Sagittarius A

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Sgr A and environs, as seen at 90 cm wavelength by the Very Large Array Labeled Map of the Milky Way Center.tif
Sgr A and environs, as seen at 90 cm wavelength by the Very Large Array
Sagittarius A
Observation data
Epoch J2000       Equinox J2000
Constellation Sagittarius
Right ascension 17h 45m 40.0409s [1]
Declination −29° 00′ 28.118″ [1]
Astrometry
Radial velocity (Rv)46 km/s
Details
Mass ~4.1 million  M☉
Radius 31.6  R☉
Age +10.000 years
Other designations
AX J1745.6-2900, SAGITTARIUS A, W 24, Cul 1742-28, SGR A, [DGW65] 96, EQ 1742-28, RORF 1742-289, [SKM2002] 28.
Database references
SIMBAD data

Sagittarius A (Sgr A) is a complex radio source at the center of the Milky Way, which contains a supermassive black hole. It is located in the constellation Sagittarius, and is hidden from view at optical wavelengths by large clouds of cosmic dust in the spiral arms of the Milky Way. The dust lane that obscures the Galactic Center from a vantage point around the Sun causes the Great Rift through the bright bulge of the galaxy.

Contents

The radio source consists of three components: the supernova remnant Sagittarius A East, the spiral structure Sagittarius A West, and a very bright compact radio source at the center of the spiral, Sagittarius A* (read "A-star"). These three overlap: Sagittarius A East is the largest, West appears off-center within East, and A* is at the center of West.

Discovery

In April 1933, Karl Jansky, considered one of the fathers of radio astronomy, discovered that a radio signal was coming from a location in the direction of the constellation of Sagittarius, towards the center of the Milky Way. [2] His observations did not extend quite as far south as we now know to be the Galactic Center. [3] 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, [4] 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. [5] The name Sagittarius A was first used in 1954 by John D. Kraus, Hsien-Ching Ko, and Sean Matt [6] when they included the object in the list of radio sources found with the Ohio State University radio telescope at 250 MHz. As was common practice at the time, sources were named by constellation with capital letters in order of brightness within each constellation, with A denoting the brightest radio source within the constellation.

Sagittarius A East

This feature is approximately 25 light-years in width and has the attributes of a supernova remnant from an explosive event that occurred between 35,000 and 100,000 BC. However, it would take 50 to 100 times more energy than a standard supernova explosion to create a structure of this size and energy. It is conjectured that Sgr A East is the remnant of the explosion of a star that was gravitationally compressed as it made a close approach to the central black hole. [7]

Sagittarius A West

Surface brightness and velocity field of the inner part of Sagittarius A West SgrAWest BEAR.jpg
Surface brightness and velocity field of the inner part of Sagittarius A West

Sgr A West has the appearance of a three-arm spiral, from the point of view of the Earth. For this reason, it is also known as the "Minispiral". This appearance and nickname are misleading, though: the three-dimensional structure of the Minispiral is not that of a spiral. It is made of several dust and gas clouds, which orbit and fall onto Sagittarius A* at velocities as high as 1,000 kilometers per second. The surface layer of these clouds is ionized. The source of ionisation is the population of massive stars (more than one hundred OB stars have been identified so far) that also occupy the central parsec.

Sgr A West is surrounded by a massive, clumpy torus of cooler molecular gas, the Circumnuclear Disk (CND). The nature and kinematics of the Northern Arm cloud of Sgr A West suggest that it once was a clump in the CND, which fell due to some perturbation, perhaps the supernova explosion responsible for Sgr A East. The Northern Arm appears as a very bright North—South ridge of emission, but it extends far to the East and can be detected as a dim extended source.

The Western Arc (outside the field of view of the image shown in the right) is interpreted as the ionized inner surface of the CND. The Eastern Arm and the Bar seem to be two additional large clouds similar to the Northern Arm, although they do not share the same orbital plane. They have been estimated to amount for about 20 solar masses each.

On top of these large scale structures (of the order of a few light-years in size), many smaller cloudlets and holes inside the large clouds can be seen. The most prominent of these perturbations is the Minicavity, which is interpreted as a bubble blown inside the Northern Arm by the stellar wind of a massive star, which is not clearly identified.

Sagittarius A*

The supermassive black hole Sagittarius A*, imaged by the Event Horizon Telescope EHT Saggitarius A black hole.tif
The supermassive black hole Sagittarius A*, imaged by the Event Horizon Telescope

Astronomers now have evidence that there is a supermassive black hole at the center of the galaxy. [9] Sagittarius A* (abbreviated Sgr A*) is agreed to be the most plausible candidate for the location of this supermassive black hole. The Very Large Telescope at Chile and Keck Telescope at Hawaii have detected stars orbiting Sgr A* at speeds greater than that of any other stars in the galaxy. One star, designated S2, was calculated to orbit Sgr A* at speeds of over 5,000 kilometers per second at its closest approach. [10]

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. [11] [12]

In September 2019, scientists found that Sagittarius A* had been consuming nearby matter at a much faster rate than usual over the previous year. Researchers speculated that this could mean that the black hole is entering a new phase, or that Sagittarius A* had stripped the outer layer of G2 when it passed through. [13]

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<span class="mw-page-title-main">Spiral galaxy</span> Class of galaxy that has spiral structures extending from their cores.

Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

<span class="mw-page-title-main">Supermassive black hole</span> Largest type of black hole

A supermassive black hole is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (M☉). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, including light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example, the Milky Way galaxy has a supermassive black hole at its center, corresponding to the radio source Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei (AGNs) and quasars.

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<span class="mw-page-title-main">Galactic Center</span> Rotational center of the Milky Way galaxy

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Andrea Mia Ghez is an American astrophysicist, Nobel laureate, and professor in the Department of Physics and Astronomy and the Lauren B. Leichtman & Arthur E. Levine chair in Astrophysics, at the University of California, Los Angeles. Her research focuses on the center of the Milky Way galaxy.

<span class="mw-page-title-main">Sagittarius A*</span> Supermassive black hole at the center of the Milky Way

Sagittarius A*, abbreviated Sgr A*, is the supermassive black hole at the Galactic Center of the Milky Way. Viewed from Earth, it is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic, visually close to the Butterfly Cluster (M6) and Lambda Scorpii.

<span class="mw-page-title-main">Carina–Sagittarius Arm</span> Minor spiral arm of the Milky Way Galaxy and one of its most pronounced arms

The Carina–Sagittarius Arm is generally thought to be a minor spiral arm of the Milky Way galaxy. Each spiral arm is a long, diffuse curving streamer of stars that radiates from the Galactic Center. These gigantic structures are often composed of billions of stars and thousands of gas clouds. The Carina–Sagittarius Arm is one of the most pronounced arms in our galaxy as many HII regions, young stars and giant molecular clouds are concentrated in it.

An astronomical radio source is an object in outer space that emits strong radio waves. Radio emission comes from a wide variety of sources. Such objects are among the most extreme and energetic physical processes in the universe.

<span class="mw-page-title-main">NGC 1365</span> Galaxy in the constellation Fornax

NGC 1365, also known as the Great Barred Spiral Galaxy, is a double-barred spiral galaxy about 75 million light-years away in the constellation Fornax.

<span class="mw-page-title-main">Milky Way</span> Galaxy containing the Solar System

The Milky Way is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye.

<span class="mw-page-title-main">Andromeda–Milky Way collision</span> Predicted galactic collision

The Andromeda–Milky Way collision is a galactic collision predicted to occur in about 4.5 billion years between the two largest galaxies in the Local Group—the Milky Way and the Andromeda Galaxy. The stars involved are sufficiently far apart that it is improbable that any of them will individually collide, though some stars will be ejected.

<span class="mw-page-title-main">S2 (star)</span> Star orbiting close to the supermassive black hole in the center of the Milky Way

S2, also known as S0–2, is a star in the star cluster close to the supermassive black hole Sagittarius A* (Sgr A*), orbiting it with a period of 16.0518 years, a semi-major axis of about 970 au, and a pericenter distance of 17 light hours – an orbit with a period only about 30% longer than that of Jupiter around the Sun, but coming no closer than about four times the distance of Neptune from the Sun. The mass when the star first formed is estimated by the European Southern Observatory (ESO) to have been approximately 14 M☉. Based on its spectral type, it probably has a mass of 10 to 15 solar masses.

<span class="mw-page-title-main">Astrophysical X-ray source</span> Astronomical object emitting X-rays

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S55 is a star that is located very close to the centre of the Milky Way, near the radio source Sagittarius A*, orbiting it with an orbital period of 12.8 years. Until 2019, when the star S62 became the new record holder, it was the star with the shortest known period orbiting the black hole at the centre of the Milky Way. This beat the record of 16 years previously set by S2. The star was identified by a University of California, Los Angeles team headed by Andrea M. Ghez. At its periapsis, its speed reaches 1.7% of the speed of light. At that point it is 246 astronomical units from the centre, while the black hole radius is only a small fraction of that size. It passed that point in 2022 and will be there again in 2034.

The Central Molecular Zone or CMZ is a region of the Milky Way Galaxy rich in an estimated 60 million solar masses (M☉) of gas within a complex of giant molecular clouds. It spans the centre of the Milky Way, and as such is in the Sagittarius constellation, between galactic longitude 1.7° and -0.7°, and latitudes -0.2° and +0.2°.

The following outline is provided as an overview of and topical guide to galaxies:

References

  1. 1 2 Reid, M.J.; Brunthaler, A. (2004). "The Proper Motion of Sagittarius A*". The Astrophysical Journal. 616 (2): 874, 883. arXiv: astro-ph/0408107 . Bibcode:2004ApJ...616..872R. doi:10.1086/424960. S2CID   16568545.
  2. ↑ "Karl Jansky: The Father of Radio Astronomy". 29 August 2012. Archived from the original on 2019-06-28. Retrieved 2019-01-27.
  3. ↑ Goss, W. M.; McGee, R. X. (1996). "The Discovery of the Radio Source Sagittarius A (Sgr A)". The Galactic Center, Astronomical Society of the Pacific Conference Series. 102: 369. Bibcode:1996ASPC..102..369G. Archived from the original on 3 March 2021. Retrieved 25 February 2021.
  4. ↑ Piddington, J. H.; Minnett, H. C. (1 December 1951). "Observations of Galactic Radiation at Frequencies of 1200 and 3000 Mc/s". Australian Journal of Scientific Research A. 4 (4): 459. Bibcode:1951AuSRA...4..459P. doi:10.1071/CH9510459. Archived from the original on 13 April 2021. Retrieved 25 February 2021.
  5. ↑ McGee, R. X.; Bolton, J. G. (1 May 1954). "Probable observation of the galactic nucleus at 400 Mc./s". Nature. 173 (4412): 985–987. Bibcode:1954Natur.173..985M. doi:10.1038/173985b0. ISSN   0028-0836. S2CID   4188235. Archived from the original on 30 January 2022. Retrieved 25 February 2021.
  6. ↑ Kraus, J. D.; Ko, H. C.; Matt, S. (December 1954). "Galactic and localized source observations at 250 megacycles per second". The Astronomical Journal . 59: 439–443. Bibcode:1954AJ.....59..439K. doi:10.1086/107059. ISSN   0004-6256. Archived from the original on June 10, 2022. Retrieved May 16, 2022– via The ADS.
  7. ↑ Maeda, Y.; Baganoff, F. K.; Feigelson, E. D.; Morris, M.; Bautz, M. W.; Brandt, W. N.; Burrows, D. N.; Doty, J. P.; Garmire, G. P.; Pravdo, S. H.; Ricker, G. R.; Townsley, L. K. (2002-05-10). "AChandra Study of Sagittarius a East: A Supernova Remnant Regulating the Activity of Our Galactic Center?" (PDF). The Astrophysical Journal. 570 (2): 671–687. arXiv: astro-ph/0102183 . Bibcode:2002ApJ...570..671M. doi:10.1086/339773. S2CID   18482187. Archived (PDF) from the original on 2022-01-20. Retrieved 2020-08-29.
  8. ↑ "Astronomers reveal first image of the black hole at the heart of our galaxy". Event Horizon Telescope. Archived from the original on 2022-05-12. Retrieved 2022-05-12.
  9. ↑ Black hole confirmed in Milky Way Archived 2009-02-15 at the Wayback Machine , Ghosh, Pallab, BBC News. 2008-12-09. Retrieved 2008-12-10.
  10. ↑ "Surfing a Black Hole - Star Orbiting Massive Milky Way Centre Approaches to within 17 Light-Hours". www.eso.org. Archived from the original on 2012-10-16. Retrieved 2011-05-16.
  11. ↑ Lemonick, Michael (6 November 2014). "Black Hole Fails to Destroy Mystery Cosmic Cloud". National Geographic . Archived from the original on 2022-12-27. Retrieved 26 December 2022.
  12. ↑ Cowen, Ron (2017-03-02). "It's Snack Time in the Cosmos". The New York Times . Archived from the original on 2017-03-02. Retrieved 26 December 2022.
  13. ↑ Devlin, Hannah (2019-09-13). "Black hole at centre of galaxy is getting hungrier, say scientists". The Guardian. ISSN   0261-3077. Archived from the original on 2019-09-17. Retrieved 2019-09-18.
  14. ↑ Takeuchi, Naoko (1997). Prety Soldier Sailor Moon: Volume 18. Tokyo: Kodansha. ISBN   4061788582.

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