If you look skyward on a clear moonless night, you can immediately see that you and all of your fellow Earthlings live in a galaxian universe. Indeed, almost all of the celestial objects that you can see with your unaided eye reside within the Milky Way – our galactic home. From pin-point stars of multiple hues to hazy nebulae and silhouetting dust clouds, the myriad denizens of our Galaxy collectively trace a vast self-gravitating system that whirls about the supermassive black hole in its center while percolating in continual self-transformation.
Photomosaic of the entire sky at optical wavelengths showing the stellar and nebular disk of the Milky Way Galaxy across the middle, the Large and Small Magellanic Clouds to the lower right, and the Andromeda Galaxy which appears as a small lens-like object to the lower left. Image courtesy of Axel Mellinger (see http://home.arcor-online.de/axel.mellinger/mwpan_aitoff.html).
Just beyond the Milky Way, the Large and Small Magellanic Clouds can be readily spotted from the Earth’s southern hemisphere. These companions (or victims) of the much larger Milky Way are but the biggest and brightest of the 15-odd galaxies ...