Hubble telescope sees nearby galaxy rotating like \'clock in the sky\'

U.S. astronomers using the Hubble Space Telescope say they've precisely measured, for the first time, the rotation rate of a galaxy. Based on the clock-like movement of its stars, the central part of a neighboring galaxy called the Large Magellanic Cloud (LMC) completes a rotation every 250 million years, the Space Telescope Science Institute reported Tuesday. Institute scientist Roeland van der Marel and Nitya Kallivayalil of the University of Virginia used Hubble to measure the average motion of hundreds of individual stars in the galaxy located 170,000 light-years away. Disk-shaped galaxies like the LMC and our own Milky Way generally rotate like a carousel. For the past century astronomers have calculated galaxy rotation rates by observing a slight shift in the spectrum of stars in the galaxy as one side of a galaxy's spinning stellar disk moves toward Earth while stars on the other side are moving away. "Determining a galaxy's rotation by measuring its instantaneous back and forth motions doesn't allow one to actually see things change over time," van der Marel said. "By using Hubble to study the stars' motions over several years, we can actually for the first time see a galaxy rotate in the plane of the sky." Hubble is the only telescope that can make this kind of observation because of its sharp resolution, its image stability, and its 24 years in space, the researchers said. "This precision is crucial, because the apparent stellar motions are so small because of the galaxy's distance," van der Marel said. "You can think of the LMC as a clock in the sky, on which the hands take 250 million years to make one revolution. We know the clock's hands move, but even with Hubble we need to stare at them for several years to see any movement."