Sagittarius A*
In 1974, radio astronomers Barry Balick and Robert L. Brown identified a bright, compact source at the center of our galaxy using the National Radio Astronomy Observatory's baseline interferometer. They published their findings on February 13 and 15 that year, marking the first clear detection of what we now call Sagittarius A*. The object sat near the border of the constellations Sagittarius and Scorpius, about 5.6 degrees south of the ecliptic. It appeared visually close to the Butterfly Cluster and Lambda Scorpii in the night sky.
Brown assigned the asterisk designation in 1982 after realizing this specific component was distinct from the larger, brighter radio source known as Sagittarius A. He chose the symbol because the emission was "exciting," echoing how excited states of atoms are denoted with asterisks in physics texts. Earlier work by Karl Jansky in April 1933 had detected radio signals coming from the direction of Sagittarius, but his observations did not extend far enough south to pinpoint the true Galactic Center. Jack Piddington and Harry Minnett later used the CSIRO radio telescope at Potts Hill Reservoir in Sydney to discover a discrete "Sagittarius-Scorpius" source, which they identified as the probable center of the Milky Way.
On the 16th of October 2002, an international team led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics reported observing the motion of star S2 near Sagittarius A* over ten years. Their data ruled out the possibility that the central object contained a cluster of dark stellar objects or degenerate fermions, strengthening evidence for a massive black hole. The team used near-infrared interferometry in the Ks-band, which is 2.1 micrometers, to reduce interference from interstellar dust.
By examining the Keplerian orbit of S2, researchers determined the mass of Sagittarius A* to be approximately four million solar masses confined within a radius no larger than 17 light-hours. Later observations of star S14 showed the mass was about 4.1 million solar masses within a volume with a radius no greater than 6.25 light-hours. Star S175 passed within a similar distance. For comparison, the Schwarzschild radius of this object is roughly 10 microarcseconds. In November 2004, astronomers discovered GCIRS 13E, a potential intermediate-mass black hole of 1,300 solar masses orbiting three light-years away.
On the 12th of May 2022, the Event Horizon Telescope Collaboration released the first image of Sagittarius A*, based on radio interferometer data collected in 2017. This marked only the second confirmed image of any black hole, following Messier 87's supermassive black hole in 2019. The process required five years of calculations to transform raw data into a coherent picture. Eight radio observatories across six geographical sites contributed to the dataset.
The resulting image revealed an overall angular size of approximately 50 microarcseconds. At Earth's distance from the Galactic Center, this translates to a diameter of about 52 microarcseconds. Radio emissions from Sagittarius A* vary on timescales of minutes, complicating analysis efforts. The observed ring structure matches theoretical predictions for hot spots orbiting close to a four-million-solar-mass black hole. Emission originates from gas and dust heated to millions of degrees as they fall toward the event horizon.
In July 2018, astronomers recorded star S2 traveling at 30% of the speed of light during its pericenter approach to Sagittarius A*. The closest point occurred in May 2018 at roughly 1,400 Schwarzschild radii from the black hole. Einstein's theory predicted that S2 would show gravitational redshift alongside velocity redshift due to proximity. Observations confirmed the effect within 10 percent measurement precision, aligning perfectly with general relativity expectations.
A paper published the 31st of October 2018, used the GRAVITY interferometer and four telescopes of the Very Large Telescope to create a virtual telescope 130 meters wide. This setup detected clumps of gas moving at 30% light speed near the black hole. Three prominent bright flares appeared exactly matching theoretical models for hot spots orbiting close to a four-million-solar-mass object. These flares likely originate from magnetic interactions in extremely hot gas orbiting very near the event horizon.
First noticed as unusual in images of the Milky Way center in 2002, the gas cloud G2 had a mass about three times that of Earth. It was confirmed in 2012 to be on a course taking it into the accretion zone of Sagittarius A*. Predictions suggested its closest approach, called perinigricon, would occur in early 2014 when the cloud reached just over 3,000 times the radius of the event horizon. Some astronomers expected complete destruction leading to significant X-ray brightening.
Nothing dramatic happened during or after the closest approach, described by Daryl Haggard as a lack of "fireworks" and a "flop." Observations published March 19 and 20, 2014, concluded G2 remained intact despite predictions for simple gas clouds. Andrea Ghez's team later suggested G2 might actually be binary stars merged into an enormous star. Alternative analysis proposed it was a dense clump within a continuous stream of matter acting like a constant breeze rather than sudden gusts hitting the disk.
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Common questions
When did radio astronomers Barry Balick and Robert L. Brown identify Sagittarius A*?
Radio astronomers Barry Balick and Robert L. Brown identified Sagittarius A in 1974 using the National Radio Astronomy Observatory's baseline interferometer. They published their findings on February 13 and 15 that year, marking the first clear detection of what we now call Sagittarius A. The object sat near the border of the constellations Sagittarius and Scorpius.
What is the mass of Sagittarius A* based on observations of star S2 and S14?
Researchers determined the mass of Sagittarius A* to be approximately four million solar masses confined within a radius no larger than 17 light-hours by examining the Keplerian orbit of star S2. Later observations of star S14 showed the mass was about 4.1 million solar masses within a volume with a radius no greater than 6.25 light-hours.
Who released the first image of Sagittarius A* and when?
The Event Horizon Telescope Collaboration released the first image of Sagittarius A* on the 12th of May 2022 based on radio interferometer data collected in 2017. This marked only the second confirmed image of any black hole following Messier 87's supermassive black hole in 2019. Eight radio observatories across six geographical sites contributed to the dataset.
How fast did star S2 travel during its pericenter approach to Sagittarius A*?
Astronomers recorded star S2 traveling at 30% of the speed of light during its pericenter approach to Sagittarius A* in July 2018. The closest point occurred in May 2018 at roughly 1,400 Schwarzschild radii from the black hole. Observations confirmed gravitational redshift alongside velocity redshift within 10 percent measurement precision.
What happened to the gas cloud G2 after its closest approach to Sagittarius A*?
Nothing dramatic happened during or after the closest approach of the gas cloud G2 to Sagittarius A*, described by Daryl Haggard as a lack of fireworks and a flop. Observations published March 19 and 20 2014 concluded G2 remained intact despite predictions for simple gas clouds. Andrea Ghez's team later suggested G2 might actually be binary stars merged into an enormous star.