Small Magellanic Cloud
Encyclopedia
The Small Magellanic Cloud (SMC) is a dwarf galaxy
. It has a diameter of about 7,000 light-year
s and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun.
Some speculate that the SMC was once a barred spiral galaxy
that was disrupted by the Milky Way to become somewhat irregular
. It contains a central bar structure.
At a distance of about 200,000 light-year
s, 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 lies a further 20 degrees to the east. The Small Magellanic Cloud is a member of the Local Group
.
islanders and indigenous Australians
. Persian astronomer Al Sufi
labelled the larger of the two clouds as Al Bakr, the White Ox. Europe
an 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 inches (35.6 cm) 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, Peru
. From 1893 and 1906, under the direction of Solon Bailey, the 24 inches (609.6 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 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 (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.
HMXB pulsars are rotating neutron stars (NSs) in binary systems with Be-type (spectral type 09-B2, luminosity classes V–III) or supergiant stellar companions. Most HMXBs are of the Be type which account for 70% in the Milky Way an 98% in the SMC (Coe et al. 2005). The Be-star equatorial disk provides a reservoir of matter that can be accreted onto the NS during periastron passage (most known systems have large orbital eccentricity) or during large-scale disk ejection episodes. This scenario leads to strings of X-ray outbursts with typical
luminosities LX=1036-1037 erg s−1, spaced at the orbital period, plus infrequent giant outbursts of greater duration and luminosity (see Negueruela 1998 for a review).
Monitoring surveys of the SMC performed with NASA's Rossi X-ray Timing Explorer (RXTE) (Laycock et al. 2005, hereafter L05; Galache et al. 2008) see X-ray pulsars in outburst at >1036 erg s−1 and have counted 50 by the end of 2008. The ROSAT and ASCA missions detected many faint X-ray point sources (e.g., Haberl & Sasaki 2000), but the typical positional uncertainties frequently made positive identification difficult. Recent studies using XMM-Newton (Haberl et al. 2008; Haberl & Pietsch 2004) and Chandra (Antoniou et al. 2009; Edge et al. 2004, and Laycock et al. 2010) have now cataloged several hundred X-ray sources in the direction of the SMC, the 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 (Lx) 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 (3U 0115-73, 2A 0116-737, SMC X-1). 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 (3A) also contains this early X-ray source within Tucana: 3A 0116-736 (2A 0116-737, SMC X-1). The SMC X-1, a HMXRB, is at J2000 right ascension
(RA) declination
(Dec) .
Two additional sources detected and listed in 3A include SMC X-2 at 3A 0042-738 and SMC X-3 at 3A 0049-726.
Dwarf galaxy
A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...
. It has a diameter of about 7,000 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun.
Some speculate that the SMC was once a barred spiral galaxy
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies...
that was disrupted by the Milky Way to become somewhat irregular
Irregular galaxy
An irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...
. It contains a central bar structure.
At a distance of about 200,000 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s, 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
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
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
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
of Tucana
Tucana
Tucana is a constellation in the southern sky, created in the late sixteenth century. Its name is Latin for the toucan, a South American bird.-History:...
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
Surface brightness
The overall brightness of an extended astronomical object such as a galaxy, star cluster, or nebula, can be measured by its total magnitude, integrated magnitude or integrated visual magnitude; a related concept is surface brightness, which specifies the brightness of a standard-sized piece of an...
, it is best viewed from a dark site away from city lights.
It forms a pair with the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
(LMC), which lies a further 20 degrees to the east. The Small Magellanic Cloud is a member of the Local Group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
.
Observation history
In the southern hemisphere, the Magellanic clouds have long been included in the lore of native inhabitants, including south seaPacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
islanders and indigenous Australians
Indigenous Australians
Indigenous Australians are the original inhabitants of the Australian continent and nearby islands. The Aboriginal Indigenous Australians migrated from the Indian continent around 75,000 to 100,000 years ago....
. Persian astronomer Al Sufi
Abd Al-Rahman Al Sufi
Abd al-Rahman al-Sufi was a Persian astronomer also known as Abd ar-Rahman as-Sufi, or Abd al-Rahman Abu al-Husayn, Abdul Rahman Sufi, Abdurrahman Sufi and known in the west as Azophi; the lunar crater Azophi and the minor planet 12621 Alsufi are named after him...
labelled the larger of the two clouds as Al Bakr, the White Ox. Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
an sailors may have first noticed the clouds during the Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...
when they were used for navigation. Portuguese
Portugal
Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...
and Dutch
Dutch Republic
The Dutch Republic — officially known as the Republic of the Seven United Netherlands , the Republic of the United Netherlands, or the Republic of the Seven United Provinces — was a republic in Europe existing from 1581 to 1795, preceding the Batavian Republic and ultimately...
sailors called them the Cape Clouds, a name that was retained for several centuries. During the circumnavigation of the Earth by Ferdinand Magellan
Ferdinand Magellan
Ferdinand Magellan was a Portuguese explorer. He was born in Sabrosa, in northern Portugal, and served King Charles I of Spain in search of a westward route to the "Spice Islands" ....
in 1519–22, they were described by Antonio Pigafetta
Antonio Pigafetta
Antonio Pigafetta was an Italian scholar and explorer from the Republic of Venice. He travelled with the Portuguese explorer Ferdinand Magellan and his crew on their voyage to the Indies. During the expedition, he served as Magellan's assistant and kept an accurate journal which later assisted him...
as dim clusters of stars. In Johann Bayer
Johann Bayer
Johann Bayer was a German lawyer and uranographer . He was born in Rain, Bavaria, in 1572. He began his study of philosophy in Ingolstadt in 1592, and moved later to Augsburg to begin work as a lawyer. He grew interested in astronomy during his time in Augsburg...
's celestial atlas Uranometria
Uranometria
Uranometria is the short title of a star atlas produced by Johann Bayer.It was published in Augsburg, Germany, in 1603 by Christophorus Mangus under the full title Uranometria : omnium asterismorum continens schemata, nova methodo delineata, aereis laminis expressa. This translates to...
, published in 1603, he named the smaller cloud, Nubecula Minor. In Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
, Nubecula means a little cloud.
Between 1834 and 1838, John Frederick William Herschel made observations of the southern skies with his 14 inches (35.6 cm) reflector from the Royal Observatory at the Cape of Good Hope
Cape of Good Hope
The Cape of Good Hope is a rocky headland on the Atlantic coast of the Cape Peninsula, South Africa.There is a misconception that the Cape of Good Hope is the southern tip of Africa, because it was once believed to be the dividing point between the Atlantic and Indian Oceans. In fact, the...
. 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
Harvard College Observatory
The Harvard College Observatory is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Department of Astronomy. It is located in Cambridge, Massachusetts, USA, and was founded in 1839...
opened an observing station at Arequipa, Peru
Arequipa Region
Arequipa is a region in southwestern Peru. It is bordered by the Ica, Ayacucho, Apurímac and Cusco regions on the north; the Puno Region on the east; the Moquegua Region on the south; and the Pacific Ocean on the west...
. From 1893 and 1906, under the direction of Solon Bailey, the 24 inches (609.6 mm) telescope at this site was used to survey photographically both the Large and Small Magellanic Clouds. Henrietta Swan Leavitt
Henrietta Swan Leavitt
Henrietta Swan Leavitt was an American astronomer. A graduate of Radcliffe College, Leavitt went to work in 1893 at the Harvard College Observatory in a menial capacity as a "computer", assigned to count images on photographic plates...
, an astronomer at the Harvard College Observatory
Harvard College Observatory
The Harvard College Observatory is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Department of Astronomy. It is located in Cambridge, Massachusetts, USA, and was founded in 1839...
, 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
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
called a "cluster variable", later called a Cepheid variable
Cepheid variable
A Cepheid is a member of a class of very luminous variable stars. The strong direct relationship between a Cepheid variable's luminosity and pulsation period, secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales.Cepheid...
after the prototype star Delta Cephei
Delta Cephei
Delta Cephei is a binary star system approximately 891 light-years away in the constellation of Cepheus . Delta Cephei is the prototype of the Cepheid variable stars, and it is among the closest stars of this type to the Sun...
, 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
Ejnar Hertzsprung
Ejnar Hertzsprung was a Danish chemist and astronomer.Hertzsprung was born in Copenhagen. In the period 1911–1913, together with Henry Norris Russell, he developed the Hertzsprung–Russell diagram....
. First he measured thirteen nearby cepheid variables to find the absolute magnitude
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
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.
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 (HMXBs) 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 (NSs) in binary systems with Be-type (spectral type 09-B2, luminosity classes V–III) or supergiant stellar companions. Most HMXBs are of the Be type which account for 70% in the Milky Way an 98% in the SMC (Coe et al. 2005). The Be-star equatorial disk provides a reservoir of matter that can be accreted onto the NS during periastron passage (most known systems have large orbital eccentricity) or during large-scale disk ejection episodes. This scenario leads to strings of X-ray outbursts with typical
luminosities LX=1036-1037 erg s−1, spaced at the orbital period, plus infrequent giant outbursts of greater duration and luminosity (see Negueruela 1998 for a review).
Monitoring surveys of the SMC performed with NASA's Rossi X-ray Timing Explorer (RXTE) (Laycock et al. 2005, hereafter L05; Galache et al. 2008) see X-ray pulsars in outburst at >1036 erg s−1 and have counted 50 by the end of 2008. The ROSAT and ASCA missions detected many faint X-ray point sources (e.g., Haberl & Sasaki 2000), but the typical positional uncertainties frequently made positive identification difficult. Recent studies using XMM-Newton (Haberl et al. 2008; Haberl & Pietsch 2004) and Chandra (Antoniou et al. 2009; Edge et al. 2004, and Laycock et al. 2010) have now cataloged several hundred X-ray sources in the direction of the SMC, the 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
Thor
In Norse mythology, Thor is a hammer-wielding god associated with thunder, lightning, storms, oak trees, strength, the protection of mankind, and also hallowing, healing, and fertility...
missile launched from Johnston Atoll
Johnston Atoll
Johnston Atoll is a atoll in the North Pacific Ocean about west of Hawaii. There are four islands located on the coral reef platform, two natural islands, Johnston Island and Sand Island, which have been expanded by coral dredging, as well as North Island and East Island , an additional two...
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 (Lx) 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
Uhuru (satellite)
Uhuru was the first satellite launched specifically for the purpose of X-ray astronomy. It was also known as the X-ray Explorer Satellite, SAS-A , SAS 1, or Explorer 42.The observatory was launched on 12 December 1970 into an initial orbit of about 560 km apogee, 520 km...
catalog lists an early X-ray source within the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Tucana
Tucana
Tucana is a constellation in the southern sky, created in the late sixteenth century. Its name is Latin for the toucan, a South American bird.-History:...
: 4U 0115-73 (3U 0115-73, 2A 0116-737, SMC X-1). Uhuru
Uhuru (satellite)
Uhuru was the first satellite launched specifically for the purpose of X-ray astronomy. It was also known as the X-ray Explorer Satellite, SAS-A , SAS 1, or Explorer 42.The observatory was launched on 12 December 1970 into an initial orbit of about 560 km apogee, 520 km...
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 (3A) also contains this early X-ray source within Tucana: 3A 0116-736 (2A 0116-737, SMC X-1). The SMC X-1, a HMXRB, is at J2000 right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...
(RA) declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
(Dec) .
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 (MMC)
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 (as seen from our perspective), 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 CloudLarge Magellanic CloudThe Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
- Magellanic CloudsMagellanic CloudsThe two Magellanic Clouds are irregular dwarf galaxies visible in the southern hemisphere, which are members of our Local Group and are orbiting our Milky Way galaxy...
- Objects within the Small Magellanic Cloud
- NGC 265NGC 265NGC 265 is an open cluster in the Small Magellanic Cloud, which is located in the constellation Tucana.- See also :* NGC 290* Open cluster* Tucana constellation* Small Magellanic Cloud- External links :**...
- NGC 290NGC 290NGC 290 is an open cluster in the Small Magellanic Cloud, which is located in the constellation Tucana.- See also :* NGC 265* Open cluster* Tucana constellation* Small Magellanic Cloud- External links :**...
- NGC 346NGC 346NGC 346 is an open cluster with associated nebula located in the Small Magellanic Cloud that appears in the constellation Tucana.-Gallery:-External links:* * *...
- NGC 347
- NGC 602NGC 602NGC 602 is a young, bright open cluster of stars located in the Small Magellanic Cloud , a satellite galaxy to the Milky Way. Radiation and shock waves from the stars have pushed away much of the lighter surrounding gas and dust that compose the nebula known as N90, and this in turn has triggered...
- NGC 265
External links
- NASA Extragalactic Database entry on the SMC
- SEDS entry on the SMC
- SMC at ESA/Hubble
- Astronomy Picture of the DayAstronomy Picture of the DayAstronomy Picture of the Day is a website provided by NASA and Michigan Technological University . According to the website, "Each day a different image or photograph of our universe is featured, along with a brief explanation written by a professional astronomer."The photograph is not necessarily...
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.