Magellanic Clouds

The Large and Small Magellanic Clouds.

The two Magellanic Clouds (or Nubeculae Magellani^[1]) are irregular dwarf galaxies visible in the southern hemisphere, which are members of our Local Group and are orbiting our Milky Way galaxy.^[2] The two galaxies are:

• Large Magellanic Cloud (LMC)
• Small Magellanic Cloud (SMC)

History

The Magellanic Clouds have been known since the earliest times to the ancient Middle Eastern peoples. The first preserved mention of the Large Magellanic Cloud is by the Persian astronomer Al Sufi.^[3][4] In 964, in his Book of Fixed Stars, he called it al-Bakr ("the Sheep") "of the southern Arabs"; he noted that the Cloud is not visible from northern Arabia and Baghdad, but can be seen at the strait of Bab el Mandeb (12°15' N), which is the southernmost point of Arabia.^[1]

In Europe, the Clouds were first observed by Italian explorers Peter Martyr d'Anghiera and Andrea Corsali at the end of the 15th century. Subsequently, they were reported by Antonio Pigafetta, who accompanied the expedition of Ferdinand Magellan on its circumnavigation of the world in 1519-1522.^[1] However, naming the clouds after Magellan did not become widespread until much later. In Bayer's Uranometria they are designated as nubecula major and nubecula minor.^[5] In the 1756 star map of the French astronomer Lacaille, they are designated as le Grand Nuage and le Petit Nuage ("the Large Cloud" and "the Small Cloud").^[6]

Characteristics

The Large Magellanic Cloud (LMC)

Part of the Small Magellanic Cloud (SMC)

The Large Magellanic Cloud and its neighbor and relative, the Small Magellanic Cloud, are conspicuous objects in the southern hemisphere, looking like separated pieces of the Milky Way to the naked eye. Roughly 21° apart in the night sky, the true distance between them is roughly 75,000 light-years. Until the discovery of the Sagittarius Dwarf Elliptical Galaxy in 1994, they were the closest known galaxies to our own. The LMC lies about 160,000 light years away,^{[7][8][9][10]} while the SMC is around 200,000.^[11] The LMC is about twice the diameter of the SMC (14,000 ly and 7,000 ly respectively). For comparison, the Milky Way is about 100,000 ly across.

Astronomers have long believed that the Magellanic Clouds have orbited the Milky Way at approximately their current distances, but evidence suggests that it is rare for them to come as close to the Milky Way as they are now.^[12] Observation and theoretical evidence suggest that the Magellanic Clouds have both been greatly distorted by tidal interaction with the Milky Way as they travel close to it. Streams of neutral hydrogen connect them to the Milky Way and to each other, and both resemble disrupted barred spiral galaxies.^[13] Their gravity has affected the Milky Way as well, distorting the outer parts of the galactic disk.

Aside from their different structure and lower mass, they differ from our Galaxy in two major ways. First, they are gas-rich; a higher fraction of their mass is hydrogen and helium compared to the Milky Way.^[14] They are also more metal-poor than the Milky Way; the youngest stars in the LMC and SMC have a metallicity of 0.5 and 0.25 times solar, respectively.^[15] Both are noted for their nebulae and young stellar populations, but as in our own Galaxy their stars range from the very young to the very old, indicating a long stellar formation history.(Chaisson and McMillan)

The Large Magellanic Cloud was host galaxy to a supernova (SN 1987A), the brightest observed in over four centuries.

A recent French paper^[16][17] referenced in a Discovery Magazine blog suggests that the clouds may be ejected from a collision that formed the Andromeda Galaxy 6 billion years ago. However, this is an unlikely scenario, and many astronomers believe the Magellanic Clouds are natural remnants of the hierarchical process of galaxy formation that formed our Milky Way Galaxy.

Mini Magellanic Cloud (MMC)

It has been proposed by astrophysicists D. S. Mathewson, V. L. Ford and N. Visvanathan that the SMC may in fact be split in two, with a smaller section of this galaxy behind the main part of the SMC (as seen from our perspective), and separated by about 30,000 ly. They suggest the reason for this is due to a past interaction with the LMC splitting the SMC, and that the two sections are still moving apart. They have dubbed this smaller remnant the Mini Magellanic Cloud.^[18][19]

In fiction

See Galaxies in fiction.

See also

• Astronomical Surveys of the Magellanic Clouds
• Irregular galaxy
• Magellanic Stream
• Magellanic Bridge

References

1. ^ ^{a b c} Allen, R. H., (1963). Star Names: Their Lore and Meaning (rep. ed.). New York, NY: Dover Publications Inc. , pp. 294-295.
2. ^ http://adsabs.harvard.edu/abs/2009MNRAS.392L..21S
3. ^ "Observatoire de Paris (Abd-al-Rahman Al Sufi)". http://messier.obspm.fr/xtra/Bios/alsufi.html. Retrieved 22 July 2011. 
4. ^ "Observatoire de Paris (LMC)". http://messier.obspm.fr/xtra/ngc/lmc.html. Retrieved 22 July 2011. 
5. ^ Bayer, J., (1661) Uranometria, pl. Aaa (49) U.S. Naval Observatory; retrieved on 2009-09-05
6. ^ de Lacaille, N. L., (1756) Planisphere contenant les Constellations Celestes, Memoires Academie Royale des Sciences pour 1752. Linda Hall Library; retrieved on 2009-09-05
7. ^ "A Cosmic Zoo in the Large Magellanic Cloud". European Southern Observatory. 1 June 2010. http://www.eso.org/public/news/eso1021/. Retrieved 29 August 2010. 
8. ^ Macri, L. M.; et al. (2006). "A New Cepheid Distance to the Maser-Host Galaxy NGC 4258 and Its Implications for the Hubble Constant". The Astrophysical Journal 652 (2): 1133–1149. arXiv:astro-ph/0608211. Bibcode 2006ApJ...652.1133M. doi:10.1086/508530. 
9. ^ Freedman, Wendy L.; Madore, Barry F. "The Hubble Constant", Annual Review of Astronomy and Astrophysics, 2010
10. ^ Majaess, Daniel J.; Turner, David G.; Lane, David J.; Henden, Arne; Krajci, Tom "Anchoring the Universal Distance Scale via a Wesenheit Template", JAAVSO, 2010
11. ^ "Little Galaxy Explored". Jet Propulsion Laboratory, California Institute of Technology. 01.05.10. http://www.spitzer.caltech.edu/images/2875-ssc2010-02a1-Little-Galaxy-Explored. Retrieved 29 August 2010. 
12. ^ Ferris, Timothy (December 2011). "Dancing in the Dark". National Geographic 220 (6): 118. 
13. ^ http://www.aao.gov.au/images/captions/misc021.html]
14. ^ http://home.insightbb.com/~lasweb/lessons/magellanic.htm Home.insightbb.com Retrieved on 2007-05-31
15. ^ http://aa.springer.de/papers/8336003/2300925/sc6.htm Aa.springer.de Retrieved on 2007-05-31
16. ^ Hammer, F.; et al. (2010). "Does M31 Result from an Ancient Major Merger?". The Astrophysical Journal 727 (1): 542–555. Bibcode 2010ApJ...725..542H. doi:10.1088/0004-637X/725/1/542. 
17. ^ Press release on Observatoire de Paris website, in French
18. ^ http://adsabs.harvard.edu/full/1986ApJ...301..664M Astrophysical Journal, Part 1, vol. 301, Feb. 15, 1986, p. 664-674.
19. ^ http://iopscience.iop.org/1538-3881/122/1/220/200523.text.html The Astronomical Journal 122:220-231 July 2001

Sources

• Eric Chaisson and Steve McMillan, Astronomy Today (Englewood Cliffs: Prentice-Hall, Inc., 1993), p. 550.
• Michael Zeilik, Conceptual Astronomy (New York: John Wiley & Sons, Inc., 1993), pp. 357–8.

External links

• Magellanic Clouds Working Group