21 octubre 2014

Small Magellanic Cloud is a nearby dwarf galaxy

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The Small Magellanic Cloud resembles a luminous cloud, but it’s really a nearby dwarf galaxy, orbiting our Milky Way.

You’ll see the Small Magellanic Cloud (SMC) from Earth’s Southern Hemisphere. It’s even farther to the south than its larger cousin, the the Large Magellanic Cloud (LMC). These two hazy patches in the southern sky are really separate galaxies from our Milky Way. They are satellite galaxies to the Milky Way, orbiting around it. Follow the links below to learn more about the Small Magellanic Cloud.

A Perseid meteor streaks between the two Magellanic Clouds in August 2013. Photo by Colin Legg.

How to see the Small Magellanic Cloud. The Small and Large Magellanic Clouds are Never seen north of about 17 degrees North latitude, the SMC not visible from North America (except far southern Mexico), northern Africa, and all of Europe and Asia (except the southern regions of India and Southeast Asia).

 From more northerly latitudes where it still can be observed, the SMC is best seen in the evening in late autumn and early winter (late night in October, mid-evening in November and December, and early evening in January). Whenever the W or M-shaped constellation Cassiopeia the Queen climbs to her highest point in the northern sky, look for the Small Magellanic Cloud to soar to its highest point in the southern sky. The Small Magellanic Cloud is located about 20 degrees from the South Celestial Pole, in the southeast corner of the constellation Tucana.

 To find where the SMC lies, look about 15 degrees below the bright far southern star Achernar in the constellation Eridanus the River. For reference, a fist-width at an arm length equals about 10 degrees. Eridanus the River meanders all the way from the Orion star Rigel to Achernar, the end of the River. The SMC is distinctly smaller and fainter than the LMC, and covers an area roughly 2.5 by 5 degrees in dimension. The overall magnitude is about +2. Since its brightness is spread over about 13 square degrees, it is somewhat harder to find than the LMC, and requires darker skies.

The Large Magellanic Clouc is found in the southeast corner of the constellation Tucana. The nearby bright star is Achernar.

SMC history and mythology. Given its faintness and location far to the South, the SMC was not known in ancient Europe and has no classical mythology associated with it. It does share in the Australian aboriginal tale of Jukara, the old couple who are fed fish from the sky river (Milky Way) by star people. The SMC is the camp of the old woman. It’s said that the LMC and SMC served as important navigation markers to the early Polynesians. The Maori of New Zealand were said to use them as predictors of winds.

 Their most famous association in western history came with the expedition of Ferdinand Magellan on his circumnavigation of the world in 1519-1522. The SMC and LMC became known as the clouds of Magellan after that time. However, later star maps still did not call them that. In Bayer’s Uranometria they are designated as nubecula major and nubecula minor. 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).

Known Milky Way galaxy satellite candidates. Some may not be true satellites, but may simply be passing near us in space. The Large and Small Magellanic Clouds are true satellites of the Milky Way, orbiting around it. Image via Wikimedia Commons

SMC science. Although the myths are sparse, the SMC holds a very important role in 20th Century astronomy. It was from stars in this galaxy that Henrietta Leavitt deduced the famous “period luminosity” relationship that allowed astronomers for the first time to gauge the distance to star clusters and nearby galaxies.

 After the LMC, this galaxy is the fourth closest galaxy to the Milky Way. The best current estimate of its distance is about 210,000 light years, or about 30 percent farther than the LMC. Classified as a dwarf irregular galaxy, it appears to some as showing indication of a distorted barred spiral structure, likely deformed by the gravitational influence of the Milky Way.

Some 15,000 to 17,000 light-years across in the longest dimension, the SMC spans nearly 5 degrees of the sky, the equivalent of 9 or 10 full moons. It may contain as many as a few hundred million stars, significantly less than the LMC and much less than the Milky Way.

 The center of the LMC is approximately RA: 0h 52m 45s, dec: -72°49’43” Bottom line: If you could view the Small Magellanic Cloud with your eye alone – in the sky visible from Earth’s Southern Hemisphere – you would say it does resemble a luminous cloud. Really, though, it is a nearby dwarf galaxy, orbiting our Milky Way.

source and credit to earthsky.org