Astronomers are mapping the motions of stars revolving around each other in the Large Magellanic Cloud (LMC). This reveals a tremendous, clock-like structure, turning once every 250 million years. Our own Milky Way galaxy rotates at about the same rate. 

Roeland van der Marel from the Space Telescope Science Institute, and Nitya Kallivayalil of the University of Virginia led the research. Observations were carried out using the Hubble Space Telescope (HST). The team spent seven years recording the motion of stars within the galaxy. 

"If we imagine a human on the moon, Hubble's precision would allow us to determine the speed at which the person's hair grows. This precision is crucial, because the apparent stellar motions are so small because of the galaxy's distance. 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," van der Marel said.

This is the first time the regular movements of stars have been used to precisely measure the rotational rate of a galaxy. The rotational velocities of galaxies can be measured through Doppler shift, provided the galaxy is positioned with its edge pointed toward Earth. The LMC, lying 170,000 light years away from Earth, is positioned with its face toward our home world. By combining new data of lateral motion with Doppler measurements, researchers were able to create the first-ever 3D map of stellar motions in a galaxy. 

In order to do this, van der Marel and his team needed to employ the Wide Field Camera 3 and Advanced Camera for Surveys aboard the space telescope. They carefully studied 22 different fields within the LMC, meticulously recording the small changes of position stars made over just a few years. 

A powerful quasar, fueled by a massive black hole, is located at the center of the Large Magellanic Cloud. Movement of stars in the structure were measured relative to this object. Our own Milky Way galaxy contains a similar black hole at its core. 

The LMC is visible in the southern hemisphere, appearing 20 times larger than the full Moon. Also visible in skies of the southern hemisphere is the Small Magellanic Cloud. This structure will be the next one studied by the astronomers, using Hubble. The two galaxies are interacting with each other and the Milky Way through gravity.