Magellanic Clouds
Encyclopedia
The two Magellanic Clouds (or Nubeculae Magellani) are irregular
dwarf
galaxies visible in the southern hemisphere
, which are members of our Local Group
and are orbit
ing our Milky Way
galaxy
. The two galaxies are:
is by the Persian astronomer Al Sufi. In 964, in his Book of Fixed Stars
, he called it al-Bakr ("the Sheep") "of the southern Arabs"; he noted that the Cloud is not visible from northern Arabia and Baghdad
, but can be seen at the strait of Bab el Mandeb (12°15' N), which is the southernmost point of Arabia.
In Europe, the Clouds were first observed by Italian explorers Peter Martyr d'Anghiera
and Andrea Corsali
at the end of the 15th century. Subsequently, they were reported by Antonio Pigafetta
, who accompanied the expedition of Ferdinand Magellan
on its circumnavigation of the world in 1519-1522. However, naming the clouds after Magellan did not become widespread until much later. 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 Large Magellanic Cloud
and its neighbor and relative, the Small Magellanic Cloud
, are conspicuous objects in the southern hemisphere, looking like separated pieces of the Milky Way
to the naked eye. Roughly 21°
apart in the night sky, the true distance between them is roughly 75,000 light-years. Until the discovery of the Sagittarius Dwarf Elliptical Galaxy
in 1994, they were the closest known galaxies to our own. The LMC lies about 160,000 light years away, while the SMC is around 200,000. The LMC is about twice the diameter of the SMC (14,000 ly and 7,000 ly respectively). For comparison, the Milky Way is about 100,000 ly across.
Astronomers have long believed that the Magellanic Clouds have orbited the Milky Way at approximately their current distances, but evidence suggests that it is rare for them to come as close to the Milky Way as they are now. Observation and theoretical evidence suggest that the Magellanic Clouds have both been greatly distorted by tidal
interaction with the Milky Way as they travel close to it. Streams of neutral hydrogen connect them to the Milky Way and to each other, and both resemble disrupted barred spiral galaxies
. Their gravity has affected the Milky Way as well, distorting the outer parts of the galactic disk.
Aside from their different structure and lower mass, they differ from our Galaxy in two major ways. First, they are gas-rich; a higher fraction of their mass is hydrogen and helium
compared to the Milky Way. They are also more metal-poor than the Milky Way; the youngest star
s in the LMC and SMC have a metallicity
of 0.5 and 0.25 times solar, respectively. Both are noted for their nebula
e and young stellar populations, but as in our own Galaxy their stars range from the very young to the very old, indicating a long stellar formation
history.(Chaisson and McMillan)
The Large Magellanic Cloud was host galaxy to a supernova
(SN 1987A
), the brightest observed in over four centuries.
A recent French paper referenced in a Discovery Magazine blog suggests that the clouds may be ejected from a collision that formed the Andromeda Galaxy 6 billion years ago. However, this is an unlikely scenario, and many astronomers believe the Magellanic Clouds are natural remnants of the hierarchical process of galaxy formation that formed our Milky Way Galaxy.
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...
dwarf
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...
galaxies visible in the southern hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
, which are members of our 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...
and are orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
ing our Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
. The two galaxies are:
- 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...
(LMC) - Small Magellanic CloudSmall Magellanic CloudThe Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun....
(SMC)
History
The Magellanic Clouds have been known since the earliest times to the ancient Middle Eastern peoples. The first preserved mention of the Large Magellanic CloudLarge 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...
is by the Persian astronomer Al Sufi. In 964, in his Book of Fixed Stars
Book of Fixed Stars
The Book of Fixed Stars is an astronomical text written by Abd al-Rahman al-Sufi around 964. The book was written in Arabic, although the author himself was Persian...
, he called it al-Bakr ("the Sheep") "of the southern Arabs"; he noted that the Cloud is not visible from northern Arabia and Baghdad
Baghdad
Baghdad is the capital of Iraq, as well as the coterminous Baghdad Governorate. The population of Baghdad in 2011 is approximately 7,216,040...
, but can be seen at the strait of Bab el Mandeb (12°15' N), which is the southernmost point of Arabia.
In Europe, the Clouds were first observed by Italian explorers Peter Martyr d'Anghiera
Peter Martyr d'Anghiera
Peter Martyr d'Anghiera was an Italian-born historian of Spain and its discoveries during the Age of Exploration...
and Andrea Corsali
Andrea Corsali
Andrea Corsali was an Italian explorer who worked in the service of Giuliano di Lorenzo de' Medici of Florence and Lorenzo II de' Medici, duke of Urbino. Corsali traveled to Asia and the south seas aboard a Portuguese merchant vessel, writing home with accounts of the lands and peoples he...
at the end of the 15th century. Subsequently, they were reported 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...
, who accompanied the expedition of 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" ....
on its circumnavigation of the world in 1519-1522. However, naming the clouds after Magellan did not become widespread until much later. In Bayer's 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...
they are designated as nubecula major and nubecula minor. In the 1756 star map of the French astronomer Lacaille
Nicolas Louis de Lacaille
Abbé Nicolas Louis de Lacaille was a French astronomer.He is noted for his catalogue of nearly 10,000 southern stars, including 42 nebulous objects. This catalogue, called Coelum Australe Stelliferum, was published posthumously in 1763. It introduced 14 new constellations which have since become...
, they are designated as le Grand Nuage and le Petit Nuage ("the Large Cloud" and "the Small Cloud").
Characteristics
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...
and its neighbor and relative, the Small Magellanic Cloud
Small Magellanic Cloud
The Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun....
, are conspicuous objects in the southern hemisphere, looking like separated pieces of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
to the naked eye. Roughly 21°
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
apart in the night sky, the true distance between them is roughly 75,000 light-years. Until the discovery of the Sagittarius Dwarf Elliptical Galaxy
Sagittarius Dwarf Elliptical Galaxy
The Sagittarius Dwarf Elliptical Galaxy is an elliptical loop-shaped satellite galaxy of the Milky Way Galaxy. It consists of four globular clusters, the main cluster being discovered in 1994...
in 1994, they were the closest known galaxies to our own. The LMC lies about 160,000 light years away, while the SMC is around 200,000. The LMC is about twice the diameter of the SMC (14,000 ly and 7,000 ly respectively). For comparison, the Milky Way is about 100,000 ly across.
Astronomers have long believed that the Magellanic Clouds have orbited the Milky Way at approximately their current distances, but evidence suggests that it is rare for them to come as close to the Milky Way as they are now. Observation and theoretical evidence suggest that the Magellanic Clouds have both been greatly distorted by tidal
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...
interaction with the Milky Way as they travel close to it. Streams of neutral hydrogen connect them to the Milky Way and to each other, and both resemble disrupted barred spiral galaxies
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...
. Their gravity has affected the Milky Way as well, distorting the outer parts of the galactic disk.
Aside from their different structure and lower mass, they differ from our Galaxy in two major ways. First, they are gas-rich; a higher fraction of their mass is hydrogen and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
compared to the Milky Way. They are also more metal-poor than the Milky Way; the youngest star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s in the LMC and SMC have a metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
of 0.5 and 0.25 times solar, respectively. Both are noted for their nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
e and young stellar populations, but as in our own Galaxy their stars range from the very young to the very old, indicating a long stellar formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
history.(Chaisson and McMillan)
The Large Magellanic Cloud was host galaxy to a supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
(SN 1987A
SN 1987A
SN 1987A was a supernova in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a nearby dwarf galaxy. It occurred approximately 51.4 kiloparsecs from Earth, approximately 168,000 light-years, close enough that it was visible to the naked eye. It could be seen from the Southern...
), the brightest observed in over four centuries.
A recent French paper referenced in a Discovery Magazine blog suggests that the clouds may be ejected from a collision that formed the Andromeda Galaxy 6 billion years ago. However, this is an unlikely scenario, and many astronomers believe the Magellanic Clouds are natural remnants of the hierarchical process of galaxy formation that formed our Milky Way Galaxy.
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
- Astronomical Surveys of the Magellanic Clouds
- Irregular galaxyIrregular galaxyAn 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...
- Magellanic StreamMagellanic StreamThe Magellanic Stream is a high-velocity cloud of gas connecting the Large and the Small Magellanic Clouds. It came into existence by a near-collision of both galaxies some 2.5 billion years ago.-Discovery and early observations:...
- Magellanic BridgeMagellanic BridgeThe Magellanic Bridge is a stream of neutral hydrogen that links the two Magellanic Clouds. It is mostly a low-metallicity gas feature, though a few stars have been found inside it. It should not be confused with the Magellanic Stream, which links the MCs to the Milky Way.-References:* SIMBAD,...
Sources
- Eric Chaisson and Steve McMillan, Astronomy Today (Englewood Cliffs: Prentice-Hall, Inc., 1993), p. 550.
- Michael Zeilik, Conceptual Astronomy (New York: John Wiley & Sons, Inc., 1993), pp. 357–8.