The Small Magellanic Cloud is a dwarf galaxy near the Milky Way. It is
classified as a dwarf irregular galaxy. It has a diameter of about 7,000
light-years, contains several hundred million stars, and has a total mass of
approximately 7 billion times the mass of the Sun. The SMC contains a central
bar structure and it is speculated that it was once a barred spiral galaxy that
was disrupted by the Milky Way to become somewhat irregular. 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 mostly in the constellation of Tucana and also partly in Hydrus and
appears as a hazy, light patch in the night sky about 3 degrees across,
looking 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, which lies a further
20 degrees to the east, and like the LMC is a member of the Local Group.
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. 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 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.
In 1891, Harvard College Observatory opened an observing station at Arequipa
in Peru. Between 1893 and 1906, under the direction of Solon Bailey, the
24-inch 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 luminosity. 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.
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 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 the Large
and Small Magellanic Clouds may be moving too fast to be orbiting the Milky
Way. Features
There is a bridge of gas connecting the Small Magellanic Cloud with the Large
Magellanic Cloud, which is evidence of tidal interaction between the galaxies.
The Magellanic Clouds have a common envelope of neutral hydrogen indicating
they have been gravitationally bound for a long time. This bridge of gas is a
star-forming site. X-ray sources
The Small Magellanic Cloud contains a large and active population of X-ray
binaries. Recent star formation has led to a large population of massive stars
and high-mass X-ray binaries which are the relics of the short-lived upper end
of the initial mass function. The young stellar population and the majority of
the known X-ray binaries are concentrated in the SMC’s Bar. HMXB
pulsars are rotating neutron stars in binary systems with Be-type or
supergiant supergiant stellar companions. Most HMXBs are of the Be
type which account for 70% in the Milky Way and 98% in the SMC. The Be-star
equatorial disk provides a reservoir of matter that can be accreted onto the
neutron star during periastron passage or during large-scale disk ejection
episodes. This scenario leads to strings of X-ray outbursts with typical
luminosities LX = 10^36–10^37 erg/s, spaced at the orbital period, plus
infrequent giant outbursts of greater duration and luminosity.
Monitoring surveys of the SMC performed with NASA's Rossi X-ray Timing Explorer
see X-ray pulsars in outburst at more than 1036 erg/s and have counted 50 by
the end of 2008. The ROSAT and ASCA missions detected many faint X-ray point
sources, but the typical positional uncertainties frequently made positive
identification difficult. Recent studies using XMM-Newton and Chandra have now
cataloged several hundred X-ray sources in the direction of the SMC, of which
perhaps half are considered likely HMXBs, and the remainder a mix of
foreground stars, and background AGN. No X-rays above background were observed
from the Magellanic Clouds during the September 20, 1966, Nike-Tomahawk
flight. Balloon observation from Mildura, Australia, on October 24, 1967,
of the SMC set an upper limit of X-ray detection. An X-ray astronomy instrument
was carried aboard a Thor missile launched from Johnston Atoll on
September 24, 1970, at 12:54 UTC for altitudes above 300 km, to search for
the Small Magellanic Cloud. The SMC was detected with an X-ray luminosity of 5 x
1038 ergs/s in the range 1.5-12 keV, and 2.5 x 1039 ergs/s in the range 5-50 keV
for an apparently extended source. The fourth Uhuru catalog lists an early
X-ray source within the constellation Tucana: 4U 0115-73. Uhuru observed the
SMC on January 1, 12, 13, 16, and 17, 1971, and detected one source located at
01149-7342, which was then designated SMC X-1. Some X-ray counts were also
received on January 14, 15, 18, and 19, 1971. The third Ariel 5 catalog also
contains this early X-ray source within Tucana: 3A 0116-736. The SMC X-1, a
HMXRB, is at J2000 right ascension 01h 15m 14s declination 73° 42′ 22″.
Two additional sources detected and listed in 3A include SMC X-2 at 3A
0042-738 and SMC X-3 at 3A 0049-726. Mini Magellanic Cloud
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, 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.
See also Small Magellanic Cloud in fiction
Large Magellanic Cloud Magellanic Clouds
Objects within the Small Magellanic Cloud:
NGC 265 NGC 290
NGC 346 NGC 347
NGC 602 References
External links Media related to Small Magellanic Cloud
at Wikimedia Commons NASA Extragalactic Database entry on the
SMC SEDS entry on the SMC
SMC at ESA/Hubble Astronomy Picture of the Day 2010
January 7 The Tail of the Small Magellanic Cloud - Likely stripped from
the galaxy by gravitational tides, the tail contains mostly gas, dust, and
newly formed stars.