Using Leavitt’s method of measuring great distances, astronomers in the next few years were able to determine the size of our galaxy, the Milky Way, which is a giant congregation of about 200 billion stars. To express such mind-boggling sizes and distances, twentieth-century astronomers adopted a new unit of distance called the light-year, the distance that light travels in a year—about six trillion miles. In these units, the nearest stars are several light-years away. The diameter of the Milky Way has been measured to be about one hundred thousand light-years. In other words, it takes a ray
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