Small Magellanic Cloud

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Small Magellanic Cloud

NGC 346, part of the Small Magellanic Cloud.
Courtesy of NASA/ESA
Observation data (J2000 epoch)
Constellation Tucana
Right ascension 00h 52m 44.8s[1]
Declination -72° 49′ 43″[1]
Redshift 158 ± 4 km/s[1]
Distance 197 ± 9 kly (61 ± 3 kpc)[2]
Type SB(s)m pec[1]
Apparent dimensions (V) 5° 20′ × 3° 5′[1]
Apparent magnitude (V) 2.7[1]
Notable features Companion dwarf to the
Milky Way
Other designations
SMC,[1] NGC 292,[1] PGC 3085,[1] Nubecula Minor[1]
See also: Galaxy, List of galaxies

The Small Magellanic Cloud (SMC) is a dwarf galaxy[3]. It contains several hundred million stars.[4]

Some speculate that the SMC was once a barred spiral galaxy that was disrupted by the Milky Way to become somewhat irregular. [5] It still contains a central bar structure.

At a distance of about 200,000 light-years, it is one of the Milky Way's nearest neighbors. It is also one of the most distant objects that can be seen with the naked eye.

With a mean declination of approximately -73 degrees, it can only be viewed from the Southern Hemisphere and the lower latitudes of the Northern Hemisphere. It is located in the constellation of Tucana and appears as a hazy, light patch in the night sky about 3 degrees across. It looks like a detached piece of the Milky Way. Since it has a very low surface brightness, it is best viewed from a dark site away from city lights.

It forms a pair with the Large Magellanic Cloud (LMC), which is positioned a further 20 degrees to the east. The Small Magellanic Cloud is a member of the Local Group.

Contents

[edit] Observation history

In the southern hemisphere, the Magellanic clouds have long been included in the lore of native inhabitants, including south sea islanders and indigenous Australians. Persian astronomer Al Sufi labelled the larger of the two clouds as Al Bakr, the White Ox. European sailors may have first noticed the clouds during the Middle Ages when they were used for navigation. Portuguese and Dutch sailors called them the Cape Clouds, a name that was retained for several centuries. During the circumnavigation of the Earth by Ferdinand Magellan in 1519–22, they were described by Antonio Pigafetta as dim clusters of stars.[6] In Johann Bayer's celestial atlas Uranometria, published in 1603, he named the smaller cloud, Nubecula Minor.[7] In Latin, Nubecula means a little cloud.[8]

Between 1834 and 1838, John Frederick William Herschel made observations of the southern skies with his 20-foot reflector from the Royal Observatory at the Cape of Good Hope. While observing the Nubecula Minor, he described it as a cloudy mass of light with an oval shape and a bright center. Within the area of this cloud he catalogued a concentration of 37 nebulae and clusters.[9]

In 1891, Harvard College Observatory opened an observing station at Arequipa, Peru. From 1893 and 1906, under the directorship of Solon Bailey, the 24-inch telescope at this site was used to photographically survey both the Large and Small Magellanic Clouds.[10] Henrietta Swan Leavitt, an astronomer at the Harvard College Observatory, used the plates from Arequipa to study of the variations in relative luminosity of stars in the SMC. In 1908, the results Henrietta's study were published, which showed that a type of variable star called a "cluster variable", later called a cepheid variable after the prototype star Delta Cephei, showed a definite relationship between the variability period and the star's luminosity.[11] This important period-luminosity relation allowed the distance to any other cepheid variable to be estimated in terms of the distance to the SMC. Hence, once the distance to the SMC was known with greater accuracy, Cepheid variables could be used as a standard candle for measuring the distances to other galaxies.[12]

Using this period-luminosity relation, in 1913 the distance to the SMC was first estimated by Ejnar Hertzsprung. First he measured thirteen nearby cepheid variables to find the absolute magnitude of a variable with a period of one day. By comparing this to the periodicity of the variables as measured by Leavitt, he was able to estimate a distance of 10,000 parsecs (30,000 light years) between the Sun and the SMC.[13] This later proved to be a gross underestimate of the true distance, but it did demonstrate the potential usefulness of this technique.[14]

[edit] See also

[edit] References

  1. ^ a b c d e f g h i j NASA/IPAC Extragalactic Database. Results for Small Magellanic Cloud. Retrieved on 2006-12-01.
  2. ^ Hilditch, R. W.; Howarth, I. D.; Harries, T. J. (2005). "Forty eclipsing binaries in the Small Magellanic Cloud: fundamental parameters and Cloud distance". Monthly Notices of the Royal Astronomical Society 357 (1): 304-324. 
  3. ^ APOD: 2005 June 17 - The Small Cloud of Magellan
  4. ^ APOD: 2005 June 17 - The Small Cloud of Magellan
  5. ^ [1]
  6. ^ Westerlund, Bengt E. (1997). The Magellanic Clouds. Cambridge University Press. ISBN 0521480701. 
  7. ^ O'Meara, Stephen James (2002). The Caldwell Objects. Cambridge University Press. ISBN 0521827965. 
  8. ^ Lewis, Charlton Thomas; Kingery, Hugh Macmaster (1918). An elementary Latin dictionary. American Book Company. 
  9. ^ Herschel, John Frederick William (1849). Outlines of Astronomy. Philadelphia: Lea & Blanchard. 
  10. ^ Longair, Malcolm S. (2006). The Cosmic Century: A History of Astrophysics and Cosmology. Cambridge University Press. ISBN 0521474361. 
  11. ^ Leavitt, Henrietta S. (1908). "1777 variables in the Magellanic Clouds". Annals of Harvard College Observatory 60: 87–108. 
  12. ^ Aparicio, Antonio; Herrero, Artemio; Sánchez, Francisco (1998). Stellar Astrophysics for the Local Group. Cambridge University Press. ISBN 0521563275. 
  13. ^ Gribbin, John R. (1999). The Birth of Time: How Astronomers Measured the Age of the Universe. Yale University Press. ISBN 0300083467. 
  14. ^ Hoffleit, Dorrit (1992). "The Selector of Highlights: A Brief Biographical Sketch of Harlow Shapley". The Journal of the American Association of Variable Star Observers 21 (2): 151-156. 

[edit] External links

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