سنة مجرية

المدار التقريبي للشمس (الدائرة الصفراء) حول مركز المجرة
المدار التقريبي للشمس (الدائرة الصفراء) حول مركز المجرة

The galactic year, also known as a cosmic year, is the duration of time required for the Sun to orbit once around the center of the Milky Way Galaxy.[1] One galactic year is approximately 225 million Earth years.[2] The Solar System is traveling at an average speed of 230 km/s (828,000 km/h) or 143 mi/s (514,000 mph) within its trajectory around the Galactic Center,[3] a speed at which an object could circumnavigate the Earth's equator in 2 minutes and 54 seconds; that speed corresponds to approximately 1/1300 of the speed of light.

The galactic year provides a conveniently usable unit for depicting cosmic and geological time periods together. By contrast, a "billion-year" scale does not allow for useful discrimination between geologic events, and a "million-year" scale requires some rather large numbers.[4]

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Timeline of the universe and Earth's history in galactic years

المقالة الرئيسية: Timeline of the early universe, Timeline of natural history, and Timeline of the far future
The orientation of the Solar System's motion

The following list assumes that 1 galactic year is 225 million years.

Time Event
Galactic
years
(gal) 		Millions
of years
(Ma)
Past (years ago)
About 61.32 gal Big Bang
About 54 gal Birth of the Milky Way
20.44 gal Birth of the Sun
17–18 gal 3937 Ma Oceans appear on Earth
16.889 gal 3800 Ma Life begins on Earth
15.555 gal 3500 Ma Prokaryotes appear
12 gal 2700 Ma Bacteria appear
10 gal 2250 Ma Eukaryian period[5][6] first appearance of eukaryotes[7] Stable continents appear
6.8 gal 1530 Ma Multicellular organisms appear
2.4 gal 540 Ma Cambrian explosion occurs
2 gal 500 Ma The first brain structure appears in worms
1.11 gal 250 Ma Permian–Triassic extinction event
0.2933 gal Cretaceous–Paleogene extinction event
0.0013 gal Emergence of anatomically modern humans
Future (years from now)
0.15 gal Mean time between impacts of asteroidal bodies in the order of magnitude of the K/Pg impactor has elapsed.[8]
1 gal All the continents on Earth may fuse into a supercontinent. Three potential arrangements of this configuration have been dubbed Amasia, Novopangaea, and Pangaea Ultima.[9]
2–3 gal Tidal acceleration moves the Moon far enough from Earth that total solar eclipses are no longer possible
4 gal Carbon dioxide levels fall to the point at which C4 photosynthesis is no longer possible. Multicellular life dies out[10]
15 gal Surface conditions on Earth are comparable to those on Venus today
22 gal The Milky Way and Andromeda Galaxy begin to collide
25 gal Sun ejects a planetary nebula, leaving behind a white dwarf
30 gal The Milky Way and Andromeda complete their merger into a giant elliptical galaxy called Milkomeda or Milkdromeda[11]
500 gal The Universe's expansion causes all galaxies beyond the Milky Way's Local Group to disappear beyond the cosmic light horizon, removing them from the observable universe[12]
2000 gal Local Group of 47 galaxies[13] coalesces into a single large galaxy[14]
Visualisation of the orbit of the Sun (yellow dot and white curve) around the Galactic Centre (GC) in the last galactic year. The red dots correspond to the positions of the stars studied by the European Southern Observatory in a monitoring programme.[15]
Visualisation of the orbit of the Sun (yellow dot and white curve) around the Galactic Centre (GC) in the last galactic year. The red dots correspond to the positions of the stars studied by the European Southern Observatory in a monitoring programme.[15]


See also

References

  1. ^ Cosmic Year Archived 2014-04-12 at the Wayback Machine, Fact Guru, University of Ottawa
  2. ^ Leong, Stacy (2002). "Period of the Sun's Orbit around the Galaxy (Cosmic Year)". The Physics Factbook.
  3. ^ http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html NASA – StarChild Question of the Month for February 2000
  4. ^ Geologic Time Scale – as 18 galactic rotations
  5. ^ El Albani, Abderrazak; Bengtson, Stefan; Canfield, Donald E.; Riboulleau, Armelle; Rollion Bard, Claire; Macchiarelli, Roberto; et al. (2014). "The 2.1 Ga Old Francevillian Biota: Biogenicity, Taphonomy and Biodiversity". PLOS ONE. 9 (6): e99438. Bibcode:2014PLoSO...999438E. doi:10.1371/journal.pone.0099438. PMC 4070892. PMID 24963687.
  6. ^ El Albani, Abderrazak; Bengtson, Stefan; Canfield, Donald E.; Bekker, Andrey; Macchiarelli, Roberto; Mazurier, Arnaud; Hammarlund, Emma U.; et al. (2010). "Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago" (PDF). Nature. 466 (7302): 100–104. Bibcode:2010Natur.466..100A. doi:10.1038/nature09166. PMID 20596019. S2CID 4331375.[dead link]
  7. ^ F. M. Gradstein (2012). The geologic time scale 2012. Volume 2 (1st ed.). Amsterdam: Elsevier. ISBN 978-0-444-59448-8. OCLC 808340848.
  8. ^ Lunar and Planetary Institute (2010), https://www.lpi.usra.edu/features/chicxulub/
  9. ^ Williams, Caroline; Nield, Ted (2007-10-17). "Pangaea, the comeback". New Scientist. Retrieved 2014-01-02.
  10. ^ Franck, S.; Bounama, C.; von Bloh, W. (2005-11-07). "Causes and timing of future biosphere extinction". Biogeosciences Discussions. Copernicus GmbH. 2 (6): 1665-1679. Bibcode:2006BGeo....3...85F. doi:10.5194/bgd-2-1665-2005. S2CID 3619702.
  11. ^ Cox, T. J.; Loeb, Abraham (2008-05-01). "The collision between the Milky Way and Andromeda". Monthly Notices of the Royal Astronomical Society. Oxford University Press (OUP). 386 (1): 461–474. arXiv:0705.1170. Bibcode:2008MNRAS.386..461C. doi:10.1111/j.1365-2966.2008.13048.x. ISSN 0035-8711. S2CID 14964036.
  12. ^ Loeb, Abraham (2011-04-18). "Cosmology with hypervelocity stars". Journal of Cosmology and Astroparticle Physics. IOP Publishing. 2011 (4): 023. arXiv:1102.0007. Bibcode:2011JCAP...04..023L. doi:10.1088/1475-7516/2011/04/023. ISSN 1475-7516. S2CID 118750775.
  13. ^ Frommert, Hartmut; Kronberg, Christine (2007-06-05). "The Local Group of Galaxies". University of Arizona. Students for the Exploration and Development of Space. Archived from the original on 2009-09-07. Retrieved 2009-10-02.
  14. ^ Adams, Fred C.; Laughlin, Gregory (1997-04-01). "A dying universe: the long-term fate and evolutionof astrophysical objects". Reviews of Modern Physics. 69 (2): 337–372. arXiv:astro-ph/9701131. Bibcode:1997RvMP...69..337A. doi:10.1103/revmodphys.69.337. ISSN 0034-6861. S2CID 12173790.
  15. ^ "Milky Way Past Was More Turbulent Than Previously Known". ESO News. European Southern Observatory. 2004-04-06. After more than 1,000 nights of observations spread over 15 years, they have determined the spatial motions of more than 14,000 solar-like stars residing in the neighbourhood of the Sun.
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