Sagittarius A *

from Wikipedia, the free encyclopedia
Radio and X-ray source
Sagittarius A *
X-ray image of Sagittarius A * and two light echoes (marked) from a previous explosion
X-ray image of Sagittarius A * and two light echoes (marked) from a previous explosion
Constellation Sagittarius
Position
equinox : J2000.0
Right ascension 17h 45m 40.0s
declination −29 ° 00 ′ 28.2 ″
Further data
distance

26673 ± 42.4 (stat.) ± 71.75 (sys.) Lj
8178 ± 13 (stat.) ± 22 (sys.) Pc

Dimensions 4.100 ± 0.034 million solar masses
diameter approx. 22.5 million km = 2.36 · 10 –6 ly
history
discovery

Bruce Balick ,
Robert Hanbury Brown

Date of discovery

February 1974

Catalog names
AX J1745.6-2900 ( ASCA )
Aladin previewer

Sagittarius A * (spoken: Sagittarius A star ; abbreviated: Sgr A * ; a region in the constellation Sagittarius ) is a source of radio waves in the center of the Milky Way . According to the current state of radio-astronomical research, it is a supermassive black hole of 4.1 million solar masses , which is about 26,670 light years from Earth. Sagittarius A * is an active galaxy nucleus in the center of the Milky Way.

history

As early as 1932, Karl Jansky found a strong radio source in the constellation Sagittarius (lat. Sagittarius ) . On February 13 and 15, 1974, astronomers Bruce Balick and Robert Hanbury Brown discovered Sagittarius A * with the help of the interferometer at the National Radio Astronomy Observatory . The asterisk "*" was added by Robert Brown to the Latin name of the constellation Sagittarius, analogous to the notation of excited states in atomic physics . He suspected that the radio source stimulated its surroundings to emit radiation. The name lasted, even if Brown's guess turned out to be wrong.

The discovery of the supermassive black hole in this region was made independently by teams led by Andrea Ghez at the Keck Observatory and Reinhard Genzel at the La Silla Observatory and Very Large Telescope in multi-year observation series from the 1990s.

In April 2019, the Event Horizon Telescope (EHT) presented recordings of the supermassive black hole in M 87 , taken two years earlier , showing its shadow. Corresponding images of Sagittarius A * with the EHT are in progress. Photos published in 2019 with a worldwide interconnection of radio telescopes including ALMA in the Global Millimeter VLBI Array (GMVA), which was observed at 3.5 mm wavelength, show a luminous disk and no shadow as with M 87. The source in the center is very symmetrical almost round shape and a major axis of 120 micro-arcseconds. Even better resolution is expected from the observations of the EHT (at 1.3 mm wavelength), where the effect of interstellar scattering is even less and can be corrected. A comparison with computer simulations (Sara Issaoun and Monika Mościbrodzka from Radboud University Nijmegen ) favors accretion disks for the source, but a jet - as seen at M 87, for example - cannot be completely ruled out, but it would have to hit the ground fairly accurately demonstrate.

Objects in the vicinity of Sgr A *

Stars

The black hole at Sagittarius A * is covered by a dark cloud in the visible light spectrum . The photograph shows the view of the southern horizon from the northern hemisphere.

In 2002, scientists (around Reinhard Genzel) doing research at the Very Large Telescope of the European Southern Observatory (ESO) were able to observe a star that had approached the Sagittarius A * region within 17 hours of light (≈18.36 billion kilometers). During their observations, the researchers were able to see a sudden U- turn of the 15 solar mass star S2 (alias S0-2). Due to the high speed of the star, this process can only be interpreted as a movement around a black hole. Due to the short distance between S2 and the enormous mass of the black hole, its orbital speed is very high; its orbital speed is up to 5000 km / s. Its orbit is relatively stable; only when S2 has approached the black hole within 16 light minutes will it be torn apart by the tidal forces. S2 only takes 15.2 years to complete one orbit around the center. For another star, the 16 times fainter star S0-102 (alias S55), the orbit was also measured in 2012 and an even shorter period of 11.5 years was measured. These two stars are so far the only objects that have been observed at such a close distance to Sgr A *.

The observation of the star S2 as it moves around Sagittarius A * was made possible by an adaptive optics system ( NAOS ) that can compensate for disruptive influences from the atmosphere. With this observation method it is now possible to rule out that Sgr A * is anything other than a supermassive black hole - for example a cluster of neutron stars .

On May 19, 2018, after an orbit lasting 16.0518 years, S2 again reached the closest approach to the black hole, which was the subject of various observation programs, including for the test of general relativity , according to the GRAVITY consortium at the Paranal Observatory (VLTI). The gravitational redshift predicted by the general theory of relativity was confirmed again.

To date (as of August 2020), a number of other stars have been discovered in the vicinity of Sgr A *, including the stars S62, S4711 and S4714, which were added in publications from 2020.

  • Of these, S62 needs 9.9 years for one orbit and approaches Sgr A * up to 2.4 billion km, which is less than the mean distance from Uranus to the sun. It reaches 6.7% of the speed of light.
  • At its closest approach of approx. 1.2 billion km, S4714 reaches about 8% of the speed of light, but because of the extreme eccentricity of its orbit of 0.985 it takes 12 years for one orbit.
  • S4711, a 150-million-year-old type B blue star , takes only 7.6 years, and its closest approximation is 21.5 billion km.

Because of the large approaching the central supermassive black hole these stars are candidates for the 20 years before the proposed new type of star " Squeezar " because you are greatly squeezed due to the strong tidal forces ( English to squeeze , squeeze ' ).

Gas cloud, G objects

In 2005, a group led by Andrea Ghez discovered a gaseous object near Sagittarius A *, which they named G1 , and researchers at the Max Planck Institute for Extraterrestrial Physics led by Stefan Gillesen discovered in 2011 the gas cloud G2 with about three times the mass of the earth , which is located on Sagittarius A * moved towards. In 2013, parts of it were observed to approach the black hole over a distance of 25 billion kilometers. The cloud was torn apart and made to glow by the ultraviolet radiation from neighboring stars. Measurements showed that the front part of the cloud had already passed the point of closest approach to the black hole at an orbit speed of more than 2800 km / s (about 1% of the speed of light). The researchers expect the measurements to provide information about the physical processes involved in approaching a black hole and the effects of extremely strong gravitational fields.

A total of six G objects were found by January 2020. They are around 100 AU in size, but expand near the black hole through tidal interaction. On the one hand they have the characteristics of gas and dust clouds, but on the other hand they behave dynamically like stars. They show both thermal emission of dust and line spectra of ionized gases. Andrea Ghez suspects that the binary star systems are merging or are on the way to merging due to the action of the black hole. Observable releases of energy are expected if the gases drawn from the object by the black hole are absorbed by the black hole.

Second black hole

Sgr A * and IRS 13 in the center of the Milky Way in the X-ray spectrum

In 2004, the Gemini telescope in Hawaii with IRS 13 discovered an object with a mass of around 1300 solar masses and accompanying Sagittarius A * at a distance of three light years. If you look closely, IRS 13 is a group of seven stars that revolve around a common center of gravity. Investigations there indicate a medium-sized black hole. This circles Sgr A * in a very small space at an unusually high speed of around 280 km / s. Another indication of the presence of a black hole is, in addition to the high speeds, the X-rays emitted by IRS 13.

More black holes

In January 2005, bursts of brightness were observed near Sgr A * using the Chandra X-ray Observatory , suggesting that there are 10,000 to 20,000 black holes within a radius of about 70 light years that make up the supermassive central black hole in Sgr A. * circle. This supports a theory that has been circulating since 2003, according to which the central black hole is "fed" via smaller holes: The small black holes in the more outlying areas of the Milky Way collect clusters of stars around them, which they then hold captive until they have spiraled into the immediate vicinity of Sgr A *. There the star clusters will eventually be broken up by the extremely large tidal forces and lose one or the other star to the supermassive black hole. The previous theory on the feeding process was based on the assumption that a huge ring-shaped gas cloud orbits the black hole and becomes heavier and heavier. As soon as a critical mass is exceeded, this cloud collapses and falls into the center of the Milky Way. Presumably, both processes play an important role in the feeding of Sgr A *.

Other objects

The magnetar PSR J1745−2900 exists in the vicinity of Sagittarius A * .

Web links

Commons : Sagittarius A *  - collection of images

Individual evidence

  1. ↑ Sagittarius A * at SIMBAD
  2. ↑ The GRAVITY Collaboration, R. Abuter, A. Amorim, M. Bauböck, JP Berger: A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty . In: Astronomy & Astrophysics . tape 625 , May 2019, ISSN  0004-6361 , p. L10 , doi : 10.1051 / 0004-6361 / 201935656 ( aanda.org [accessed November 22, 2019]).
  3. ↑ Measure the size of the black hole in the Milky Way. Science ORF, January 1, 2010.
  4. ↑ Geoffrey C. Bower, Heino Falcke, Robeson M. Herrnstein, Jun-Hui Zhao, WM Goss, Donald C. Backer: Detection of the Intrinsic Size of Sagittarius A * Through Closure Amplitude Imaging. In: Science . April 30, 2004, Volume 304, No. 5671, pp. 704-708, doi: 10.1126 / science.1094023 .
  5. ↑ A monster in the sights. Retrieved September 8, 2019 . In: Image of Science . , December 10, 2008.
  6. ^ Ronald Smothers: Commemorating a Discovery in Radio Astronomy . In: The New York Times , June 9, 1998. Retrieved September 13, 2015. 
  7. ↑ Fulvio Melia: The Galactic Supermassive Black Hole . Princeton University Press, Princeton 2007, ISBN 0-691-13129-5 .
  8. ^ WM Goss, Robert L. Brown, KY Lo: The Discovery of Sgr A *. In: The central 300 parsecs of the Milky Way (= Conference proceedings ), 2003, arxiv : astro-ph / 0305074
  9. ↑ Jun-Hui Zhao, Mark R. Morris, and William Miller Goss: A New Perspective of the Radio-Bright Zone at the Galactic Center: Feedback from Nuclear Activities. The Astrophysical Journal, 2016, doi: 10.3847 / 0004-637X / 817/2/171 .
  10. ↑ Looking into nothing, Der Spiegel, No. 16, 2019, p. 96
  11. ^ Lifting of the veil around the black hole in the center of our Milky Way , MPG Radioastronomie, January 21, 2019
  12. ↑ Sara Issaoun, H. Falcke, Anton census, SS Doeleman, T. Krichbaum u. a .: The Size, Shape, and Scattering of Sagittarius A * at 86 GHz: First VLBI with ALMA, Astroph. J. 2019 doi: 10.3847 / 1538-4357 / aaf732 , arXiv: 1901.06226
  13. ↑ R. Schödel, Reinhard Genzel u. a .: A star in a 15.2-year orbit around the supermassive black hole at the center of the Milky Way. In: Nature. 419, October 17, 2002, pp. 694-696, doi: 10.1038 / nature01121 .
  14. ^ S. Gillessen, F. Eisenhauer, S. Trippe, T. Alexander, R. Genzel, F. Martins, T. Ott: Monitoring stellar orbits around the Massive Black Hole in the Galactic Center . In: Astroph. Journal. Volume 692, 2009, p. 1075, preprint 2008, arxiv : 0810.4674 .
  15. ^ Gravity, ESO
  16. ↑ Gravity Collaboration, R. Abuter et al. a., Detection of the gravitational redshift in the orbit of the star S2 near the Galactic center massive black hole, Astronomy & Astrophysics, Volume 615, 2018, L 15, Abstract
  17. ↑ F. Eisenhauer et al. : SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month . In: The Astrophysical Journal . 628, No. 1, July 20, 2005, pp. 246-259. arxiv : astro-ph / 0502129 . bibcode : 2005ApJ ... 628..246E . doi : 10.1086 / 430667 .
  18. ↑ S. Gillessen et al. : An Update on Monitoring Stellar Orbits in the Galactic Center . In: The Astrophysical Journal . 837, No. 1, February 2017, p. 30. arxiv : 1611.09144 . bibcode : 2017ApJ ... 837 ... 30G . doi : 10.3847 / 1538-4357 / aa5c41 .
  19. ↑ Florian Peißker, Andreas Eckart, Marzieh Parsa: S62 on a 9.9 year orbit around SgrA * . In: The Astrophysical Journal . 889, No. 1, January 24, 2020, p. 61. arxiv : 2002.02341 . doi : 10.3847 / 1538-4357 / ab5afd .
  20. ↑ Florian Peißker, Andreas Eckart, Michal Zajaček, Basel Ali, Marzieh Parsa: S62 and S4711: Indications of a Population of Faint Fast-moving Stars inside the S2 Orbit — S4711 on a 7.6 yr Orbit around Sgr A * , in: The Astrophysical Journal, Volume 899, No. 1, August 11, 2020, doi: 10.3847 / 1538-4357 / ab9c1c
    Michelle Starr: We Just Found The Fastest Star in The Milky Way, Traveling at 8% The Speed ​​of Light , on: science alert , August 13, 2020
  21. ↑ Nadja Podbregar: The fastest star in the Milky Way discovered , on: scinexx.de from August 17, 2020
  22. ↑ Torn by the black hole. European Southern Observatory, July 17, 2013 (press release).
  23. ↑ Anna Ciurlo, Andrea Ghez, Alexander Stephan a. a .: A population of dust-enshrouded objects orbiting the Galactic black hole , Nature, Volume 577, January 15, 2020, pp. 337-340, abstract
  24. ↑ Jean-Pierre Maillard, Thibaut Paumard, Susan R. Stolovy, François Rigaud: The nature of the Galactic Center source IRS 13 revealed by high spatial resolution in the infrared . In: Astron. and Astrophys. Volume 423, 2004, p. 155, arxiv : astro-ph / 0404450 . New black hole discovered . Raumfahrer.net, November 9, 2004.
  25. ↑ 2005 . After a lecture by Michael Muno.