While the LMC was long considered to be a satellite galaxy of the Milky Way, its status as a galaxy is now strongly disputed. Moreover, its status as a satellite galaxy of the Milky Way is also being disputed, since its total space velocity of 293 km/sec (relative to the galactocentric rest frame) is thought by many, if not most investigators, to be too high for the Milky Way to gravitationally hold on to.
Note that the calculations that determined the LMC’s total space velocity took into account that the LMC and SMC (Small Magellanic Cloud) are orbiting each other since they are gravitationally bound, and that their common center of mass is orbiting the Milky Way.
Quick Facts:
• Constellation: Dorado/Mensa
• Coordinates: RA 05h 23m 34.5s|Dec. -69° 45′ 22″
• Distance: 163,000 light years
• Mass: 1010 solar masses
• Object type: SB(s)m (Magellanic spiral galaxy)
• Apparent diameter: 10.75 degrees × 9.17 degrees
• Effective diameter: 14,000 light years
• Apparent magnitude: 0.9
• Other designations: LMC, ESO 56- G 115, PGC 17223, Nubecula Major
Visibility
Having a declination of about -70 degrees, the LMC is only visible in the Southern Celestial Hemisphere, and then only under dark skies from latitudes south of 20 degrees North. Located on the Dorado/Mensa border, the LMC appears as a faint, cloud-like object that stretches across about 10 degrees, which is 20 times as big as the full Moon.
However, an observer in the LMC would see the Milky Way at about magnitude -2.0., which is about 14 times as bright as the LMC is to an observer on Earth. Moreover, to an observer in the LMC the Milky Way would stretch across about 36 degrees, which is equivalent to 70 full Moons. In addition, since the LMC has a very high galactic latitude, an observer in the LMC would see the Milky Way at an oblique angle, which means that such an observer would see structure and detail in the Milky Way that is forever hidden from observers on Earth.
Physical Properties
Although the LMC contains a central bar, this bar is offset from the geometric centre, which suggests that the LMC may once have been a dwarf spiral galaxy, but that its spiral structure was likely disrupted or distorted by tidal interactions with both the Milky Way, and the Small Magellanic Could. This is supported by the fact that the LMC and SMC are enveloped by a common envelope of neutral hydrogen gas, which is conclusive evidence that the LMC and SMC have been gravitationally bound for a very long time. Note that there is also a bridge of gas and dust that connects the LMC to the SMC, with almost the entire bridge being an active star-forming region.
While initial surveys of the LMC that were conducted during September of 1966 did not detect X-ray emissions from the LMC that rose above the ambient level, a second survey conducted a few days later also failed to detect any X-ray emissions in the 8 to 80 kilo electron-Volt range. However, a subsequent X-ray survey of the constellation Dorado in the 1.5 to 10.5 kilo electron-Volt range revealed an X-ray source that extended across 12 degrees, which turned out be consistent with the dimensions of the LMC. This source also turned out to be reasonably luminous, having an emission rate of 4 x 1038 ergs/s that extended out to distance of about 50 kilo parsecs, which translates into 163,000 light years, which in turn, means that the X-ray emissions of the LMC reach Earth, or very close to Earth.
Star Formation
Like many other irregular galaxies, the LMC contains large volumes of dust and gas, and stars are currently being formed at a high rate all over the galaxy. In fact, despite its diminutive size, the LMC contains the Tarantula Nebula, which is one of the largest, and most active star-forming regions in the entire Local Group of Galaxies.
In terms of variety, the LMC contains a large selection of objects, which prompted one astronomer to describe the galaxy as “[an] astronomical treasure-house, a great celestial laboratory for the study of the growth and evolution of the stars.” This is borne out by the fact that collectively, several surveys of the galaxy have found about 60 globular clusters in a wide range of masses, more than 400 planetary nebulae, more than 700 open star clusters, and many hundreds of thousands of giant and super giant stars that cover the full range of possible masses. Moreover, the closest observed supernova, designated Supernova 1987a, occurred in the LMC.
Notable Features
In addition to its vast store of celestial objects, the LMC also contains a nitrogen-rich supernova remnant, named the Lionel-Murphy SNR (N86), which is named after a famous Australian High Court Judge. Because the Judge is known for his interest in science, astronomers at the Australian National University’s Mount Stromlo Observatory named the supernova remnant after him in honor of not only his interest in science, but also because the object faintly resembles the Judges’ large nose.