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Explanation: How big is our universe? This question, among others, was debated by two leading astronomers in 1920 in what has since become known as astronomy's Great Debate. Many astronomers then believed that our Milky Way Galaxy was the entire universe. Many others, though, believed that our galaxy was just one of many. In the Great Debate, each argument was detailed, but no consensus was reached. The answer came over three years later with the detected variation of single spot in the Andromeda Nebula, as shown on the original glass discovery plate digitally reproduced here. When Edwin Hubble compared images,...

It's been nearly a century since scientists first theorized that the Universe was expanding, and that the farther away a galaxy was from us, the faster it appears to recede. This isn't because galaxies are physically moving away from us, but rather because the Universe is full of gravitationally-bound objects, and the fabric of space that those objects reside in is expanding. But this picture, which held sway from the 1920s onward, has been recently revised. It's been only 20 years since we first realized that this expansion was speeding up, and that as time goes on, individual galaxies will...

WILLIAMS BAY, Wis. -- After more than 100 years of operation, the Yerkes Observatory in Williams Bay, Wisconsin, is closing. Owned by the University of Chicago, the observatory is slated to cease operations Oct. 1 “Unfortunately, operating Yerkes no longer makes sense for the university from a programmatic or cost standpoint," said David Fithian, U of C's executive vice president, in a statement. "Drawing to a close our operations there is the first step in a collaborative process to determine the ultimate disposition of the buildings and property. We currently have no specific plans nor have we approached any potential...

Allan R. Sandage, who spent his life measuring the universe, becoming the most influential astronomer of his generation, died Saturday at his home in San Gabriel, Calif. He was 84. The cause was pancreatic cancer, according to an announcement by the Carnegie Observatories, where he had spent his whole professional career. Over more than six decades, Dr. Sandage was like one of those giant galaxies that sit at the center of a cluster of galaxies, dominating cosmic weather. He wrote more than 500 papers, ranging across the cosmos, covering the evolution and behavior of stars, the birth of the Milky...