The Magellanic Clouds
Our galaxy's brightest satellite systems are the Magellanic Clouds, obvious naked-eye objects deep in the southern skies. They are shown together here in a 45-minute exposure on Ektachrome 400 taken from Cerro Tololo, Chile (and scanned at fairly high density), using a Canon 50mm lens wide open at Near the Small Cloud, what looks like a bright star is in fact the brilliant nearby globular cluster 47 Tucanae . There are, of course, many wonderful features to be found within the Clouds. For example, they have a population of very blue and very populous star clusters, sometimes called "Blue Globulars" to honor the suspicion that they might, in fact, be progenitors of what will one day resemble galactic globular clusters. Similar star clusters are found to be numerous in starburst galaxis, especially interacting and merging systems. One particularly striking example is NGC 1978 in the LMC. There are several regions for which HST images are available as pretty pictures on the Web, aside from vast number in the archive. This includes the three-ring circus of ejecta around SN 1987A (with the rings clearly distinguished and the evolution of the expanding fireball followed with WFPC2, and the blast wave dynamics and remnant chemistry explored with STIS). In addition, one can see data on other supernova remnants, such as the oxygen-rich N132D , on the immense central star cluster R136 of 30 Doradus, and the contrasting star clusters of NGC 1850 and its unnamed neighbor.
This image has been fairly popular, showing up in textbooks and even the occasional childrens' book. Unfortunately for me, NOAO at the time had a policy of taking employees' images into their files and not keeping a name associated with them... It is also available for download in a 1600x2300-pixel TIFF file (11 Mbytes).
The Magellanic Clouds continue to play a very important role in our understanding of the Universe. The fact that they are well observable only from southern latitudes was an important driver in the eastablishment of southern stations of such facilities as the Harvard College Observatory, and made looking at them a major part of the overall programs of the later, larger southern-hemisphere telescopes. They might as well have been tailor-made laboratories to test our ideas about the evolution of stars - huge collections of stars of all ages and luminosity, all at just about the same distance from us so that we can compare them to each other free of the uncertainties due to distance which enter so easily when we study various populations in our own galaxy.
It was from observations in the Magellanic Clouds that Henrietta Leavitt discovered (1912) the relation between pulsation period and average luminosity for Cepheid variable stars. These stars are intrinsically very bright so we can see them in distant galaxies; in fact, it was by identifying these in small dwarf galaxies and eventually the great spiral in Andromeda that Edwin Hubble (in a series or projects in the 1920s) went on to demonstrate the existence of other galaxies well beyond the Milky Way. The telescope bearing his name has been used to extend this technique beyond 70 million light-years distance, and the Cepheids in the Magellanic Clouds are still the zero point for these distance measurements.
As irregular systems much smaller and less massive than our own Milky Way, the Clouds provide us with environments to see how their makeup and surroundings can change the life cycles of stars. Each of these has had less starbirth over time than our galaxy, and has retained a smaller mass of heavy elements produced in preceding generations of stars. This makes their stars more like those in our galaxy were early in its history, and more like what we see by looking deep into the Universe at early epochs. For example, the most massive stars blow away large amounts of gas in winds, driven as their radiation pushes on the outer gas layers - most efficiently on atoms of heavier elements such as carbon, nitrogen, and oxygen, so that these winds are much less pronounced for stars in the Clouds.
The Large Cloud provided us with an unexpected boon in 1987, when the light from the explosion of a star 150,000 years earlier finally reached us. This was the nearest supernova explosion since the 18th century (and it't not completely clear that anyone actually saw that one). For the first time, we knew what star had blown up (now observing a moment of silence in honor of Sanduleak -69 202) and what kind it had been. Experiments around the world detected the fleeting neutrinos produced as a neutron star was formed in the heart of the explosion, a spectacular confirmation of what had been a purely theoretical notion. Further observations of the explosion remnant have allowed astronomers to map the three-dimensional distribution of gas and dust in its vicinity (and incidentally, find the reflected light from supernovae in the Large Cloud which would have been seen on Earth only prior to Magellan's voyage). As the blast remnant develops, the Hubble and Chandra observatories are now watching gas once expelled by the star being heated and evaporating as the explosion's cooling fireball reaches it.
The list goes on. The Large Cloud was one of the favored locations to watch for gravitational lensing by dark stellar objects in the halo of our galaxy, by the MACHO collaboration (using the Great Melbourne Telecope, since destroyed by a bush fire). These results have sharpened and deepened the mystery of dark matter in galaxies.
Finally, radio telescopes have shown that our own galaxy is bad for the health of the Magellanic Clouds. They both lie in a stream of hydrogen and fugitive stars pulled out of them by the tidal forces from the Milky Way's gravity. We see signs that some even smaller former satellite galaxies have not survived even that well, but the Clouds' epochs are numbered.
Some old books used the terms Nebecula Major and Minor to refer to them, but in contemporary usage, in all the langages I know well enough to read, they now bear Magellan's name. Naturally his crew discovered them only in the same sense that European explorers of that era discovered the Americas, Australia, New Zealand, or Hawaii, but for astronomers that was enough. Their recognition became knit into the globally-connected community of understanding and they became, in astronomers' odd way, part of our world.
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