Large Magellanic Cloud

Large Magellanic Cloud
The Large Magellanic Cloud
Observation data (J2000 epoch)
Constellation	Mensa/Dorado
Right ascension	05h 23m 34.5s[1]
Declination	-69° 45′ 22″[1]
Redshift	278 ± 3 km/s[1]
Distance	157 kly (48.5 kpc)[2]
Type	SB(s)m[1]
Apparent dimensions (V)	10.75° × 9.17°[1]
Apparent magnitude (V)	0.9[1]
Other designations
LMC, ESO 56- G 115, PGC 17223[1]
See also: Galaxy, List of galaxies

The Large Magellanic Cloud (LMC) is a nearby irregular galaxy, once thought to be a satellite of our own.^[3][4] At a distance of slightly less than 50 kiloparsecs (≈160,000 light-years), the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal (~ 16 kiloparsecs) and Canis Major Dwarf Galaxy (~ 12.9 kiloparsecs) lying closer to the center of the Milky Way. It has a mass equivalent to approximately 10 billion times the mass of our Sun (10¹⁰ solar masses), making it roughly 1/10 as massive as the Milky Way, and a diameter of about 14,000 light-years.^[5] The LMC is the fourth largest galaxy in the Local Group, the first, second and third largest places being taken by Andromeda Galaxy (M31), our own Milky Way Galaxy, and the Triangulum Galaxy (M33), respectively.

While the LMC is often considered an irregular type galaxy (the NASA Extragalactic Database lists the Hubble sequence type as Irr/SB(s)m), the LMC contains a very prominent bar in its center, suggesting that it may have previously been a barred spiral galaxy. The LMC's irregular appearance is possibly the result of tidal interactions with both the Milky Way, and the Small Magellanic Cloud (SMC).

It is visible as a faint "cloud" in the night sky of the southern hemisphere, straddling the border between the constellations of Dorado and Mensa.

History

The first recorded mention of the Large Magellanic Cloud was by the Persian astronomer, Abd Al-Rahman Al Sufi (later known in Europe as "Azophi"), in his Book of Fixed Stars around 964 AD.^[6][7]

The next recorded observation was in 1503–4 by Amerigo Vespucci in a letter about his third voyage. In this letter he mentions "three Canopes, two bright and one obscure"; "bright" refers to the two Magellanic Clouds, and "obscure" refers to the Coalsack.^[8]

Fernão de Magalhães, better known as Ferdinand Magellan, on his voyage in 1519, was the first to bring the LMC into common Western knowledge. The galaxy now bears his name.^[7]

Geometry

The Large Magellanic Cloud has a prominent central bar and a spiral arm.^[9] The central bar seems to be warped so that the east and west ends are nearer the Milky Way than the middle.^[10]

The LMC was long considered to be a planar galaxy that could be assumed to lie at a single distance from us. However, in 1986, Caldwell and Coulson^[11] found that field Cepheid variables in the northeast portion of the LMC lie closer to the Milky Way than Cepheids in the southwest portion. More recently, this inclined geometry for fields stars in the LMC has been confirmed via observations of Cepheids,^[12] core helium burning red clump stars^[13] and the tip of the red giant branch.^[14] All three of these papers find an inclination of ~ 35^o, where a face on galaxy has an inclination of 0^o. Further work on the structure of the LMC using the kinematics of carbon stars showed that the LMC's disk is both thick^[14] and flared.^[15] Regarding the distribution of star clusters in the LMC, Schommer et al.^[16] measured velocities for ~80 clusters and found that the LMC's cluster system has kinematics consistent with the clusters moving in a disk-like distribution. These results were confirmed by Grocholski et al.,^[17] who calculated distances to a number of clusters and showed that the LMC's cluster system is in fact distributed in the same plane as the field stars.

Distance

Location of the Large Magellanic Cloud with respect to the Milky Way and other satellite galaxies.

Determining a precise distance to the LMC, as with any other galaxy, was challenging due to the use of standard candles for calculating distances, with the primary problem being that many of the standard candles are not as 'standard' as one would like; in many cases, the age and/or metallicity of the standard candle plays a role in determining the intrinsic luminosity of the object. The distance to the LMC has been calculated using a variety of standard candles, with Cepheid variables being one of the most popular. Cepheids have been shown to have a relationship between their absolute luminosity and the period over which their brightness varies. However, Cepheids appear to suffer from a metallicity effect, where Cepheids of different metallicities have different period-luminosity relations. Unfortunately, the Cepheids in the Milky Way typically used to calibrate the period-luminosity relation are more metal rich than those found in the LMC.

In the era of 8-meter-class telescopes, eclipsing binaries have been found throughout the Local Group. Parameters of these systems can be measured without mass or compositional assumptions. The light echoes of supernova 1987A are also geometric measurements, without any stellar models or assumptions.

Recently, the Cepheid absolute luminosity has been re-calibrated using Cepheid variables in the galaxy NGC 4258 that cover a range of metallicities.^[2] Using this improved calibration, they find an absolute distance modulus of (m − M)₀ = 18.41, or 48 kpc (~157,000 light years). This distance, which is slightly shorter than the typically assumed distance of 50 kpc, has been confirmed by other authors (see references to the NGC 4258 paper).

By cross-correlating different measurement methods, one can bound the distance; the residual errors are now less than the estimated size parameters of the LMC. Further work involves measuring the position of a target star or star system within the galaxy (i.e., toward or away from the observer).

Features

Like many irregular galaxies, the LMC is rich in gas and dust, and it is currently undergoing vigorous star formation activity.^[18] It is home to the Tarantula Nebula, the most active star-forming region in the Local Group.

The LMC is full of a wide range of galactic objects and phenomena that make it aptly known as an "astronomical treasure-house, a great celestial laboratory for the study of the growth and evolution of the stars," as described by Robert Burnham, Jr.^[19] Surveys of the galaxy have found roughly 60 globular clusters, 400 planetary nebulae, and 700 open clusters, along with hundreds of thousands of giant and supergiant stars.^[20] Supernova 1987a—the nearest supernova in recent years—was also located in the Large Magellanic Cloud. The Lionel-Murphy SNR is nitrogen-abundant supernova remnant (SNR) N86 in the Large Magellanic Cloud named by astronomers at the Australian National University's Mount Stromlo Observatory in acknowledgement of Australian High Court Justice Lionel Murphy's interest in science and because of SNR N86's perceived resemblance to his large nose.^[21]

View from the LMC

From a viewpoint in the LMC, the Milky Way would be a spectacular sight. The galaxy's total apparent magnitude would be -2.0—over 14 times brighter than the LMC appears to us on Earth—and it would span about 36° across the sky, which is the width of over 70 full moons. Furthermore, because of the LMC's high galactic latitude, an observer there would get an oblique view of the entire galaxy, free from the interference of interstellar dust which makes studying in the Milky Way's plane difficult from Earth.^[22] The Small Magellanic Cloud would be about magnitude 0.6, substantially brighter than the LMC appears to us.

Gallery

LH 95 stellar nursery in Large Magellanic Cloud. Credit: NASA/ESA	SNR B0544-6910 in the LMC. Credit: ESO	SNR 0543-689 in the LMC. Credit: ESO	N44 region in the Large Magellanic Cloud. Credit: ESO
DEM L 159 and two clusters KMHK 840 (top left) and KMHK 831 (bottom right). Credit: ESO

See also

• Large Magellanic Cloud in fiction

Notes

Further reading

• "Single measurement throws out everything we thought we knew". physorg.com, September 17, 2007.

External links

Wikimedia Commons has media related to: Large Magellanic Cloud

• NASA Extragalactic Database
• Encyclopedia of Astronomy entry
• SEDS LMC page

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Coordinates: 05^h 23^m 34.5^s, −69° 45′ 22″