ABOUT THIS IMAGE:

This ancient stellar jewelry box, a globular cluster called NGC 6397, glitters with the light from hundreds of thousands of stars. Astronomers using the Hubble Space Telescope to gauge the distance to this brilliant stellar grouping, obtained the first precise measurement ever made to an ancient globular cluster.

The new measurement sets the cluster's distance at 7,800 light-years away, with just a 3 percent margin of error. NGC 6397 is one of the closest globular clusters to Earth. By measuring an accurate distance to NGC 6397, astronomers then calculated a precise age for the cluster. The cluster is 13.4 billion years old, which means it was born shortly after the big bang. NGC 6397 is one of about 150 globular clusters that orbit outside of our Milky Way galaxy's comparatively younger starry disk. These spherical, densely packed swarms of hundreds of thousands of stars are our galaxy's first homesteaders.

The cluster's blue stars are near the end of their lives. These stars have used up their hydrogen fuel that makes them shine. Now they are converting helium to energy in their cores, which fuses at a higher temperature and appears blue. The reddish glow is from red giant stars that have consumed their hydrogen fuel and have expanded in size. The myriad small white objects include stars like our Sun.

This image is composed of a series of observations taken from July 2004 to June 2005 with Hubble's Advanced Camera for Surveys. The research team used Hubble's Wide Field Camera 3 to measure the distance to the cluster.

When you want to know the size of a room, you use a measuring tape to calculate its dimensions.
But you can't use a tape measure to cover the inconceivably vast distances in space. And, until now, astronomers did not have an equally precise method to accurately measure distances to some of the oldest objects in our universe - ancient swarms of stars outside the disk of our galaxy called globular clusters.

Estimated distances to our Milky Way galaxy's globular clusters were achieved by comparing the brightness and colors of stars to theoretical models and observations of local stars. But the accuracy of these estimates varies, with uncertainties hovering between 10 percent and 20 percent.

Using NASA's Hubble Space Telescope, astronomers were able to use the same sort of trigonometry that surveyors use to precisely measure the distance to NGC 6397, one of the closest globular clusters to Earth. The only difference is that the angles measured in Hubble's camera are infinitesimal by earthly surveyors' standards.

The new measurement sets the cluster's distance at 7,800 light-years away, with just a 3 percent margin of error, and provides an independent estimate for the age of the universe. The Hubble astronomers calculated NGC 6397 is 13.4 billion years old and so formed not long after the big bang. The new measurement also will help astronomers improve models of stellar evolution.