Sagittarius A*| Supermassive Black Hole at the Heart of our Galaxy
Black holes are among the most profound predictions of Einstein’s theory of general relativity. Originally studied as a mere mathematical consequence of the theory rather than as physically relevant objects, they soon became thought of as generic and sometimes inevitable outcomes of the gravitational collapse that initially forms a galaxy.
In fact, most physicists have suspected that our own galaxy revolves around a supermassive black hole at its center. There are other ideas too — such as “dark matter” (an invisible substance thought to make up most of the matter in the universe). But now an international team of astronomers, including a team that I led from the University of Central Lancashire, has unveiled the first image of the object lurking at the center of the Milky Way — and it is a supermassive black hole.
This means there is now overwhelming evidence for the black hole, dubbed Sagittarius A*. While it might seem a little scary to be so close to such a beast, it is in fact some 26,000 light-years away, which is reassuringly far. In fact, because the black hole is so far away from Earth, it appears to us to have about the same size in the sky as a donut would have on the Moon. Sagittarius A* also seems rather inactive — it is not devouring a lot of matter from its surroundings.
What is SAGITTARIUS A*?
Sagittarius A*, often abbreviated to Sgr A* and pronounced “Sagittarius A star”, it is a Supermassive Black Hole at the center of the Milky Way Galaxy, located in the constellation Sagittarius. It is a strong source of radio waves and is embedded in the larger Sagittarius A complex. Most of the radio radiation is from a synchrotron mechanism, indicating the presence of free electrons and magnetic fields. Sagittarius A* is a compact, extremely bright point source. X-ray, infrared, spectroscopic, and radio interferometric investigations have indicated the very small dimensions of this region. Infrared observations of stars orbiting the position of Sagittarius A* demonstrate the presence of a black hole with a mass equivalent to 4,310,000 Suns. (For these infrared observations, American astronomer Andrea Ghez and German astronomer Reinhard Genzel were awarded the 2020 Nobel Prize for Physics.) These properties are similar to those of other galaxies with active nuclei (e.g., Seyfert galaxies) but on a smaller scale.
A gas cloud, G2, passed through the Sagittarius A* region in 2014 and managed to do so without disappearing beyond the event horizon, as theorists predicted would happen. Rather, it disintegrated, suggesting that G2 and a previous gas cloud, G1, were star remnants with larger gravitational fields than gas clouds.
QUICK FACTS: SAGITTARIUS A*
Also known as: Sgr A*.
Distance from Earth: 26,000 light-years.
Size: 4.6 million times the mass of the Sun.
Mass: 8.26×1036 kg (4.154±0.014)×106M☉
Type of object: Supermassive black hole.
Location in the sky: Sagittarius Constellation.
Location in the universe: Center of our Milky Way galaxy.
Did you know: In 2018, researchers found evidence for thousands of stellar-mass black holes located within 3 light-years of Sagittarius A* at the center of our Milky Way galaxy..
Observation and Description
Black holes are notoriously difficult to spot, usually only inferred by the effects they have on their environment. This is because not only do they not emit light, but black holes also trap photons behind a boundary called the event horizon, making studying them directly in optical light near impossible. Observing Sagittarius A* from Earth is made even more difficult due to the fact that it is shrouded by a thick screen of intervening dust.
On May 12, 2022, the first image of Sagittarius A* was released by the Event Horizon Telescope Collaboration. The image, which is based on radio interferometer data taken in 2017, confirms that the object contains a black hole. This is the second image of a black hole. This image took five years of calculations to process. The data was collected by eight radio observatories at six geographical sites. Radio images are produced from data by aperture synthesis, usually from night long observations of stable sources. The radio emission from Sgr A* varies on the order of minutes, complicating the analysis.
Their result gives an overall angular size for the source of 51.8±2.3 μas). At a distance of 26,000 light-years (8,000 parsecs), this yields a diameter of 51.8 million kilometers (32.2 million miles). For comparison, Earth is 150 million kilometers(1.0 astronomical unit; 93 million miles) from the Sun, and Mercury is 46 million km (0.31 AU; 29 million mi) from the Sun at perihelion. The proper motion of Sgr A* is approximately −2.70 mas per year for the right ascension and −5.6 mas per year for the declination. The telescope’s measurement of these black holes tested Einstein’s theory of relativity more rigorously than has previously been done, and the results match perfectly.
In 2019, measurements made with the High-resolution Airborne Wideband Camera-Plus (HAWC+) mounted in the SOFIA aircraft revealed that magnetic fields cause the surrounding ring of gas and dust, temperatures of which range from −280 to 17,500 °F (99.8 to 9,977.6 K; −173.3 to 9,704.4 °C), to flow into an orbit around Sagittarius A*, keeping black hole emissions low.
Astronomers have been unable to observe Sgr A* in the optical spectrum because of the effect of 25 magnitudes of extinction by dust and gas between the source and Earth.
SAGITTARIUS A* FIRST IMAGE
An image of the supermassive black hole at the center of the Milky Way, a behemoth dubbed Sagittarius A*, was revealed by the Event Horizon Telescope on May 12, 2022.
The image was captured using observations of light — the light emitted by matter that’s heated up as it hurtles toward the center of Sag A*. This technique gives scientists a view of essentially the shadow of the black hole.
Capturing an image of a black hole is no easy task and requires a global network of observatories that coordinates to act like a telescope the size of Earth — the Event Horizon Telescope (EHT).
There is still a great deal to learn about Sagittarius A* but the first image of the Milky Way’s central black hole could reveal further secrets held by the cosmic object that has shaped our galaxy.
The History of Discovery
Theories surrounding Sagittarius A* and its massive occupant date back to the early 1930s when Karl Jansky found a radio signal emitted from a location in the direction of the Sagittarius constellation directed towards the center of the Milky Way.
The galactic center compact radio source Sagittarius A* was then identified in February 1974 by astronomers Bruce Balick and Robert L. Brown. It was during the 1980s that astronomers formulated the idea that the central compact object was likely to be a black hole of a size — until then — unimaginable.
Karl Jansky, considered a father of radio astronomy, discovered in April 1933 that a radio signal was coming from a location in the direction of the constellation of Sagittarius, towards the center of the Milky Way. The radio source later became known as Sagittarius A. His observations did not extend quite as far south as we now know to be the Galactic Center. Observations by Jack Piddington and Harry Minnett using the CSIRO radio telescope at Potts Hill Reservoir, in Sydney discovered a discrete and bright “Sagittarius-Scorpius” radio source, which after further observation with the 80-foot (24-metre) CSIRO radio telescope at Dover Heights was identified in a letter to Nature as the probable Galactic Center.
Later observations showed that Sagittarius A actually consists of several overlapping sub-components; a bright and very compact component, Sgr A*, was discovered on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the baseline interferometer of the National Radio Astronomy Observatory. The name Sgr A* was coined by Brown in a 1982 paper because the radio source was “exciting”, and excited states of atoms are denoted with asterisks.
Since the 1980s, it has been evident that the central component of Sgr A* is likely a black hole. In 1994, infrared and submillimetre spectroscopy studies by a Berkeley team involving Nobel Laureate Charles H. Townes and future Nobel Prize Winner Reinhard Genzel showed that the mass of Sgr A* was tightly concentrated and of the order 3 million Suns.
On October 16, 2002, an international team led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics reported the observation of the motion of the star S2 near Sagittarius A* throughout a period of ten years. According to the team’s analysis, the data ruled out the possibility that Sgr A* contains a cluster of dark stellar objects or a mass of degenerate fermions, strengthening the evidence for a massive black hole. The observations of S2 used near-infrared (NIR) interferometry (in the Ks-band, i.e. 2.1 μm) because of reduced interstellar extinction in this band. SiO masers were used to align NIR images with radio observations, as they can be observed in both NIR and radio bands. The rapid motion of S2 (and other nearby stars) easily stood out against slower-moving stars along the line-of-sight so these could be subtracted from the images.
The VLBI radio observations of Sagittarius A* could also be aligned centrally with the NIR images, so the focus of S2’s elliptical orbit was found to coincide with the position of Sagittarius A*. From examining the Keplerian orbit of S2, they determined the mass of Sagittarius A* to be 4.1±0.6 million solar masses, confined in a volume with a radius no more than 17 light-hours (120 AU [18 billion km; 11 billion mi]). Later observations of the star S14 showed the mass of the object to be about 4.1 million solar masses within a volume with radius no larger than 6.25 light-hours (45 AU [6.7 billion km; 4.2 billion mi]). S175 passed within a similar distance. For comparison, the Schwarzschild radius is 0.08 AU (12 million km; 7.4 million mi). They also determined the distance from Earth to the Galactic Center (the rotational center of the Milky Way), which is important in calibrating astronomical distance scales, as 8,000 ± 600 parsecs (30,000 ± 2,000 light-years). In November 2004, a team of astronomers reported the discovery of a potential intermediate-mass black hole, referred to as GCIRS 13E, orbiting 3 light-years from Sagittarius A*. This black hole of 1,300 solar masses is within a cluster of seven stars. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.[citation needed]
After monitoring stellar orbits around Sagittarius A* for 16 years, Gillessen et al. estimated the object’s mass at 4.31±0.38 million solar masses. The result was announced in 2008 and published in The Astrophysical Journal in 2009. Reinhard Genzel, team leader of the research, said the study has delivered “what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the Galactic Center show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt.”
On January 5, 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sgr A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sgr A*, according to astronomers.
On 13 May 2019, astronomers using the Keck Observatory witnessed a sudden brightening of Sgr A*, which became 75 times brighter than usual, suggesting that the supermassive black hole may have encountered another object.
Conclusive evidence that the compact object Sagittarius A* is a supermassive black hole was delivered in 2018 when emissions caused by magnetic interactions from hot gas clumps close to the black hole moving at around 30% the speed of light were observed by astronomers using the European Southern Observatory (ESO)’s Very Large Telescope (VLT).
Over the next decade, astronomers continued to rule out other possible candidates for this object — including tightly clustered stars — which strengthened the idea that Sagittarius A* is a supermassive black hole.
The publication of the picture of the Sagittarius A* black hole is a tremendously exciting achievement by the collaboration.
This work should impact efforts using radio telescopes to observe and understand the “shadow” cast by the event horizon of Sgr A* against the background of surrounding, glowing matter. It will also be useful for understanding the impact that orbiting stars and gas clouds might make with the matter flowing towards and away from the black hole.
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