Sagittarius A (Sgr A) is a complex radio source located at the centre of the Milky Way Galaxy. It lies in the direction of Sagittarius constellation, near the border with Scorpius.
The radio source consists of the supernova remnant Sagittarius A East, the spiral structure Sagittarius A West, and a bright compact radio source at the centre of the spiral structure, called Sagittarius A*.
Sagittarius A* (pronounced “Sagittarius A-star”) is the most plausible candidate for the location of the supermassive black hole at the centre of our galaxy. The black hole at the centre of the Milky Way lies at a distance of 26,000 light years from Earth.
Sgr A can’t be seen in optical wavelengths because it is hidden from view by large dust clouds in the Milky Way’s spiral arms.
Center of our Milky Way Galaxy, located in the constellation of Sagittarius. In visible light the lion’s share of stars are hidden behind thick clouds of dust. This obscuring dust becomes increasingly transparent at infrared wavelengths. This 2MASS image reveals multitudes of otherwise hidden stars, penetrating all the way to the central star cluster of the Galaxy. This central core, seen in the upper left portion of the image, is about 25,000 light years away and is thought to harbor a supermassive black hole. The reddening of the stars here and along the Galactic Plane is due to scattering by the dust; it is the same process by which the sun appears to redden as it sets. – Atlas Image mosaic obtained as part of the Two Micron All Sky Survey (2MASS), a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
The Galactic centre is best observed in infrared light and radio waves.
The three components of Sagittarius A overlap. The supernova remnant Sgr A East is the largest component. The spiral structure Sgr A West appears within Sgr A East, while Sgr A* lies at the centre of Sgr A West.
Sagittarius A East
The supernova remnant Sagittarius A East is a non-thermal radio source located within parsecs of the Milky Way’s centre. The size of its radio shell is the smallest of the known mixed-morphology supernova remnants.
Sgr A East is about 25 light years wide and is believed to have formed after a supernova explosion that occurred between 35,000 and 100,000 BCE. However, the size of the object indicates that it would have taken 50 to 100 times more energy than a standard supernova event to form a remnant this wide.
For this reason, the star that exploded in the supernova event is conjectured to have been gravitationally compressed because it had made a close approach to the Milky Way’s central black hole.
Sagittarius A West
The spiral structure Sagittarius A West is sometimes called the “Minispiral” because it appears as a three-arm spiral when observed from Earth.
The Sagittarius A West complex of ionized gas, here observed in the Bracket gamma line of ionized Hydrogen, has the apparent shape of a three-arm spiral. The image above was produced using data obtained with the BEAR spectro-imager on the Canada-France-Hawaii Telescope. The color shades represent different radial velocities, explicited on the spectrum in the upper-left inset, which corresponds to the selected pixel. Image: Thibaut Paumard
However, the object’s appearance is misleading because its three-dimensional structure is not that of a spiral, but it is made of clouds of dust and gas that orbit Sgr A* and fall onto it at great velocities, up to 1,000 km/s. The clouds’ surface layer is ionized by a hundred or more massive OB stars found in this region.
The Sgr A West structure is surrounded by a Circumnuclear Disk (CDN), a massive clump of molecular gas.
Sagittarius A* is believed to be the location of the supermassive black hole in the centre of the Milky Way Galaxy. Astronomers have detected stars orbiting Sgr A* at speeds much greater that those of any other stars in the Milky Way. One of these stars, designated S2, was observed spinning around Sgr A* at speeds of over 5,000 km/s when it made its closest approach to the object.
Sagittarius A* has a diameter of 44 million kilometres, roughly equalling the distance from Mercury to the Sun (46 million km).
Sgr A* emits a large amount of IR, gamma-rays and X-rays. It appears motionless, but there are clouds of dust and gas orbiting it, which provides a clue to the nature of the object. Astronomers calculated its mass using Kepler’s laws and measuring the period and semi-major axis of the orbit of a star that came within 17 light hours of the object. They arrived at approximately 4 million solar masses. The only kind of object that can be that massive and have a radius of about 100 astronomical units is a black hole.
The object was discovered on February 13 and 15, 1974 by astronomers Robert Brown and Bruce Balick at the National Radio Astronomy Observatory.
The SWEEPS target area is in the Sagittarius constellation, toward the center of the Milky Way galaxy. The target area is a rare transparent window to the distant central bulge stars located approximately 27,000 light-years away from Earth. The location of the SWEEPS area is indicated on this Milky Way image in blue. Image: NASA, ESA, Z. Levay (STScI) and A. Fujii
The motion of the star S2 over a period of 10 years was reported on October 16, 2002 by an international team of scientists led by Rainer Schödel of the Max Planck Institute for Extraterrestrial Physics. Their observations using near-infrared (NIR) interferometry strenghtened the theory that Sgr A* was the location of a massive black hole.
Location: 17h 45 m 40.0409s (right ascension), -29°0’28.118” (declination)
Distance: 25,900 ± 1,400 light years (7,940 ± 420 parsecs)
Diameter: 44 million kilometres
Angular diameter: 37 μas
Mass: 4.31 ± 0.38 million solar masses
The discovery of an intermediate-mass black hole candidate, designated GCIRS 13E, was reported in November 2004. The object was detected orbiting three light years from Sgr A*. The black hole was detected within a cluster of seven stars and its mass was estimated at 1,300 solar masses.
In 2008, the results of 16-year long observations of stellar orbits around Sgr A* by Gillessen et al. were announced and published in The Astrophysical Journal in 2009. The team estimated the object’s mass to be 4.31 ± 0.38 million solar masses.
Reinhard Genzel, who led the research, said that it provided “what is now considered to be the best empirical evidence that super-massive black holes do really exist. The stellar orbits in the galactic centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt.”
Sagittarius A* is not exactly centred on the black hole. If it were, we would be able to see the object magnified as a result of gravitational lensing, a phenomenon that occurs because light of a distant source gets lensed, or bent by the gravity of an extremely massive object in the foreground. (The Einstein Cross in Pegasus constellation is a good example.)
NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, has captured these first, focused views of the supermassive black hole at the heart of our galaxy in high-energy X-ray light. The background image, taken in infrared light, shows the location of our Milky Way’s humongous black hole, called Sagittarius A*. In the main image, the brightest white dot is the hottest material located closest to the black hole, and the surrounding pinkish blob is hot gas, likely belonging to a nearby supernova remnant. The time series at right shows a flare caught by NuSTAR over an observing period of two days in July; the middle panel shows the peak of the flare, when the black hole was consuming and heating matter to temperatures up to 180 million degrees Fahrenheit (100 million degrees Celsius). The main image is composed of light seen at four different X-ray energies. Blue light represents energies of 10 to 30 kiloelectron volts (keV); green is 7 to 10 keV; and red is 3 to 7 keV. The time series shows light with energies of 3 to 30 keV. The background image of the central region of our Milky Way was taken at shorter infrared wavelengths by NASA’s Spitzer Space Telescope. Image: NASA
As we don’t see the object enlarged beyond its size, this indicates that the radio emissions of Sgr A* are not centred on the black hole, but come from a bright spot in the area around it, near the event horizon. In other words, the emission doesn’t come from the black hole itself, but from the material that the black hole is about to swallow up.
The black hole itself can’t be seen, but observations of nearby objects are only consistent if there is one present in the vicinity of Sagittarius A*.
The NASA/ESA Hubble Space Telescope, the Spitzer Space Telescope and the Chandra X-ray Observatory have collaborated to produce an unprecedented image of the central region of our Milky Way galaxy. In this image, observations using infrared light and X-ray light see through the obscuring dust and reveal the intense activity near the galactic core. Note that the centre of the galaxy is located within the bright white region to the right of and just below the middle of the image. The entire image width covers about one-half a degree, about the same angular width as the full moon. Each telescope’s contribution is presented in a different colour: Yellow represents the near-infrared observations of Hubble. They outline the energetic regions where stars are being born as well as reveal hundreds of thousands of stars. Red represents the infrared observations of Spitzer. The radiation and winds from stars create glowing dust clouds that exhibit complex structures from compact, spherical globules to long, stringy filaments. Blue and violet represent the X-ray observations of Chandra. X-rays are emitted by gas heated to millions of degrees by stellar explosions and by outflows from the supermassive black hole in the galaxy’s centre. The bright blue blob on the left side is emission from a double star system containing either a neutron star or a black hole. When these views are brought together, this composite image provides one of the most detailed views ever of our galaxy’s mysterious core. Image: NASA, ESA, SSC, CXC and STScI
This image, not unlike a pointillist painting, shows the star-studded centre of the Milky Way towards the constellation of Sagittarius. The crowded centre of our galaxy contains numerous complex and mysterious objects that are usually hidden at optical wavelengths by clouds of dust — but many are visible here in these infrared observations from Hubble. However, the most famous cosmic object in this image still remains invisible: the monster at our galaxy’s heart called Sagittarius A*. Astronomers have observed stars spinning around this supermassive black hole (located right in the centre of the image), and the black hole consuming clouds of dust as it affects its environment with its enormous gravitational pull. Image: NASA, ESA, and G. Brammer
In a 16-year long study, using several of ESO’s flagship telescopes, a team of German astronomers has produced the most detailed view ever of the surroundings of the monster lurking at our Galaxy’s heart — a supermassive black hole. The research has unravelled the hidden secrets of this tumultuous region by mapping the orbits of almost 30 stars, a five-fold increase over previous studies. One of the stars has now completed a full orbit around the black hole. By watching the motions of 28 stars orbiting the Milky Way’s most central region with admirable patience and amazing precision, astronomers have been able to study the supermassive black hole lurking there. It is known as “Sagittarius A*”. The new research marks the first time that the orbits of so many of these central stars have been calculated precisely and reveals information about the enigmatic formation of these stars — and about the black hole to which they are bound. Image: ESO, Stefan Gillessen, Reinhard Genzel, Frank Eisenhauer
This Chandra image of Sgr A* and the surrounding region is based on data from a series of observations lasting a total of about one million seconds, or almost two weeks. Such a deep observation has given scientists an unprecedented view of the supernova remnant near Sgr A* (known as Sgr A East) and the lobes of hot gas extending for a dozen light years on either side of the black hole. These lobes provide evidence for powerful eruptions occurring several times over the last ten thousand years. The image also contains several mysterious X-ray filaments, some of which may be huge magnetic structures interacting with streams of energetic electrons produced by rapidly spinning neutron stars. Such features are known as pulsar wind nebulas. Image: NASA/CXC/MIT/F. Baganoff, R. Shcherbakov et al.
The centre of our Milky Way galaxy is located in the southern constellation Sagittarius (The Archer) and is “only” 26,000 light-years away. On high-resolution images, it is possible to discern thousands of individual stars within the central, one light-year wide region. Using the motions of these stars to probe the gravitational field, observations over the last decade have shown that a mass of about 3 million times that of the Sun is concentrated within a radius of only 10 light-days of the compact radio and X-ray source Sgr A* (Sagittarius A) at the centre of the star cluster. This means that SgrA* is the most likely counterpart of the black hole believed to exist at the centre of our Galaxy. This image was obtained in mid-2002 with the NACO instrument at the 8.2-m VLT Yepun telescope. It combines frames in three infrared wavebands between 1.6 and 3.5 µm. The compact objects are stars and their colours indicate their temperature (blue =”hot”, red =”cool”). There is also diffuse infrared emission from interstellar dust between the stars. Image: ESO