The Small Magellanic Cloud (SMC) is a dwarf galaxy near the Milky Way. Classified as a dwarf irregular galaxy, the SMC has a D25 isophotal diameter of about 5.78 kiloparsecs (18,900 light-years), and contains several hundred million stars. It has a total mass of approximately 7 billion solar masses. At a distance of about 200,000 light-years, the SMC is among the nearest intergalactic neighbors of the Milky Way and is one of the most distant objects visible to the naked eye. The SMC is visible from the entire Southern Hemisphere and can be fully glimpsed low above the southern horizon from latitudes south of about 15° north. The galaxy is located across the constellation of Tucana and part of Hydrus, appearing as a faint, hazy patch resembling a detached piece of the Milky Way. The SMC has an average apparent diameter of about 4.2° (8 times the Moon's) and thus covers an area of about 14 square degrees (70 times the Moon's). Since its surface brightness is very low, this deep-sky object is best seen on clear moonless nights and away from city lights. The SMC forms a pair with the Large Magellanic Cloud (LMC), which lies 20° to the east, and, like the LMC, is a member of the Local Group. It is currently a satellite of the Milky Way but is likely a former satellite of the LMC.
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 mentions them in his Book of Fixed Stars, repeating a quote by the polymath Ibn Qutaybah, but had not observed them himself. 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 1519–1522, they were described by Antonio Pigafetta as dim clusters of stars. In Johann Bayer's celestial atlas Uranometria, published in 1603, he named the smaller cloud, Nubecula Minor. In Latin, Nubecula means a little cloud.
Between 1834 and 1838, John Frederick William Herschel made observations of the southern skies with his 14-inch (36 cm) reflector from the Royal Observatory. 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. In 1891, the Harvard College Observatory opened an observing station at Arequipa in Peru. Between 1893 and 1906, under the direction of Solon Bailey, the 24-inch (610 mm) telescope at this site was used to survey photographically both the Large and Small Magellanic Clouds. Henrietta Swan Leavitt, an astronomer at the Harvard College Observatory, used the plates from Arequipa to study the variations in relative luminosity of stars in the SMC. In 1908, the results of her 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 apparent brightness. Leavitt realized that since all the stars in the SMC are roughly the same distance from Earth, this result implied a similar relationship between period and absolute brightness. This important period-luminosity relation allowed the distance to any other Cepheid variable to be estimated in terms of the distance to the SMC. She hoped a few Cepheid variables could be found close enough to Earth so that their parallax, and hence distance from Earth, could be measured. This soon happened, allowing Cepheid variables to be used as standard candles, facilitating many astronomical discoveries. 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. This later proved to be a gross underestimate of the true distance, but it did demonstrate the potential usefulness of this technique. Announced in 2006, measurements with the Hubble Space Telescope suggest that either the Large and Small Magellanic Clouds may be moving too fast to be orbiting the Milky Way, or that the Milky Way Galaxy is more massive than was thought.
Features
The SMC contains a central bar structure, and astronomers speculate that it was once a barred spiral galaxy that was disrupted by the Milky Way to become somewhat irregular. There is a bridge of gas connecting the Small Magellanic Cloud with the Large Magellanic Cloud (LMC), which is evidence of tidal interaction between the galaxies. This bridge of gas is a star-forming site. The Magellanic Clouds have a common envelope of neutral hydrogen, indicating they have been gravitationally bound for a long time. In 2017, using the Dark Energy Survey plus MagLiteS data, a stellar over-density associated with the Small Magellanic Cloud was discovered, which is probably the result of interactions between the SMC and LMC.
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