This article is about the astronomical point in the Milky Way. For the general concept, seeGalaxy § Center. 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 CenterA starchart of the night sky towards the Galactic Center
There are around 10 millionstars within oneparsec of the Galactic Center, dominated byred giants, with a significant population of massivesupergiants andWolf–Rayet stars from star formation in the region around 1 million years ago. The core stars are a small part within the much wider central region, calledgalactic bulge.
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 telescope on 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 dust along 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, andradio wavelengths.
In the early 1940sWalter Baade atMount Wilson Observatory took advantage ofwartime blackout conditions 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.[9] This gap has been known asBaade's Window ever since.[10]
AtDover Heights in 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 A andCentaurus 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.[11]
In July 2022, astronomers reported the discovery of massive amounts ofprebiotic molecules, including some associated withRNA, in the Galactic Center of the Milky Way galaxy.[13][14]
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 System and the Galactic Center is not certain,[15] although estimates since 2000 have remained within the range 24–28.4kilolight-years (7.4–8.7kiloparsecs).[16] The latest estimates from geometric-based methods andstandard candles yield the following distances to the Galactic Center:
An accurate determination of the distance to the Galactic Center as established fromvariable stars (e.g.RR Lyrae variables) orstandard candles (e.g.red-clump stars) 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 bulge owing tointerstellar extinction; and an uncertainty in characterizing how a mean distance to a group ofvariable stars found in the direction of the Galactic bulge relates to the distance to the Galactic Center.[26][27]
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°.[24][26][28] Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.[29] The bar is delineated by red-clump stars (see alsored giant); however,RR Lyrae variables do not trace a prominent Galactic bar.[26][30][31] The bar may be surrounded by a ring called the5-kpc ring that contains a large fraction of the molecular hydrogen present in the Milky Way, and most of the Milky Way'sstar formation activity. Viewed from theAndromeda Galaxy, it would be the brightest feature of the Milky Way.[32]
A study in 2008 which linkedradio telescopes in Hawaii, Arizona and California (Very-long-baseline interferometry) measured the diameter of Sagittarius A* to be 44 million kilometers (0.3AU).[4][34] For comparison, the radius of Earth's orbit around theSun is about 150 million kilometers (1.0AU), whereas the distance ofMercury from 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 Physics in 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[35][36] or 4.1 million solar masses.[25]
On 5 January 2015, NASA reported observing anX-ray flare 400 times brighter than usual, a record-breaker, from Sagittarius A*. The unusual event may have been caused by the breaking apart of anasteroid falling into the black hole or by the entanglement ofmagnetic field lines within gas flowing into Sagittarius A*, according to astronomers.[37]
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 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.[38] TermedFermi oreRosita bubbles,[39] they extend up to about 25,000light years above and below the Galactic Center.[38] 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.[38] The 2014Bruno Rossi Prize went toTracy Slatyer,Douglas Finkbeiner, and Meng Su "for their discovery, in gamma rays, of the large unanticipated Galactic structure called theFermi bubbles".[40]
The origin of the bubbles is being researched.[41][42] The bubbles are connected and seemingly coupled, via energy transport, to the galactic core by columnar structures of energetic plasma termedchimneys.[43] In 2020, for the first time, the lobes were seen in visible light[44] and optical measurements were made.[45] By 2022, detailed computer simulations further confirmed that the bubbles were caused by the Sagittarius A* black hole.[46][39]
The central cubicparsec around Sagittarius A* contains around 10 millionstars.[47] Although most of them are old redgiant stars, the Galactic Center is also rich inmassive stars. More than 100OB andWolf–Rayet stars have been identified there so far.[48] They seem to have all been formed in a singlestar formation event a few million years ago. The existence of these relatively young stars was a surprise to experts, who expected thetidal forces from the central black hole to prevent their formation.[49]
Thisparadox of youth is even stronger for stars that are on very tight orbits around Sagittarius A*, such asS2 andS0-102. The scenarios invoked to explain this formation involve either star formation in a massivestar cluster offset from the Galactic Center that would have migrated to its current location once formed, or star formation within a massive, compact gasaccretion disk around 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.[50] Most of these 100 young, massive stars seem to be concentrated within one or two disks, rather than randomly distributed within the central parsec.[51][52] 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-year region around the Galactic Center has revealed an accumulating ring with a mass several million times that of theSun and near the critical density forstar formation.
They predict that in approximately 200 million years, there will be an episode ofstarburst in 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.[53]
Several suggestions have been put forward to explain this puzzling observation, but none is completely satisfactory.[54][55] 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.
The galactic center is suspected to have a large population ofstellar mass black holes. There are probably around 25,000 stellar massblack holes in the central parsecs of the galactic center as a result of dynamical friction and migration.[56][57] Theses black holes has a major effect on thestellar population of the galactic center and theS cluster. They limit the number of massiveO-type stars through stellar collisions.[58]
In May 2021, NASA published new images of the Galactic Center, based on surveys fromChandra X-ray Observatory and other telescopes.[59] 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 Observatory and other telescopes. In the first image,X-rays from Chandra are orange, green, and purple, showing different X-ray energies, and the radio data fromMeerKAT are 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[60]
Red giant stars coexist with white, Sun-like stars.[61]
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 onESO'sVery Large Telescope
Infra-red image of the center of the Milky Way revealing a new population of massive stars
^Baade, W (1946). "A Search for the Nucleus of Our Galaxy".Publications of the Astronomical Society of the Pacific.58 (343): 249.Bibcode:1946PASP...58..249B.doi:10.1086/125835.
^Ng, Y. K; Bertelli, G; Chiosi, C; Bressan, A (1996). "The galactic structure towards the Galactic Center. III. A study of Baade's Window: Discovery of the bar population?".Astronomy and Astrophysics.310: 771.Bibcode:1996A&A...310..771N.
^Malkin, Zinovy (2012). "The current best estimate of the Galactocentric distance of the Sun based on comparison of different statistical techniques".arXiv:1202.6128 [astro-ph.GA].
^Alcock, C.; Allsman, R. A.; Alves, D. R.; Axelrod, T. S.; Becker, A. C.; Basu, A.; Baskett, L.; Bennett, D. P.; Cook, K. H.; Freeman, K. C.; Griest, K.; Guern, J. A.; Lehner, M. J.; Marshall, S. L.; Minniti, D.; Peterson, B. A.; Pratt, M. R.; Quinn, P. J.; Rodgers, A. W.; Stubbs, C. W.; Sutherland, W.; Vandehei, T.; Welch, D. L. (January 1998). "The RR Lyrae Population of the Galactic Bulge from the MACHO Database: Mean Colors and Magnitudes".The Astrophysical Journal.492 (1):190–199.arXiv:astro-ph/9706292.Bibcode:1998ApJ...492..190A.doi:10.1086/305017.S2CID16244436.
^Schödel, R.; Ott, T.; Genzel, R.; Hofmann, R.; Lehnert, M.; Eckart, A.; Mouawad, N.; Alexander, T.; Reid, M. J.; Lenzen, R.; Hartung, M.; Lacombe, F.; Rouan, D.; Gendron, E.; Rousset, G.; Lagrange, A.-M.; Brandner, W.; Ageorges, N.; Lidman, C.; Moorwood, A. F. M.; Spyromilio, J.; Hubin, N.; Menten, K. M. (October 2002). "A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way".Nature.419 (6908):694–696.arXiv:astro-ph/0210426.Bibcode:2002Natur.419..694S.doi:10.1038/nature01121.PMID12384690.S2CID4302128.
^Merritt, David (May 2011). Morris, Mark; Wang, Daniel Q.; Yuan, Feng (eds.). "Dynamical Models of the Galactic Center".The Galactic Center: A Window to the Nuclear Environment of Disk Galaxies. The Galactic Center: A Window on the Nuclear Environment of Disk Galaxies.439. San Francisco: 142.arXiv:1001.5435.Bibcode:2011ASPC..439..142M.
![A small portion of a gigapixel color mosaic of the Milky Way's heart[60]](/mt/?noimg=&dark=on&url=https%3a%2f%2fwikipedia.org%2fwiki%2fFile%3aLights_out_in_the_galactic_centre.jpg)
![Red giant stars coexist with white, Sun-like stars.[61]](/mt/?noimg=&dark=on&url=https%3a%2f%2fwikipedia.org%2fwiki%2fFile%3aHubble_captures_glittering_crowded_hub_of_our_Milky_Way.jpg)
![White Dwarfs in Milky Way's Central Hub[62]](/mt/?noimg=&dark=on&url=https%3a%2f%2fwikipedia.org%2fwiki%2fFile%3aHubble_Spots_White_Dwarfs_in_Milky_Way%2527s_Central_Hub.jpg)
![Detection of an unusually bright X-ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy[37]](/mt/?noimg=&dark=on&url=https%3a%2f%2fwikipedia.org%2fwiki%2fFile%3aX-RayFlare-BlackHole-MilkyWay-20140105.jpg)
