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The Magellanic Clouds, Two Satellite Galaxies of the Milky Way

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The Magellanic Clouds, Two Satellite Galaxies of the Milky Way The Magellanic Clouds, two satellite galaxies of the Milky Way Domingos Soares I would like to suggest an exciting adventure through the southern hemi- sphere skies: to see, with the naked eye, an entire galaxy! And if you are lucky, two! They are the LARGE MAGELLANIC CLOUD and the SMALL MAGELLANIC CLOUD! Both are dwarf galaxies, with irregular shapes, satellites of our own galaxy, the Milky Way. But that is not too bad for someone to begin in the fabulous world of galaxies. Galaxies are large ensembles of stars that are kept together by their mutual gravitational attraction. The Sun is just one of the hundreds of billions of stars of our galaxy, which we call Milky Way. The gravitational attraction is one of the fundamental interaction in na- ture | matter attracts matter | and is responsible, for example, for the weight we have and for the presence of the Moon on our skies. The stability of the universe is kept by the perfect balance between the gravitational at- traction and the motion of the bodies. If the Moon, for some reason, stopped moving, it would \fall" into the Earth. If the stars stopped, they would \fall" into each other. Galaxies are many times called the fundamental \blocks" of the structure of the universe. The universe (all that exists) can be studied on its \large scale" as if it were a set of \points", in which every \point" is, in reality, one galaxy. On such studies, one tries to understand the evolution of the universe as a whole. That is why the phenomenal galaxies can be treated as if they were simple points. The investigation of the origin and evolution of the universe is made in the branch of science | in fact, a subdivision of Astrophysics | called Cosmology. The Milky Way has many small companions (satellite galaxies), which go along with it in its journey through the cosmic space. Two of them are 1 especial because are linked to the ancient Portuguese explorers of the oceans and may be easily seen on the southern hemisphere skies. They are the Large Magellanic Cloud and the Small Magellanic Cloud. Those \clouds" were sighted (and recorded) in 1520 by the Portuguese navigator Ferdinand Magellan (1480-1521). He was the first inhabitant of the northern hemisphere to refer to them in his voyage annotations. This is the reason for the name given to those galaxies, which make a breathtaking scenery in the sky, and that have a great importance in the history of scientific discoveries in Astronomy. Being galaxies, the Magellanic Clouds are constituted by stars. But as in every galaxy, between the stars, there is also much gas (mainly hydro- gen, the most abundant chemical element in the whole universe) and much \dust", clusters of particles of dimensions much larger than the most simple molecules. The most common particles are microscopic fragments of graphite, the same of our pencils. These dust particles can be organic macromolecules as well. The Magellanic Clouds are located at a distance of 180,000 light-years and are amongst the first in the list of the closest galaxies to the Milky Way. The galaxies \Canis Majoris Dwarf" (discovered in 2003) and \Sagittarius Dwarf" (discovered in 1994) are our closest neighbors, located at approx- imately 50,000 and 90,000 light-years from our galaxy, respectively. They are, nevertheless, very difficult to see because they blend with the stars, gas and dust of our own galaxy, since they overlap with the region of the largest concentration of matter of the Milky Way. These and other satellite galaxies | they are about twenty! | are in constant motion around the Milky Way, to which they are bound by the gravitational force. With the exception of the Milky Way's satellite galaxies, all other galaxies are at many millions of light-years from us. For example, the Andromeda galaxy, which is very similar to the Milky Way and the closest among the most distant, is at a distance of 2 million light-years. The distance of 1 light-year is the distance that a light ray travels during the time period of 1 year. It is indeed a very long distance! To have an idea of such a distance suffice it to say that the Sun is located at a distance of 8 light-minutes from the Earth, that is, the solar light takes approximately 8 minutes from the surface of the Sun to the surface of the Earth. This is equivalent to 94 million miles! The Small Magellanic Cloud is a bit farther from us than the Large Cloud. The two clouds are connected by an \invisible" cloud of hydrogen, which can only be \seen" with the aid of an especial telescope called \radio telescope", 2 that detects the radio waves emitted by the atoms of hydrogen. The Magellanic Clouds photographed by David Malin, using equipment of the Anglo-Australian Observatory, located in Australia. The Large Cloud is seen at the upper right corner of the picture and the Small Cloud at the lower left corner. We need clear skies to see the Magellanic Clouds. This is only possible outside the pollution (including light pollution) that characterizes large cities. Those who are in regions of darker skies can have a good sight of these neighbors of ours. Nights without the Moon, or with low brightness Moon, are particularly favorable because the sky brightness is enormously reduced, making it easy to observe such a low brightness celestial objects. How to find the Magellanic Clouds in the sky? It is very easy if you are in the southern hemisphere. Find the two brightest stars of the sky: Sirius (\alpha", that is, \the most bright" of the constellation of Canis Major, and Canopus (alpha of the constellation of Carina). In the end and beginning of each year, they can be easily located in the direction of the southeast horizon, 3 around 9 p.m. Next to Sirius, in the east direction, is the constellation of Orion, the Hunter, with the well-known Orion's belt stars, the Three Sisters. Orion's brightest star is Rigel. It is very close to Sirius. The angular separation, that is, the separation seen in the sky, between Sirius and Rigel is more or less two thirds of the separation between Sirius and Canopus. The latter, in its turn, is closest to the south horizon than Sirius. And the Magellanic Clouds? Sirius and Canopus \point" to them. Imagine a straight line beginning in Sirius and passing through Canopus. At a bit less than half the separa- tion between Sirius and Canopus, measured from Canopus, lies the fabulous LARGE MAGELLANIC CLOUD! Go down a little bit farther and there you will find, fainter, the SMALL MAGELLANIC CLOUD! And do not forget that they are galaxies! Millions of stars together due to their mutual gravity; and they, by their turn, are bound to our galaxy's gravity! 4 Picture by Akira Fujii, where one can see the Magellanic Clouds and the stars that point to them, Sirius and Canopus. The brightest star of the whole sky, Sirius, is up and to the left. The Orion constellation is to the right of Sirius and Rigel is its brightest star. Orion's belt is the three aligned stars just above and to the left of Rigel. Canopus, the second brightest star of the sky, is well below Sirius and both point to the Large Magellanic Cloud. The Small Cloud is fainter and is below the Large Cloud, to the right. Use this photo to find the Clouds from your observation site. East is to the right and South downwards. 5 Southern hemisphere observers, willing to see the Magellanic Clouds, do not need to wait until the end of the year, when the hours of observation are much more convenient. In September, for example, they will have to schedule their observations by dawn! Sirius and Canopus are well placed in the directions mentioned above and the Magellanic Clouds can be seen. But | it is worthwhile repeat |, unfortunately, one will have too much difficulty to observe the Clouds in big cities' skies, because of the combination of a polluted atmosphere and of excessive artificial illumination. However, even so, the \pointers", the stars Sirius and Canopus, can be easily seen, as well as Rigel. Those who are in the outskirts of big cities will be able to see the Clouds without major burdens. Simply cheer yourself up and rise very early! But do not give up! There are two factors that quickly put the Clouds in their perfect conditions of observation. First, notice that during the passing of a night the positions of the objects on the sky change. Due to the Earth's rotational motion around its own axis, people in the southern hemisphere see the objects on the sky rotate around the \south celestial pole" (a point where Earth's rotation axis points to). Besides that, the Earth has also its annual movement of translation around the Sun. The combined movements of rotation and translation of the Earth result in that the \rise" of the stars on the horizon \moves forward" 4 minutes each day and 2 hours every month. Because of that, in a few months after our September example, Sirius, Canopus and the Clouds will be \rising" at about 6 p.m. and will be optimal for observation around 9 p.m. Those who are not willing to get up very early in September, have just to wait for two months and the following months. The conditions will be then excellent for a true tour through the galaxies! 6.
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