 million kilometers.) S2’s orbital dynamics enabled better mass and radius calculations and tightened estimates of the distance between Earth and the galactic center.

Since 2020, star S4714 has been shown to orbit even closer, passing within 12.6 AU of Sgr A*, roughly equal to the distance between Saturn and the Sun. Whipping around the black hole at 53.5 million mph (86 million km/h) — about 8 percent of the speed of light — it takes 9.9 years to circle Sagittarius A*.

But the pivotal moment came in May 2022, when the first picture of Sgr A* was published in Astrophysical Journal Letters, the second time (after M87 in 2019) that a black hole had been directly imaged. Using data from the Event Horizon Telescope very-long-baseline array of worldwide radio observatories, the image revealed the fuzzy reddish-orange accretion disk encircling a central blob of the most ethereal blackness; truly an object from which light could never escape.

Balick, then approaching his 80th year, was astounded but what he saw did not surprise him. “If it looked much different,” he told Mashable, “then general relativity would need a major makeover.” But the joy of seeing an object he helped discover a half-century earlier was tinged with sadness – Balick dearly wished that Brown had also been alive to see it.