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Galactic Center

Coordinates:Sky map17h45m40.04s,−29° 00′ 28.1″
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This article is about the astronomical point in the Milky Way. For the general concept, seeGalaxy § Center.For wider central region, seeGalactic bulge.For the book series, seeGalactic Center Saga.

The Galactic Center, as seen by one of the 2MASS infrared telescopes, is located in the bright upper left portion of the image.
Marked location of the Galactic Center
A starchart of the night sky towards the Galactic Center

TheGalactic Centeris thebarycenterof theMilky Wayand a corresponding point on therotational axisof the galaxy.[1][2]Itscentral massive objectis asupermassive black holeof about 4 millionsolar masses,which is calledSagittarius A*,[3][4][5]a compactradio sourcewhich is almost exactly at the galactic rotational center.[clarification needed]The Galactic Center is approximately 8 kiloparsecs (26,000 ly) away from Earth[3]in the direction of theconstellationsSagittarius,Ophiuchus,andScorpius,where the Milky Way appears brightest, visually close to theButterfly Cluster(M6) or the starShaula,south to thePipe Nebula.

There are around 10 millionstarswithin oneparsecof the Galactic Center, dominated byred giants,with a significant population of massivesupergiantsandWolf–Rayet starsfrom star formation in the region around 1 million years ago. The core stars are a small part within the much widergalactic bulge.

Discovery[edit]

This pan video gives a closer look at a huge image of the central parts of the Milky Way made by combining thousands of images from ESO'sVISTA telescopeon Paranal in Chile and compares it with the view in visible light. Because VISTA has a camera sensitive to infrared light, it can see through much of the dust blocking the view in visible light, although many more opaque dust filaments still show up well in this picture.

Because ofinterstellar dustalong the line of sight, the Galactic Center cannot be studied atvisible,ultraviolet,or soft (low-energy)X-raywavelengths.The available information about the Galactic Center comes from observations atgamma ray,hard (high-energy) X-ray,infrared,submillimetre, andradiowavelengths.

Immanuel Kantstated inUniversal Natural History and Theory of the Heavens(1755) that a large star was at the center of the Milky Way Galaxy, and thatSiriusmight be the star.[6]Harlow Shapleystated in 1918 that the halo ofglobular clusterssurrounding the Milky Way seemed to be centered on the star swarms in the constellation of Sagittarius, but the darkmolecular cloudsin the area blocked the view for optical astronomy.[7]

In the early 1940sWalter BaadeatMount Wilson Observatorytook advantage ofwartime blackoutconditions in nearby Los Angeles, to conduct a search for the center with the 100-inch (250 cm)Hooker Telescope.He found that near the starAlnasl(Gamma Sagittarii), there is a one-degree-wide void in the interstellar dust lanes, which provides a relatively clear view of the swarms of stars around the nucleus of the Milky Way Galaxy.[8]This gap has been known asBaade's Windowever since.[9]

AtDover Heightsin Sydney, Australia, a team of radio astronomers from the Division of Radiophysics at theCSIRO,led byJoseph Lade Pawsey,used "sea interferometry"to discover some of the first interstellar and intergalactic radio sources, includingTaurus A,Virgo AandCentaurus A.By 1954 they had built an 80-foot (24 m) fixed dish antenna and used it to make a detailed study of an extended, extremely powerful belt of radio emission that was detected in Sagittarius. They named an intense point-source near the center of this beltSagittarius A,and realised that it was located at the very center of the Galaxy, despite being some 32 degrees south-west of the conjectured galactic center of the time.[10]

In 1958 theInternational Astronomical Union(IAU) decided to adopt the position of Sagittarius A as the true zero coordinate point for the system ofgalactic latitude and longitude.[11]In theequatorial coordinate systemthe location is:RA17h45m40.04s,Dec−29° 00′ 28.1″ (J2000epoch).

In July 2022, astronomers reported the discovery of massive amounts ofprebiotic molecules,including some associated withRNA,in the Galactic Center of theMilky Way Galaxy.[12][13]

Distance to the Galactic Center[edit]

Animation of a barred galaxy like the Milky Way showing the presence of an X-shaped bulge. The X-shape extends to about one half of the bar radius. It is directly visible when the bar is seen from the side, but when the viewer is close to the long axis of the bar it cannot be seen directly and its presence can only be inferred from the distribution of brightnesses of stars along a given direction.

The exact distance between theSolar Systemand the Galactic Center is not certain,[14]although estimates since 2000 have remained within the range 24–28.4kilolight-years(7.4–8.7kiloparsecs).[15]The latest estimates from geometric-based methods andstandard candlesyield the following distances to the Galactic Center:

  • 7.4±0.2(stat) ± 0.2(syst)or7.4±0.3 kpc(≈24±1kly)[15]
  • 7.62±0.32 kpc(≈24.8±1 kly)[16]
  • 7.7±0.7 kpc(≈25.1±2.3 kly)[17]
  • 7.94 or8.0±0.5 kpc(≈26±1.6 kly)[18][19][20]
  • 7.98±0.15(stat) ± 0.20(syst)or8.0±0.25 kpc(≈26±0.8 kly)[21]
  • 8.33±0.35 kpc(≈27±1.1 kly)[5]
  • 8.0±0.3 kpc(≈25.96±0.98 kly)[22]
  • 8.7±0.5 kpc(≈28.4±1.6 kly)[23]
  • 8.122±0.031 kpc(≈26.49±0.1 kly)[24]
  • 8.178±0.013(stat) ± 0.022(syst)kpc (≈26.67±0.1 kly)[3]

An accurate determination of the distance to the Galactic Center as established fromvariable stars(e.g.RR Lyrae variables) orstandard candles(e.g.red-clumpstars) is hindered by numerous effects, which include: an ambiguousreddening law;a bias for smaller values of the distance to the Galactic Center because of a preferential sampling of stars toward the near side of theGalactic bulgeowing tointerstellar extinction;and an uncertainty in characterizing how a mean distance to a group ofvariable starsfound in the direction of the Galactic bulge relates to the distance to the Galactic Center.[25][26]

The nature of the Milky Way'sbar,which extends across the Galactic Center, is also actively debated, with estimates for its half-length and orientation spanning between 1–5 kpc (short or a long bar) and 10–50°.[23][25][27]Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.[28]The bar is delineated by red-clump stars (see alsored giant); however,RR Lyrae variablesdo not trace a prominent Galactic bar.[25][29][30]The bar may be surrounded by a ring called the5-kpcringthat contains a large fraction of the molecular hydrogen present in the Milky Way, and most of the Milky Way'sstar formationactivity. Viewed from theAndromeda Galaxy,it would be the brightest feature of the Milky Way.[31]

Supermassive black hole[edit]

Thesupermassive black holeSagittarius A*,imaged by theEvent Horizon Telescope.[32]

The complexastronomical radio sourceSagittarius Aappears to be located almost exactly at the Galactic Center and contains an intense compact radio source,Sagittarius A*,which coincides with asupermassive black holeat the center of the Milky Way.Accretionof gas onto theblack hole,probably involving anaccretion diskaround it, would release energy to power the radio source, itself much larger than the black hole.

A study in 2008 which linkedradio telescopesin Hawaii, Arizona and California (Very-long-baseline interferometry) measured the diameter of Sagittarius A* to be 44 million kilometers (0.3AU).[4][33]For comparison, the radius of Earth's orbit around theSunis about 150 million kilometers (1.0AU), whereas the distance ofMercuryfrom the Sun at closest approach (perihelion) is 46 million kilometers (0.3 AU). Thus, the diameter of the radio source is slightly less than the distance from Mercury to the Sun.

Scientists at theMax Planck Institute for Extraterrestrial Physicsin Germany using Chilean telescopes have confirmed the existence of a supermassive black hole at the Galactic Center, on the order of 4.3 millionsolar masses.[5]Later studies have estimated a mass of 3.7 million[34][35]or 4.1 million solar masses.[24]

On 5 January 2015, NASA reported observing anX-rayflare 400 times brighter than usual, a record-breaker, from Sagittarius A*. The unusual event may have been caused by the breaking apart of anasteroidfalling into the black hole or by the entanglement ofmagnetic field lineswithin gas flowing into Sagittarius A*, according to astronomers.[36]

There is a supermassive black hole in the bright white area to the right of the center of this wide (scrollable) image. This composite photograph covers about half of a degree.

Gamma- and X-ray emitting Fermi bubbles[edit]

Galactic gamma- and X-ray bubbles
Gamma- and X-ray bubbles at the Milky Way galaxy center: Top: illustration; Bottom: video.

In November 2010, it was announced that two large elliptical lobe structures of energeticplasma,termedbubbles,which emit gamma- and X-rays, were detected astride the Milky Way galaxy's core.[37]TermedFermioreRositabubbles,[38]they extend up to about 25,000light yearsabove and below the Galactic Center.[37]The galaxy's diffuse gamma-ray fog hampered prior observations, but the discovery team led by D. Finkbeiner, building on research by G. Dobler, worked around this problem.[37]The 2014Bruno Rossi Prizewent toTracy Slatyer,Douglas Finkbeiner, and Meng Su "for their discovery, in gamma rays, of the large unanticipated Galactic structure called theFermi bubbles".[39]

The origin of the bubbles is being researched.[40][41]The bubbles are connected and seemingly coupled, via energy transport, to the galactic core by columnar structures of energetic plasma termedchimneys.[42]In 2020, for the first time, the lobes were seen in visible light[43]and optical measurements were made.[44]By 2022, detailed computer simulations further confirmed that the bubbles were caused by the Sagittarius A* black hole.[45][38]

Stellar population[edit]

The Galactic Center of theMilky Wayand ameteor

The central cubicparsecaround Sagittarius A* contains around 10 millionstars.[46]Although most of them are old redgiant stars,the Galactic Center is also rich inmassive stars.More than 100OBandWolf–Rayet starshave been identified there so far.[47]They seem to have all been formed in a singlestar formationevent a few million years ago. The existence of these relatively young stars was a surprise to experts, who expected thetidal forcesfrom the central black hole to prevent their formation.[48]

Thisparadox of youthis even stronger for stars that are on very tight orbits around Sagittarius A*, such asS2andS0-102.The scenarios invoked to explain this formation involve either star formation in a massivestar clusteroffset from the Galactic Center that would have migrated to its current location once formed, or star formation within a massive, compact gasaccretion diskaround the central black-hole. Current evidence favors the latter theory, as formation through a large accretion disk is more likely to lead to the observed discrete edge of the young stellar cluster at roughly 0.5 parsec.[49]Most of these 100 young, massive stars seem to be concentrated within one or two disks, rather than randomly distributed within the central parsec.[50][51]This observation however does not allow definite conclusions to be drawn at this point.

Star formation does not seem to be occurring currently at the Galactic Center, although the Circumnuclear Disk of molecular gas that orbits the Galactic Center at two parsecs seems a fairly favorable site for star formation. Work presented in 2002 by Antony Stark and Chris Martin mapping the gas density in a 400-light-yearregion around the Galactic Center has revealed an accumulating ring with a mass several million times that of theSunand near the critical density forstar formation.

They predict that in approximately 200 million years, there will be an episode ofstarburstin the Galactic Center, with many stars forming rapidly and undergoing supernovae at a hundred times the current rate. This starburst may also be accompanied by the formation of galacticrelativistic jets,as matter falls into the centralblack hole.It is thought that the Milky Way undergoes a starburst of this sort every 500 million years.

In addition to the paradox of youth, there is a "conundrum of old age" associated with the distribution of the old stars at the Galactic Center. Theoretical models had predicted that the old stars—which far outnumber young stars—should have a steeply-rising density near the black hole, a so-calledBahcall–Wolf cusp.Instead, it was discovered in 2009 that the density of the old stars peaks at a distance of roughly 0.5 parsec from Sgr A*, then falls inward: instead of a dense cluster, there is a "hole", orcore,around the black hole.[52]

Several suggestions have been put forward to explain this puzzling observation, but none is completely satisfactory.[53][54]For instance, although the black hole would eat stars near it, creating a region of low density, this region would be much smaller than a parsec. Because the observed stars are a fraction of the total number, it is theoretically possible that the overall stellar distribution is different from what is observed, although no plausible models of this sort have been proposed yet.

Gallery[edit]

In May 2021, NASA published new images of the Galactic Center, based on surveys fromChandra X-ray Observatoryand other telescopes.[55]Images are about 2.2 degrees (1,000 light years) across and 4.2 degrees (2,000 light years) long.

A panorama of the Galactic Center builds on previous surveys fromChandra X-ray Observatoryand other telescopes. In the first image,X-raysfrom Chandra are orange, green, and purple, showing different X-ray energies, and the radio data fromMeerKATare gray. The next images show single (broadband) colors, with Chandra X-ray data in pink and MeerKAT radio data in blue.
Composite labeled image.
Composite image.
X-ray and Radio single color composite.
Radio single color.
  • A small portion of a gigapixel color mosaic of the Milky Way's heart.[56]
    A small portion of a gigapixel color mosaic of the Milky Way's heart.[56]
  • Red giant stars coexist with white, Sun-like stars.[57]
    Red giant stars coexist with white, Sun-like stars.[57]
  • White Dwarfs in Milky Way's Central Hub.[58]
    White Dwarfs in Milky Way's Central Hub.[58]
  • The center of the Milky Way – image taken by ISAAC, the VLT's near- and mid-infrared spectrometer and camera.
    The center of the Milky Way – image taken by ISAAC, theVLT's near- and mid-infrared spectrometer and camera.
  • Infrared image from Spitzer Space Telescope.
    Infrared image fromSpitzer Space Telescope.
  • A view of the night sky near Sagittarius, enhanced to show better contrast and detail in the dust lanes. The principal stars in Sagittarius are indicated in red.
    A view of the night sky nearSagittarius,enhanced to show better contrast and detail in the dust lanes. The principal stars in Sagittarius are indicated in red.
  • The central parts of the Milky Way, as observed in the near-infrared with the NACO instrument on ESO's Very Large Telescope.
    The central parts of the Milky Way, as observed in the near-infrared with the NACO instrument onESO'sVery Large Telescope.
  • Infra-red image of the center of the Milky Way revealing a new population of massive stars.
    Infra-red image of the center of the Milky Way revealing a new population of massive stars.
  • Detection of an unusually bright X-ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy.[36]
    Detection of an unusually brightX-rayflare fromSagittarius A*,asupermassive black holein the center of theMilky Waygalaxy.[36]
  • The center of the Milky Way, as imaged by 64 radio telescopes of the South African MeerKAT array.
    The center of the Milky Way, as imaged by 64 radio telescopes of the South AfricanMeerKATarray.
The surroundings of the Galactic Center (Top view map).
The surroundings of the Galactic Center (Top view map).

See also[edit]

Notes and references[edit]

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