Light zips along through interstellar space at 186,000 miles (300,000 kilometers) per second (more than 66 trips across the entire United States, in one second). It’s one of the most commonly used celestial yardsticks, the distance light travels in one year. Ok, fine, but what the heck is a light-year? Another galaxy, IC 1101, spans as much as 4 million light-years. Our neighboring Andromeda galaxy, for example, is some 220,000 light-years wide. That sounds huge, and it is, at least until we start comparing it to other galaxies. Our galaxy probably contains 100 to 400 billion stars, and is about 100,000 light-years across. Groups of them are bound into clusters of galaxies, and these into superclusters the superclusters are arranged in immense sheets stretching across the universe, interspersed with dark voids and lending the whole a kind of spiderweb structure. Based on the deepest images obtained so far, it’s one of about 2 trillion galaxies in the observable universe. Our galaxy is a gravitationally bound collection of stars, swirling in a spiral through space. To get a better sense, for instance, of the true distances to exoplanets – planets around other stars – we might start with the theater in which we find them, the Milky Way galaxy. When we talk about the enormity of the cosmos, it’s easy to toss out big numbers – but far more difficult to wrap our minds around just how large, how far, and how numerous celestial bodies really are. Make the jump to light-years as we cruise through the Milky Way galaxy.
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