Galactic Center

The Galactic Center, or Galactic Centre, is the rotational center of the Milky Way. It is 8,122 ± 31 parsecs (26,490 ± 100 ly) away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest. It coincides with the compact radio source Sagittarius A*.

There are around 10 million stars within one parsec of the Galactic Center, dominated by red giants. There is also a significant population of massive supergiants and Wolf-Rayet stars from a star formation event around one million years ago. There is a supermassive black hole of 4.100 ± 0.034 million solar masses at the Galactic Center, which powers the Sagittarius A* radio source.

Galactic Cntr full cropped
The Galactic Center, as seen by one of the 2MASS infrared telescopes, is located in the bright upper left portion of the image.


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's VISTA 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 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, sub-millimetre and radio wavelengths.

Immanuel Kant stated in General 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.[1] 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 astronomers.[2] 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 100-inch (250 cm) 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 our Milky Way Galaxy.[3] This gap has been known as Baade's Window ever since.[4]

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 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 belt Sagittarius A, and realised that it was located at the very center of our Galaxy, despite being some 32 degrees south-west of the conjectured galactic center of the time.[5]

In 1958 the International Astronomical Union (IAU) decided to adopt the position of Sagittarius A as the true zero co-ordinate point for the system of galactic latitude and longitude.[6] In the equatorial coordinate system the location is: RA  17h 45m 40.04s, Dec −29° 00′ 28.1″ (J2000 epoch).

Distance to the Galactic Center

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 the Solar System and the Galactic Center is not certain,[7] although estimates since 2000 have remained within the range 24–28.4 kilolight-years (7.4–8.7 kiloparsecs).[8] The latest estimates from geometric-based methods and standard candles yield the following distances to the Galactic Center:

  • 7.4±0.2(stat) ± 0.2(syst) or 7.4±0.3 kpc (≈24±kly)[8]
  • 7.62±0.32 kpc (≈24.8±1 kly)[9]
  • 7.7±0.7 kpc (≈25.1±2.3 kly)[10]
  • 7.94 or 8.0±0.5 kpc (≈26±1.6 kly)[11][12][13]
  • 7.98±0.15(stat) ± 0.20(syst) or 8.0±0.25 kpc (≈26±0.8 kly)[14]
  • 8.33±0.35 kpc (≈27±1.1 kly)[15]
  • 8.7±0.5 kpc (≈28.4±1.6 kly)[16]

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 countless 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.[17][18]

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°.[16][17][19] Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.[20] The bar is delineated by red-clump stars (see also red giant); however, RR Lyr variables do not trace a prominent Galactic bar.[17][21][22] 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, as well as most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.[23]

Supermassive black hole

Center of the Milky Way Galaxy IV – Composite
There is a supermassive black hole in the bright white area to the right of the center of the image. This composite photograph covers about half of a degree.

The complex astronomical radio source Sagittarius A appears to be located almost exactly at the Galactic Center (approx. 18 hrs, −29 deg), 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 a disk around it, would release energy to power the radio source, itself much larger than the black hole. The latter is too small to see with present instruments.

Artist impression of a supermassive black hole at the centre of a galaxy
Artist impression of a supermassive black hole at the center of a galaxy[24]

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).[25][26] 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.[15]

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.[27]

Stellar population

Milky Way Galaxy and a meteor
Galactic Center of the Milky Way and a meteor

The central cubic parsec around Sagittarius A* contains around 10 million stars.[28] 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.[29] 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. 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.[30] Most of these 100 young, massive stars seem to be concentrated within one or two disks, rather than randomly distributed within the central parsec.[31][32] 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 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 also 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.[33] Several suggestions have been put forward to explain this puzzling observation, but none is completely satisfactory.[34][35] 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 than what is observed, although no plausible models of this sort have yet been proposed.


Lights out in the galactic centre

A small portion of a gigapixel color mosaic of the Milky Way’s heart.[36]

Hubble captures glittering crowded hub of our Milky Way

Red giant stars coexist with white, Sun-like stars.[37]

Hubble Spots White Dwarfs in Milky Way's Central Hub

White Dwarfs in Milky Way's Central Hub[38]

Center Milky Way

The center of the Milky Way - image taken by ISAAC, the VLT's near- and mid-infrared spectrometer and camera

Milky Way IR Spitzer

Infrared image from Spitzer Space Telescope

Milky way 2 md

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

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

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[27]

Space SKA telescope image of Galactic Center

The center of the Milky Way, as imaged by 64 radio telescopes in the South African wilderness (via MeerKAT array).

See also

Notes and references

  1. ^ Ley, Willy (August 1965). "The Galactic Giants". For Your Information. Galaxy Science Fiction. pp. 130–142.
  2. ^ Shapley, H (1918). "Studies based on the colors and magnitudes in stellar clusters. VII. The distances, distribution in space, and dimensions of 69 globular clusters". Astrophysical Journal. 48: 154. Bibcode:1918ApJ....48..154S. doi:10.1086/142423.
  3. ^ 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.
  4. ^ 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.
  5. ^ Pawsey, J. L (1955). "A Catalogue of Reliably Known Discrete Sources of Cosmic Radio Waves". Astrophysical Journal. 121: 1. Bibcode:1955ApJ...121....1P. doi:10.1086/145957.
  6. ^ Blaauw, A.; Gum, C.S.; Pawsey, J.L.; Westerhout, G. (1960). "The new IAU system of galactic coordinates (1958 revision)". Monthly Notices of the Royal Astronomical Society. 121 (2): 123–131. Bibcode:1960MNRAS.121..123B. doi:10.1093/mnras/121.2.123.
  7. ^ Malkin, Zinovy M. (February 2013). "Analysis of Determinations of the Distance between the Sun and the Galactic Center". Astronomy Reports. 57 (2): 128–133. arXiv:1301.7011. Bibcode:2013ARep...57..128M. CiteSeerX doi:10.1134/S1063772913020078. Russian original Малкин, З. М. (2013). "Об определении расстояния от Солнца до центра Галактики". Astronomicheskii Zhurnal (in Russian). 90 (2): 152–157. doi:10.7868/S0004629913020072.
  8. ^ a b Francis, Charles; Anderson, Erik (June 2014). "Two estimates of the distance to the Galactic Centre". Monthly Notices of the Royal Astronomical Society. 441 (2): 1105–1114. arXiv:1309.2629. Bibcode:2014MNRAS.441.1105F. doi:10.1093/mnras/stu631.
  9. ^ Eisenhauer, F.; Genzel, R.; Alexander, T.; Abuter, R.; Paumard, T.; Ott, T.; Gilbert, A.; Gillessen, S.; Horrobin, M.; Trippe, S.; Bonnet, H.; Dumas, C.; Hubin, N.; Kaufer, A.; Kissler-Patig, M.; Monnet, G.; Ströbele, S.; Szeifert, T.; Eckart, A.; Schödel, R.; Zucker, S. (2005). "SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month". Astrophysical Journal. 628 (1): 246–259. arXiv:astro-ph/0502129. Bibcode:2005ApJ...628..246E. doi:10.1086/430667.
  10. ^ Majaess, D.J.; Turner, D.G.; Lane, D.J. (2009). "Characteristics of the Galaxy according to Cepheids". MNRAS. 398 (1): 263–270. arXiv:0903.4206. Bibcode:2009MNRAS.398..263M. doi:10.1111/j.1365-2966.2009.15096.x.
  11. ^ Reid, Mark J. (1993). "The distance to the center of the Galaxy". Annual Review of Astronomy and Astrophysics. 31 (1): 345–372. Bibcode:1993ARA&A..31..345R. doi:10.1146/annurev.aa.31.090193.002021.
  12. ^ Eisenhauer, F.; Schödel, R.; Genzel, R.; Ott, T.; Tecza, M.; Abuter, R.; Eckart, A.; Alexander, T. (2003). "A Geometric Determination of the Distance to the Galactic Center". The Astrophysical Journal. 597 (2): L121–L124. arXiv:astro-ph/0306220. Bibcode:2003ApJ...597L.121E. doi:10.1086/380188.
  13. ^ Horrobin, M.; Eisenhauer, F.; Tecza, M.; Thatte, N.; Genzel, R.; Abuter, R.; Iserlohe, C.; Schreiber, J.; Schegerer, A.; Lutz, D.; Ott, T.; Schödel, R. (2004). "First results from SPIFFI. I: The Galactic Center" (PDF). Astronomische Nachrichten. 325 (2): 120–123. Bibcode:2004AN....325...88H. doi:10.1002/asna.200310181. Archived from the original (PDF) on 2007-06-21. Retrieved 2007-05-10.
  14. ^ Malkin, Zinovy (2012). "The current best estimate of the Galactocentric distance of the Sun based on comparison of different statistical techniques". 1202: arXiv:1202.6128. arXiv:1202.6128. Bibcode:2012arXiv1202.6128M.
  15. ^ a b Gillessen, S.; Eisenhauer; Trippe; Alexander; Genzel; Martins; Ott (2009). "Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center". The Astrophysical Journal. 692 (2): 1075–1109. arXiv:0810.4674. Bibcode:2009ApJ...692.1075G. doi:10.1088/0004-637X/692/2/1075.
  16. ^ a b Vanhollebeke, E.; Groenewegen, M. A. T.; Girardi, L. (April 2009). "Stellar populations in the Galactic bulge. Modelling the Galactic bulge with TRILEGAL". Astronomy and Astrophysics. 498 (1): 95–107. arXiv:0903.0946. Bibcode:2009A&A...498...95V. doi:10.1051/0004-6361/20078472.
  17. ^ a b c Majaess, D (March 2010). "Concerning the Distance to the Center of the Milky Way and Its Structure". Acta Astronomica. 60 (1): 55–74. arXiv:1002.2743. Bibcode:2010AcA....60...55M.
  18. ^ Vovk, Olga "Universe at a glance blog: Milky Way: Distance to the Galactic Centre". April 2011
  19. ^ Cabrera-Lavers, A.; González-Fernández, C.; Garzón, F.; Hammersley, P. L.; López-Corredoira, M. (December 2008). "The long Galactic bar as seen by UKIDSS Galactic plane survey". Astronomy and Astrophysics. 491 (3): 781–787. arXiv:0809.3174. Bibcode:2008A&A...491..781C. doi:10.1051/0004-6361:200810720.
  20. ^ Nishiyama, Shogo; Nagata, Tetsuya; Baba, Daisuke; Haba, Yasuaki; Kadowaki, Ryota; Kato, Daisuke; Kurita, Mikio; Nagashima, Chie; Nagayama, Takahiro; Murai, Yuka; Nakajima, Yasushi; Tamura, Motohide; Nakaya, Hidehiko; Sugitani, Koji; Naoi, Takahiro; Matsunaga, Noriyuki; Tanabé, Toshihiko; Kusakabe, Nobuhiko; Sato, Shuji (March 2005). "A Distinct Structure inside the Galactic Bar". Astrophysical Journal. 621 (2): L105–L108. arXiv:astro-ph/0502058. Bibcode:2005ApJ...621L.105N. doi:10.1086/429291.
  21. ^ 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". Astrophysical Journal. 492 (1): 190–199. arXiv:astro-ph/9706292. Bibcode:1998ApJ...492..190A. doi:10.1086/305017.
  22. ^ Kunder, Andrea; Chaboyer, Brian (December 2008). "Metallicity Analysis of MACHO Galactic Bulge RR0 Lyrae Stars from their Light Curves". The Astronomical Journal. 136 (6): 2441–2452. arXiv:0809.1645. Bibcode:2008AJ....136.2441K. doi:10.1088/0004-6256/136/6/2441.
  23. ^ Staff (September 12, 2005). "Introduction: Galactic Ring Survey". Boston University. Retrieved 2007-05-10.
  24. ^ "ALMA Reveals Intense Magnetic Field Close to Supermassive Black Hole". ESO Press Release. European Southern Observatory. Retrieved 21 April 2015.
  25. ^ Doeleman, Sheperd S.; et al. (2008). "Event-horizon-scale structure in the supermassive black hole candidate at that Galactic Centre". Nature. 455 (7209): 78–80. arXiv:0809.2442. Bibcode:2008Natur.455...78D. doi:10.1038/nature07245. PMID 18769434.
  26. ^ Reynolds, Christopher S. (2008). "Bringing black holes into focus". Nature. 455 (7209): 39–40. Bibcode:2008Natur.455...39R. doi:10.1038/455039a. PMID 18769426.
  27. ^ a b Chou, Felicia; Anderson, Janet; Watzke, Megan (5 January 2015). "RELEASE 15-001 - NASA's Chandra Detects Record-Breaking Outburst from Milky Way's Black Hole". NASA. Retrieved 6 January 2015.
  28. ^ "Lecture 31: The Center of Our Galaxy".
  29. ^ Mauerhan, J. C.; Cotera, A.; Dong, H. (2010). "Isolated Wolf-Rayet Stars and O Supergiants in the Galactic Center Region Identified Via Paschen-α Excess". The Astrophysical Journal. 725 (1): 188–199. arXiv:1009.2769. Bibcode:2010ApJ...725..188M. doi:10.1088/0004-637X/725/1/188.
  30. ^ Støstad, M.; Do, T.; Murray, N.; Lu, J.R.; Yelda, S.; Ghez, A. (2015). "Mapping the Outer Edge of the Young Stellar Cluster in the Galactic Center". The Astrophysical Journal. 808 (2): 106. arXiv:1504.07239. Bibcode:2015ApJ...108..106S. doi:10.1088/0004-637X/808/2/106.
  31. ^ "UCLA Galactic Center Group".
  32. ^ "Galactic Center".
  33. ^ Buchholz, R. M.; Schödel, R.; Eckart, A. (May 2009). "Composition of the galactic center star cluster: Population analysis from adaptive optics narrow band spectral energy distributions". Astronomy and Astrophysics. 499 (2): 483–501. arXiv:0903.2135. Bibcode:2009A&A...499..483B. doi:10.1051/0004-6361/200811497.
  34. ^ 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. San Francisco. 439: 142. arXiv:1001.5435. Bibcode:2011ASPC..439..142M.
  35. ^ Chown, Marcus (Sep 2010). "Something's been eating the stars". New Scientist. 207 (2778): 30–33. Bibcode:2010NewSc.207...30M. doi:10.1016/S0262-4079(10)62278-6.
  36. ^ "Lights out in the galactic centre". Retrieved 30 April 2018.
  37. ^ "Hubble captures glittering crowded hub of our Milky Way". Retrieved 15 January 2018.
  38. ^ "Hubble Spots White Dwarfs in Milky Way's Central Hub". Retrieved 9 November 2015.

Further reading

  • Eckart, A.; Schödel, R.; Straubmeier, C. (2005). The Black Hole at the Center of the Milky Way. London: Imperial College Press. ISBN 978-1-86094-567-0.
  • Melia, Fulvio (2003). The Black Hole in the Center of Our Galaxy. Princeton: Princeton University Press. ISBN 978-0-691-09505-9.
  • Melia, Fulvio (2007). The Galactic Supermassive Black Hole. Princeton: Princeton University Press. ISBN 978-0-691-13129-0.

External links

Coordinates: Sky map 17h 45m 40.04s, −29° 00′ 28.1″

Baade's Window

Baade's Window is an area of the sky with relatively low amounts of interstellar "dust" along the line of sight from the Earth. This area is considered an observational "window" as the normally obscured Galactic Center of the Milky Way is visible in this direction. It is named for astronomer Walter Baade who first recognized its significance. This area corresponds to one of the brightest visible patches of the Milky Way.


GCIRS 8* (Galactic Centre IRS 8*) is a young massive star in the Galactic Center region, discovered in May 2006. IRS 8 is an infra-red source identified as a bowshock. The star causing the bowshock has been classified as an O5-O6 giant or supergiant several hundred thousand times as luminous as the sun. It is estimated to be 3.5 million years old, although if it is a binary then it would probably be older. The mass is estimated at 38 M☉.

GCRT J1745-3009

GCRT J1745-3009 is a transient, bursting low-frequency radio source which lies in the direction of the galactic center.

Galactic Center Saga

The Galactic Center Saga is a series of books by author Gregory Benford detailing a galactic war between mechanical and biological life.

In the Ocean of Night (1977) — 1977 Nebula Award nominee, 1978 Locus Award nomineeAcross the Sea of Suns (1984)Great Sky River (1987) — 1988 Nebula NomineeTides of Light (1989) — 1990 Locus Award nomineeFurious Gulf (1994)Sailing Bright Eternity (1996)"A Hunger for the Infinite" a novella published in the anthology Far Horizons

Galactic habitable zone

In astrobiology and planetary astrophysics, the galactic habitable zone is the region of a galaxy in which life might most likely develop. More specifically, the concept of a galactic habitable zone incorporates various factors, such as metallicity and the rate of major catastrophes such as supernovae, to calculate which regions of the galaxy are more likely to form terrestrial planets, initially develop simple life, and provide a suitable environment for this life to evolve and advance. According to research published in August 2015, very large galaxies may favor the birth and development of habitable planets more than smaller galaxies such as the Milky Way. In the case of the Milky Way, its galactic habitable zone is commonly believed to be an annulus with an outer radius of about 10 kiloparsecs and an inner radius close to the Galactic Center (with both radii lacking hard boundaries).Galactic habitable-zone theory has been criticized due to an inability to quantify accurately the factors making a region of a galaxy favorable for the emergence of life. In addition, computer simulations suggest that stars may change their orbits around the galactic center significantly, therefore challenging at least part of the view that some galactic areas are necessarily more life-supporting than others.

Galactic plane

The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms galactic plane and galactic poles usually refer specifically to the plane and poles of the Milky Way, in which Planet Earth is located.

Some galaxies are irregular and do not have any well-defined disk. Even in the case of a barred spiral galaxy like the Milky Way, defining the galactic plane is slightly imprecise and arbitrary since the stars are not perfectly coplanar. In 1959, the IAU defined the position of the Milky Way's north galactic pole as exactly RA = 12h 49m, Dec = 27° 24′ in the then-used B1950 epoch; in the currently-used J2000 epoch, after precession is taken into account, its position is RA 12h 51m 26.282s, Dec 27° 07′ 42.01″. This position is in Coma Berenices, near the bright star Arcturus; likewise, the south galactic pole lies in the constellation Sculptor.

The "zero of longitude" of galactic coordinates was also defined in 1959 to be at position angle 123° from the north celestial pole. Thus the zero longitude point on the galactic equator was at 17h 42m 26.603s, −28° 55′ 00.445″ (B1950) or 17h 45m 37.224s, −28° 56′ 10.23″ (J2000), and its J2000 position angle is 122.932°. The galactic center is located at position angle 31.72° (B1950) or 31.40° (J2000) east of north.

Gregory Benford

Gregory Benford (born January 30, 1941) is an American science fiction author and astrophysicist who is Professor Emeritus at the Department of Physics and Astronomy at the University of California, Irvine. He is a contributing editor of Reason magazine.Benford wrote the Galactic Center Saga science fiction novels, beginning with In the Ocean of Night (1977). The series postulates a galaxy in which sentient organic life is in constant warfare with sentient electromechanical life.

In 1969 he wrote "The Scarred Man", the first story about a computer virus, published in 1970.

Messier 107

Messier 107 or M107, also known as NGC 6171, is a very loose globular cluster in the constellation Ophiuchus, and is the last globular cluster in the Messier Catalogue. It was discovered by Pierre Méchain in April 1782, then independently by William Herschel in 1793. Herschel described it as a "globular cluster of stars, large, very rich, very much compressed, round, well resolved, clearly consisting of stars". It was not until 1947 that Helen Sawyer Hogg added it and three other objects discovered by Méchain to the list of Messier objects. The cluster is located 2.5° south and slightly west of the star Zeta Ophiuchi.M107 is close to the galactic plane at a distance of about 20,900 light-years from Earth and 9.8 kly (3.0 kpc) from the Galactic Center. The orbit of this cluster carries it through the galaxy between 9.2–12.4 kly (2.82–3.79 kpc) from the Galactic Center, with the perigalactic distance laying within the galactic bar region. It is an Oosterhoff type I cluster with a metallicity of –0.95 and is considered part of the halo population. There are 22 known RR Lyrae variable stars in this cluster and a probable SX Phoenicis variable.

Messier 37

Messier 37 (also known as M37 or NGC 2099) is the richest open cluster in the constellation Auriga. It is the brightest of three open clusters in Auriga and was discovered by the Italian astronomer Giovanni Battista Hodierna before 1654. M37 was missed by French astronomer Guillaume Le Gentil when he rediscovered M36 and M38 in 1749. French astronomer Charles Messier independently rediscovered M37 in September 1764 but all three clusters were recorded by Hodierna. It is classified as Trumpler type I,1,r or I,2,r.

M37 is located in the antipodal direction, opposite from the Galactic Center as seen from Earth. Estimates of its age range from 347 million to 550 million years. It has 1,500 times the mass of the Sun and contains over 500 identified stars, with roughly 150 stars brighter than magnitude 12.5. M37 has at least a dozen red giants and its hottest surviving main sequence star is of stellar classification B9 V. The abundance of elements other than hydrogen and helium, what astronomers term metallicity, is similar to, if not slightly higher than, the abundance in the Sun.At its estimated distance of around 4,500 light-years (1,400 parsecs) from Earth, the cluster's angular diameter of 24 arcminutes corresponds to a physical extent of about 20–25 ly (6.1–7.7 pc). The tidal radius of the cluster, where external gravitational perturbations begin to have a significant influence on the orbits of its member stars, is about 46–59 ly (14–18 pc). This cluster is following an orbit through the Milky Way with a period of 219.3 Ma and an eccentricity of 0.22. This will bring it as close as 19.6 kly (6.0 kpc) to, and as distant as 30.7 kly (9.4 kpc) from, the Galactic Center. It reaches a peak distance above the galactic plane of 0.29 kly (0.089 kpc) and will cross the plane with a period of 31.7 Ma.

Milky Way

The Milky Way is the galaxy that contains our Solar System. The name describes 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. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος (galaxías kýklos, "milky circle"). From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.

The Milky Way is a barred spiral galaxy with a diameter between 150,000 and 200,000 light-years (ly). It is estimated to contain 100–400 billion stars and more than 100 billion planets. The Solar System is located at a radius of 26,490 (± 100) light-years from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A*, assumed to be a supermassive black hole of 4.100 (± 0.034) million solar masses.

Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much (about 90%) of the mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing electromagnetic radiation. This conjectural mass has been termed "dark matter". The rotational period is about 240 million years at the radius of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.

NGC 17

NGC 17, also known as NGC 34, is a spiral galaxy in the constellation Cetus. It is the result of a merger between two disk galaxies, resulting in a recent starburst in the central regions and continuing starforming activity. The galaxy is still gas-rich, and has a single galactic nucleus. It lies 250 million light years away. It was discovered in 1886 by Frank Muller and then observed again later that year by Lewis Swift.

Due to the major merger event NGC 17 has no defined spiral arms like the Milky Way galaxy. Unlike the Milky Way, the center bar nucleus is also distorted. The merger destroyed any galactic habitable zone that may have been there before the merger. For the Milky Way, the galactic habitable zone is commonly believed to be an annulus with an outer radius of about 10 kiloparsecs and an inner radius close to the Galactic Center, both of which lack hard boundaries.

NGC 2419

NGC 2419 (also known as Caldwell 25) is a globular cluster in the constellation Lynx. It was discovered by William Herschel on December 31, 1788. NGC 2419 is at a distance of about 300,000 light years from the solar system and at the same distance from the galactic center.

NGC 2419 bears the nickname "the Intergalactic Wanderer," which was bestowed when it was erroneously thought not to be in orbit around the Milky Way. Its orbit takes it further away from the galactic center than the Magellanic Clouds, but it can (with qualifications) be considered as part of the Milky Way. At this great distance it takes three billion years to make one trip around the galaxy.The cluster is dim in comparison to more famous globular clusters such as M13. Nonetheless, NGC 2419 is a 9th magnitude object and is readily viewed, in good sky conditions, with good quality telescopes as small as 102mm (four inches) in aperture. Intrinsically it is one of the brightest and most massive globular clusters of our galaxy, having an absolute magnitude of -9.42 and being 900,000 times more massive than our Sun.It was proposed that NGC 2419 could be, as Omega Centauri, the remnant of a dwarf spheroidal galaxy disrupted and accreted by the Milky Way. However, later research seems to disprove that possibility.Astronomer Leos Ondra has noted that NGC 2419 would be the "best and brightest" for any observers in the Andromeda Galaxy, looking for globular clusters in our galaxy since it lies outside the obscuring density of the main disk. This is analogous to the way the cluster G1 can be seen orbiting outside of the Andromeda Galaxy from Earth.

NGC 45

NGC 45 is a low surface brightness spiral galaxy in the constellation of Cetus. It was discovered on 11 November 1835 by the English astronomer John Herschel.

Unlike the Milky Way, NGC 45 has no clear defined spiral arms, and its center bar nucleus is also very small and distorted. NGC 45 thus does not have a galactic habitable zone. For the Milky Way, the galactic habitable zone is commonly believed to be an annulus with an outer radius of about 10 kiloparsecs and an inner radius close to the Galactic Center, both of which lack hard boundaries.

NGC 6101

NGC 6101 (also known as Caldwell 107) is a globular cluster in the constellation Apus, which was discovered by James Dunlop and catalogued by him as Δ68. It is located at a distance of about 47,600 light-years from the Sun and about 36,500 light-years from the galactic center of the Milky Way. It requires a telescope of at least 20 cm (7.9 in) aperture to resolve individual stars. Research revealed this cluster to contain an unexpected large number of black holes.

NGC 6139

NGC 6139 is a globular cluster of the Milky Way in the constellation Scorpius. It is located 3.6 kiloparsecs (10 kilolight-years) from the Galactic Center (less than half the distance of the Sun from the Galactic Center).

Orion Arm

The Orion Arm is a minor spiral arm of the Milky Way some 3,500 light-years (1,100 parsecs) across and approximately 10,000 light-years (3,100 parsecs) in length, containing the Solar System, including the Earth. It is also referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion Bridge, and formerly, the Local Spur and Orion Spur.

The arm is named for the Orion constellation, which is one of the most prominent constellations of Northern Hemisphere winter (Southern Hemisphere summer). Some of the brightest stars and most famous celestial objects of the constellation (e.g. Betelgeuse, Rigel, the three stars of Orion's Belt, the Orion Nebula) are within it as shown on the interactive map below.

The arm is between the Carina–Sagittarius Arm (the local portions of which are toward the Galactic Center) and the Perseus Arm (the local portion of which is the main outer-most arm and one of two major arms of the galaxy).

Long thought to be a minor structure, namely a "spur" between the two arms mentioned, evidence was presented in mid 2013 that the Orion Arm might be a branch of the Perseus Arm, or possibly an independent arm segment.Within the arm, the Solar System is close to its inner rim, in a relative cavity in the arm's Interstellar Medium known as the Local Bubble, about halfway along the arm's length, approximately 8,000 parsecs (26,000 light-years) from the Galactic Center.

Pistol Star

The Pistol Star is a blue hypergiant star; one of the most luminous known in the Milky Way. It is one of many massive young stars in the Quintuplet cluster in the Galactic Center region. The star owes its name to the shape of the Pistol Nebula, which it illuminates. It is located approximately 25,000 light years from Earth in the direction of Sagittarius. It would be visible to the naked eye as a fourth magnitude star if it were not for the interstellar dust that completely hides it from view in visible light.

S2 (star)

S2, also known as S0–2, is a star that is located close to the radio source Sagittarius A*, orbiting it with an orbital period of 16.0518 years, a semi-major axis of about 970 au, and a pericenter distance of 17 light hours (18 Tm or 120 au) – 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 10-15 solar masses.Its changing apparent position has been monitored since 1995 by two groups (at UCLA and at the Max Planck Institute for Extraterrestrial Physics) as part of an effort to gather evidence for the existence of a supermassive black hole in the center of the Milky Way galaxy. The accumulating evidence points to Sagittarius A* as being the site of such a black hole. By 2008, S2 had been observed for one complete orbit.A team of astronomers mainly from the Max Planck Institute for Extraterrestrial Physics used observations of S2's orbital dynamics around Sgr A* to measure the distance from the Earth to the galactic center. They determined the distance to be 7.94 ± 0.42 kiloparsecs, in close agreement with prior determinations of the distance by other methods.S2 was precisely tracked during its May 2018 close approach to Sagittarius A*, with results in accord with general relativity predictions.

Sagittarius A*

Sagittarius A* (pronounced "Sagittarius A-star", standard abbreviation Sgr A*) is a bright and very compact astronomical radio source at the center of the Milky Way, near the border of the constellations Sagittarius and Scorpius. It is part of a larger astronomical feature known as Sagittarius A. Sagittarius A* is the location of a supermassive black hole, like those that are now generally accepted to be at the centers of most if not all spiral and elliptical galaxies. Observations of a number of stars, most notably the star S2, orbiting around Sagittarius A* have been used to show the presence of, and produce data about, the Milky Way's central supermassive black hole, and have led to the conclusion that Sagittarius A* is the site of that black hole.

Galactic core
Spiral arms
Satellite galaxies

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