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HubbleSite Hubble Makes the First Precise Distance Measurement to an Ancient Globular Star Cluster

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Space Telescope Science



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.

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