Small Magellanic Cloud
The SMC is one of the Milky Way’s closest galactic neighbors. Even though it is a small, or so-called dwarf galaxy, the SMC is so bright that it is visible to the unaided eye from the Southern Hemisphere and near the equator. Many navigators, including Ferdinand Magellan who lends his name to the SMC, used it to help find their way across the oceans
The Small Magellanic Cloud (SMC) is an irregular dwarf galaxy and a companion to our own Milky Way Galaxy. Unlike spiral and elliptical galaxies, irregular galaxies lack any appearance of organized structure. Like its neighboring Large Magellanic Cloud (LMC), the SMC appears as a huge and diffuse cloud in the southern nighttime sky. Both of these galaxies are named in honor of the explorer Ferdinand Magellan, who noted their presence in becoming the first to sail around the world nearly 500 years ago.
The Small Magellanic Cloud is roughly 7,000 light-years in diameter (about 7% of the Milky Way’s diameter) and contains about 7 billion solar masses (less than one percent of the mass of the Milky Way). While it is about half the size of its companion, the Large Magellanic Cloud, it contains nearly as many stars (about 7 billion versus 10 billion), meaning it has a higher stellar density.
However, the star formation rate is currently lower for the Small Magellanic Cloud. This is likely because it has less free gas than its larger sibling, and, therefore, had periods of more rapid formation in the past.
Located 200,000 light years away, the SMC has a mass of approximately 3 billion solar masses. Some observations suggest that it might be a barred disk galaxy, deformed by gravitational interactions with the Milky Way and the Large Magellanic Cloud (LMC). The existence of the Magellanic Stream, a tidal tail of gas that has been stripped from the SMC/LMC system and now extends half way around the Milky Way, provides further evidence for interactions between these three galaxies.
It was while studying variable stars in the Small Magellanic Cloud that Henrietta Leavitt discovered the period-luminosity of Cepheid variable stars. This has become one of the most important relations in determining distances to objects in the Universe, and forms the first rung of the extragalactic distance ladder.
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).
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.