Sagittarius A*

• 1. Introduction

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  Sagittarius A* sits at the very heart of the Milky Way, a supermassive black hole anchoring an entire galaxy of hundreds of billions of stars. Its mass is roughly 4.3 million times that of our Sun, yet light cannot escape it. What we see instead is the glow of gas and dust, heated to millions of degrees as it spirals inward, broadcasting across the universe in radio waves and infrared. For decades, astronomers could not photograph it directly. Dust and gas between us and the Galactic Center impose 25 magnitudes of extinction, blocking any view in visible light. The questions this object raises cut to the heart of physics itself: how do we know something exists if we can only see what it does to everything around it? How did a black hole of such immense mass come to sit at the center of our own galaxy? And what happens to the stars, the gas clouds, even the light that wanders too close?

• 2. A Name With An Asterisk

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  Karl Jansky, considered one of the fathers of radio astronomy, picked up a faint radio signal in April 1933 coming from the direction of the constellation Sagittarius. He traced it toward the center of the Milky Way, though his observations did not quite reach far enough south to land on what we now call the Galactic Center. That took decades more of careful work. Jack Piddington and Harry Minnett, using the CSIRO radio telescope at Potts Hill Reservoir in Sydney, detected a discrete and bright source they called "Sagittarius-Scorpius". Further observations with the 80-foot CSIRO telescope at Dover Heights pinned it down as the probable Galactic Center in a letter to Nature. The object we now call Sagittarius A*, however, is a compact component nested inside the larger source. Bruce Balick and Robert L. Brown discovered it on the 13th and the 15th of February, 1974, using the baseline interferometer of the National Radio Astronomy Observatory. Brown coined the name Sgr A* in a 1982 paper, explaining that the source was "exciting", and in chemistry, excited atomic states are marked with asterisks. The playful logic behind that asterisk has stuck ever since.

• 3. The Stars That Told The Story

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  Orbiting so close to Sagittarius A* that their paths can be tracked year to year, a cluster of stars known collectively as the S stars became the most powerful tools astronomers had for measuring the black hole. On the 16th of October, 2002, an international team led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics published ten years of observations of the star S2. The data ruled out the possibility that Sgr A* contained a cluster of dark stellar objects or a mass of degenerate fermions. Using the Keplerian orbit of S2, the team determined that the object's mass reached 4.1 million solar masses, confined within a volume no larger than 17 light-hours in radius, or about 120 AU across. For comparison, the Schwarzschild radius of such a black hole is just 0.08 AU. A later observation of the star S14 tightened the radius constraint further, to no more than 6.25 light-hours, or 45 AU. The team also placed the Galactic Center at a distance of 8,000 plus or minus 600 parsecs from Earth, a figure important for calibrating astronomical distance scales across the cosmos. In July 2018, S2 was clocked at 7,650 kilometers per second during its close approach, reaching about 2.55% of the speed of light, and the gravitational redshift predicted by Einstein's general theory of relativity was detected in agreement with theory, to within the 10 percent measurement precision.

• 4. The 2022 Portrait

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  On the 12th of May, 2022, the Event Horizon Telescope Collaboration released the first image of Sagittarius A*, showing the glowing ring of its accretion disk. The Event Horizon Telescope is not a single instrument; it is a worldwide network of radio observatories working together, and for this project data were collected by eight observatories at six geographical sites. The underlying observations were made in 2017, but the image took five years of calculations to process. What makes Sgr A* unusually difficult to image is that its radio emission varies on the order of minutes. Stars like the Sun take hours for light to cross their diameters; the radio brightness of Sgr A* fluctuates in the time it takes to brew a pot of coffee. That variability complicated every stage of the analysis. The resulting image gives the source an angular size of 51.8 microarcseconds. At a distance of 26,000 light-years, that angular size translates to a physical diameter of 51.8 million kilometers, smaller than the distance from Earth to the Sun and only a little larger than Mercury's closest approach to the Sun. The 2022 image was the second confirmed image of a black hole, after the portrait of the supermassive black hole at the center of Messier 87 was released in 2019.

• 5. The Gas Cloud That Fizzled

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  First noticed in images of the Galactic Center taken in 2002, the gas cloud G2 drew intense attention when a 2012 paper published in Nature confirmed it was likely on a course taking it into the accretion zone of Sgr A. G2 carries a mass about three times that of Earth. Predictions placed its closest approach, called a perinigricon, in early 2014, at a distance of just over 3,000 times the radius of the event horizon, or roughly 260 AU from the black hole. Many astronomers expected a dramatic brightening as the cloud was shredded. Simulations were run by groups at ESO and Lawrence Livermore National Laboratory before the encounter. Observations were confirmed with Chandra, XMM, the VLA, INTEGRAL, Swift, and Fermi. Daryl Haggard described the anticipation as feeling "more like an experiment" than a typical observation. What actually happened was anticlimactic: nothing notable was observed during or after the closest approach. Observers from the UCLA Galactic Center Group, working with data from the 19th and the 20th of March, 2014, found that G2 was still intact. An analysis published on the 21st of July, 2014, based on Very Large Telescope data, proposed that G2 was not an isolated cloud but a dense clump within a continuous gas stream, with the earlier-passing cloud G1 sharing a nearly identical orbit. Andrea Ghez and colleagues proposed a different explanation entirely: that G2 is a pair of binary stars that had merged into an extremely large star, held together by its own gravity against the tidal forces of Sgr A.

• 6. Flares, Speeds, and a Runaway Star

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  On the 5th of January, 2015, NASA reported an X-ray flare from Sgr A* that was 400 times brighter than usual, a record at the time. Astronomers suggested it could have been caused by an asteroid breaking apart as it fell into the black hole, or by the entanglement of magnetic field lines within infalling gas. On the 13th of May, 2019, observers at the Keck Observatory watched the region around Sgr A become 75 times brighter than usual in a sudden brightening, possibly the result of the black hole encountering another object. In a paper published on the 31st of October, 2018, astronomers using the GRAVITY interferometer and the four telescopes of the Very Large Telescope created a virtual telescope 130 meters in diameter and detected clumps of gas moving at about 30% of the speed of light. Three bright flares matched theoretical predictions for hot spots orbiting close to a black hole of four million solar masses. A separate observation, reported in July 2019, found a star designated S5-HVS1 speeding through the constellation Grus, about 29,000 light-years from Earth, at 1,755 kilometers per second. That is 0.006 times the speed of light, and the star may have been flung outward after an encounter with Sagittarius A. Another record holder, S4714, has an orbital period of 12 years and an eccentricity of 0.985, which drives it within 12.6 AU of the black hole at speeds reaching about 8% of the speed of light.

• 7. A Nobel Prize at the Center

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  In 1994, infrared and sub-millimetre spectroscopy studies by a Berkeley team that included Nobel Laureate Charles H. Townes and a then-future Nobel Prize winner, Reinhard Genzel, showed that the mass of Sgr A* was tightly concentrated and on the order of 3 million solar masses. That early estimate pointed unmistakably toward a black hole. After monitoring stellar orbits for 16 years, Genzel's group published a mass estimate of 4.31 million solar masses in 2008, with the work appearing in The Astrophysical Journal in 2009. Genzel described it as "the best empirical evidence that supermassive black holes do really exist", saying that the stellar orbits in the Galactic Center showed that the central mass concentration of four million solar masses "must be a black hole, beyond any reasonable doubt." In 2020, the Nobel Prize in Physics recognized this work directly. Reinhard Genzel and Andrea Ghez each received a quarter share of the prize for their discovery that Sagittarius A* is a supermassive compact object for which a black hole was the only explanation. The remaining half went to Sir Roger Penrose for his proof that black hole formation is a robust prediction of general relativity. In November 2004, a separate discovery added another dimension to the picture: a potential intermediate-mass black hole of 1,300 solar masses, designated GCIRS 13E, was found orbiting just 3 light-years from Sgr A* within a cluster of seven stars, hinting at the mechanism by which supermassive black holes may grow over cosmic time.