Galactic Center
Template:Short description Template:About Template:Use American English Template:Use dmy dates
The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy.<ref name="NYT-20220131">Template:Cite news</ref><ref name="ARX-20220128">Template:Cite journal</ref> Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*,<ref name="GRAVITY"/><ref name="EHS"/><ref name="gillessenetal2009"/> a compact radio source which is almost exactly at the galactic rotational center.Template:Clarification needed The Galactic Center is approximately Template:Convert away from Earth<ref name="GRAVITY" /> in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest, visually close to the Butterfly Cluster (M6) or the star Shaula, south to the Pipe Nebula.
There are around 10 million stars within one parsec of the Galactic Center, dominated by red giants, with a significant population of massive supergiants and Wolf–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, called galactic bulge.
DiscoveryEdit
Because of interstellar dust along the line of sight, the Galactic Center cannot be studied at visible, ultraviolet, or soft (low-energy) X-ray wavelengths. The available information about the Galactic Center comes from observations at gamma ray, hard (high-energy) X-ray, infrared, submillimetre, and radio wavelengths.
Immanuel Kant stated in Universal Natural History and Theory of the Heavens (1755) that a large star was at the center of the Milky Way galaxy, and that Sirius might be the star.<ref name="ley196508">Template:Cite magazine</ref> Harlow Shapley stated in 1918 that the halo of globular clusters surrounding the Milky Way seemed to be centered on the star swarms in the constellation of Sagittarius, but the dark molecular clouds in the area blocked the view for optical astronomy.<ref name=shapley>Template:Cite journal</ref>
In the early 1940s Walter Baade at Mount Wilson Observatory took advantage of wartime blackout conditions in nearby Los Angeles, to conduct a search for the center with the Template:Convert Hooker Telescope. He found that near the star Alnasl (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.<ref name=baade>Template:Cite journal</ref> This gap has been known as Baade's Window ever since.<ref name=ng>Template:Cite journal</ref>
At Dover Heights in Sydney, Australia, a team of radio astronomers from the Division of Radiophysics at the CSIRO, led by Joseph Lade Pawsey, used "sea interferometry" to discover some of the first interstellar and intergalactic radio sources, including Taurus A, Virgo A and Centaurus A. By 1954 they had built an Template:Convert 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 belt Sagittarius 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.<ref name=pawsey>Template:Cite journal</ref>
In 1958 the International Astronomical Union (IAU) decided to adopt the position of Sagittarius A as the true zero coordinate point for the system of galactic latitude and longitude.<ref name=iau>Template:Cite journal</ref> In the equatorial coordinate system the location is: RA Template:RA, Dec Template:DEC (J2000 epoch).
In July 2022, astronomers reported the discovery of massive amounts of prebiotic molecules, including some associated with RNA, in the Galactic Center of the Milky Way galaxy.<ref name="SA-20220708">Template:Cite news</ref><ref name="FASS-20220708">Template:Cite journal</ref>
Distance to the Galactic CenterEdit
The exact distance between the Solar System and the Galactic Center is not certain,<ref name="malkin13">Template:Cite journal Russian original Template:Cite journal</ref> although estimates since 2000 have remained within the range Template:Convert.<ref name="francis14"/> The latest estimates from geometric-based methods and standard candles yield the following distances to the Galactic Center:
- Template:Val or Template:Val (Template:Val)<ref name="francis14">Template:Cite journal</ref>
- Template:Val (Template:Val)<ref name="eisenhauer05">Template:Cite journal</ref>
- Template:Val (Template:Val)<ref name="majaess2009">Template:Cite journal</ref>
- 7.94 or Template:Val (Template:Val)<ref name="distance1">Template:Cite journal</ref><ref name="distance2">Template:Cite journal</ref><ref name="distance3">Template:Cite journal</ref>
- Template:Val or Template:Val (Template:Val)<ref name="malkin12">Template:Cite arXiv</ref>
- Template:Val (Template:Val)<ref name="gillessenetal2009">Template:Cite journal</ref>
- Template:Val (Template:Val)<ref name="camarillo2018">Template:Cite journal</ref>
- Template:Val (Template:Val)<ref name="vanhollebeke09">Template:Cite journal</ref>
- Template:Val (Template:Val)<ref name=Abuter>Template:Cite journal</ref>
- Template:Val kpc (Template:Val)<ref name="GRAVITY">Template:Cite journal</ref>
An accurate determination of the distance to the Galactic Center as established from variable stars (e.g. RR Lyrae variables) or standard candles (e.g. red-clump stars) is hindered by numerous effects, which include: an ambiguous reddening 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 the Galactic bulge owing to interstellar extinction; and an uncertainty in characterizing how a mean distance to a group of variable stars found in the direction of the Galactic bulge relates to the distance to the Galactic Center.<ref name="majaess10">Template:Cite journal</ref><ref name="vovk">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
The nature of the Milky Way's bar, 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°.<ref name="vanhollebeke09"/><ref name="majaess10"/><ref name="Cabrera-Lavers08">Template:Cite journal</ref> Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.<ref name="nishiyama06">Template:Cite journal</ref> The bar is delineated by red-clump stars (see also red giant); however, RR Lyrae variables do not trace a prominent Galactic bar.<ref name="majaess10"/><ref name="alcock98">Template:Cite journal</ref><ref name="kunder08">Template:Cite journal</ref> The bar may be surrounded by a ring called the 5-kpc ring that contains a large fraction of the molecular hydrogen present in the Milky Way, and most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.<ref name="fn14">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Supermassive black holeEdit
The complex astronomical radio source Sagittarius A appears to be located almost exactly at the Galactic Center and contains an intense compact radio source, Sagittarius A*, which coincides with a supermassive black hole at the center of the Milky Way. Accretion of gas onto the black hole, probably involving an accretion disk around it, would release energy to power the radio source, itself much larger than the black hole.
A study in 2008 which linked radio telescopes in Hawaii, Arizona and California (Very-long-baseline interferometry) measured the diameter of Sagittarius A* to be 44 million kilometers (0.3 AU).<ref name="EHS">Template:Cite journal</ref><ref name="BBHF">Template:Cite journal</ref> For comparison, the radius of Earth's orbit around the Sun is about 150 million kilometers (1.0 AU), whereas the distance of Mercury 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 the Max 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 million solar masses.<ref name="gillessenetal2009"/> Later studies have estimated a mass of 3.7 million<ref name=Ghez>Template:Cite journal</ref><ref name=Schödel>Template:Cite journal</ref> or 4.1 million solar masses.<ref name=Abuter />
On 5 January 2015, NASA reported observing an X-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 an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A*, according to astronomers.<ref name="NASA-20150105">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Gamma- and X-ray emitting Fermi bubblesEdit
Template:Multiple image In November 2010, it was announced that two large elliptical lobe structures of energetic plasma, termed bubbles, which emit gamma- and X-rays, were detected astride the Milky Way galaxy's core.<ref name="bubbles">Template:Cite news</ref> Termed Fermi or eRosita bubbles,<ref name="relics">Template:Cite journal</ref> they extend up to about 25,000 light years above and below the Galactic Center.<ref name="bubbles"/> 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.<ref name="bubbles"/> The 2014 Bruno Rossi Prize went to Tracy Slatyer, Douglas Finkbeiner, and Meng Su "for their discovery, in gamma rays, of the large unanticipated Galactic structure called the Fermi bubbles".<ref name="Rossi2014">Template:Cite news</ref>
The origin of the bubbles is being researched.<ref name="origin">Template:Cite journal</ref><ref>Template:Cite news</ref> The bubbles are connected and seemingly coupled, via energy transport, to the galactic core by columnar structures of energetic plasma termed chimneys.<ref name="chimneys">Template:Cite journal</ref> In 2020, for the first time, the lobes were seen in visible light<ref>Template:Cite journal</ref> and optical measurements were made.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> By 2022, detailed computer simulations further confirmed that the bubbles were caused by the Sagittarius A* black hole.<ref>Template:Cite news</ref><ref name="relics"/>
Stellar populationEdit
The central cubic parsec around Sagittarius A* contains around 10 million stars.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Although most of them are old red giant stars, the Galactic Center is also rich in massive stars. More than 100 OB and Wolf–Rayet stars have been identified there so far.<ref>Template:Cite journal</ref> They seem to have all been formed in a single star formation event a few million years ago. The existence of these relatively young stars was a surprise to experts, who expected the tidal forces from the central black hole to prevent their formation.<ref>Template:Cite journal</ref>
This paradox of youth is even stronger for stars that are on very tight orbits around Sagittarius A*, such as S2 and S0-102. The scenarios invoked to explain this formation involve either star formation in a massive star cluster offset from the Galactic Center that would have migrated to its current location once formed, or star formation within a massive, compact gas accretion 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.<ref>Template:Cite journal</ref> Most of these 100 young, massive stars seem to be concentrated within one or two disks, rather than randomly distributed within the central parsec.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 the Sun and near the critical density for star formation.
They predict that in approximately 200 million years, there will be an episode of starburst 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 galactic relativistic jets, as matter falls into the central black 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-called Bahcall–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", or core, around the black hole.<ref>Template:Cite journal</ref>
Several suggestions have been put forward to explain this puzzling observation, but none is completely satisfactory.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.
Stellar black holesEdit
The galactic center is suspected to have a large population of stellar mass black holes. There are probably around 25,000 stellar mass black holes in the central parsecs of the galactic center as a result of dynamical friction and migration.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Theses black holes has a major effect on the stellar population of the galactic center and the S cluster. They limit the number of massive O-type stars through stellar collisions.<ref>Template:Cite journal</ref>
GalleryEdit
In May 2021, NASA published new images of the Galactic Center, based on surveys from Chandra X-ray Observatory and other telescopes.<ref>Template:Cite journal</ref> Images are about 2.2 degrees (1,000 light years) across and 4.2 degrees (2,000 light years) long.
- Lights out in the galactic centre.jpg
CitationClass=web }}</ref>
- Hubble captures glittering crowded hub of our Milky Way.jpg
CitationClass=web }}</ref>
- Hubble Spots White Dwarfs in Milky Way's Central Hub.jpg
CitationClass=web }}</ref>
- Center Milky Way.jpg
The center of the Milky Way – image taken by ISAAC, the VLT's near- and mid-infrared spectrometer and camera
- Milky Way IR Spitzer.jpg
Infrared image from Spitzer Space Telescope
- Milky way 2 md.jpg
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.
- Centre of the Milky Way.jpg
The central parts of the Milky Way, as observed in the near-infrared with the NACO instrument on ESO's Very Large Telescope
- An Infrared View of the Galaxy.jpg
Infra-red image of the center of the Milky Way revealing a new population of massive stars
- X-RayFlare-BlackHole-MilkyWay-20140105.jpg
Detection of an unusually bright X-ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy<ref name="NASA-20150105" />
- Space SKA telescope image of Galactic Center.jpg
The center of the Milky Way, as imaged by 64 radio telescopes of the South African MeerKAT array
See alsoEdit
Notes and referencesEdit
Further readingEdit
Press
- {{#invoke:citation/CS1|citation
|CitationClass=web }}
External linksEdit
- UCLA Galactic Center Group Template:Webarchive
- Max Planck Institute for Extraterrestrial Physics Galactic Center Group
- The Galactic Supermassive Black Hole
- The Black Hole at the Center of the Milky Way
- The dark heart of the Milky Way
- Animation showing orbits of stars near the center of the Milky Way galaxy
- Zooming in on the center of the Milky Way
- Dramatic Increase in Supernova Explosions Looms
- APOD:
- A simulation of the stars orbiting the Milky Way's central massive black hole
- Galactic Center on arxiv.org