Magellanic Clouds

Magellanic Clouds are two small Galaxies that are the nearest external galaxies to our own. Both clouds are easily visible to the naked eye, appearing like isolated offshoots of the Milky Way with apparent diameters of about 8º (the Large Magellanic Cloud) and 3º (the Small Magellanic Cloud). Their distances are estimated to be about 170,000 and 190,000 light years respectively. The clouds are sufficiently close to the Earth for detailed observations to be made of stars and nebulae; such observations are not possible for more distant galaxies. For example, the Period–Luminosity relationship for Cepheid variables was first established in 1912 by Henrietta Leavitt’s studies of the Small Magellanic Cloud.

The Large Magellanic cloud is a Small Galaxy that is the nearest external galaxy to our own. It is about 170,000 light years away and has just 5% to 10% of the mass of our Galaxy. It extends over about 8º of the sky within the constellations of Dorado and Mensa, where it is easily visible to the naked eye, appearing like a detached portion of the Milky Way. The Large Magellanic Cloud (LMC), along with the Small Magellanic Cloud, is named after Ferdinand Magellan, who observed them during his voyage around the world in 1519, but they were known before that date. The LMC’s physical diameter is 25,000 to 30,000 light years. At first sight it appears to be an irregular galaxy, but it has a central bar and possibly a spiral arm, so it is generally classed with the spiral galaxies. The Tarantula Nebula complex is found within the LMC and is one of the largest HII regions known. The complex is a vigorous star-forming region, and this, together with other star forming regions, may be the result of a close passage between the LMC and our Galaxy some 200 million years ago. There is also an enormous bridge of material between the Galaxy and the Magellanic Clouds. Known as the Magellanic Stream, this bridge is one of the largest high-velocity clouds known and probably resulted from the same close passage.

Young objects, such as hot stars and Cepheids, in the LMC lie in a thin disk, which is seen nearly face-on (an angle to the plane of the sky of about 27º). Older objects, such as planetary nebulae, form a somewhat thicker disk structure, but there is no evidence at present for a spherical halo such as is formed by very old objects in our own Galaxy. In our Galaxy, Globular Clusters are all old objects. However, the cluster NGC1866 in the LMC, though globular in form, is a young system. Several such ‘blue globulars’ exist in the clouds. No similar objects are known in our own Galaxy.

The Small Magellanic Cloud (SMC) is also small galaxy and is the second nearest external galaxy to us. It is about 190,000 light years away and has about 2% of the mass of our Galaxy. It extends over about 3º of the sky within the constellation of Tucana. It is an irregular galaxy with a Hubble Classification of Irr I. The SMC’s physical width is about 15,000 light years, but it is highly elongated with the long axis along the line of sight. Its depth is therefore some 60,000 light years. The curious, long, twisted structure that has been found for the galaxy suggests that strong tidal forces have distorted the SMC. The SMC and the LMC orbits our Galaxy roughly at right angles to the plane of the Milky Way, and it may at some time in the future be disrupted and captured by our Galaxy.

Questions to Ponder

• What Triggers Star Formation in the Magellanic Clouds?
• How far away is the Large Magellanic Cloud from the Earth?
• What is the shape of the Large Magellanic Cloud?